diff options
| author | nectar <nectar@FreeBSD.org> | 2002-09-18 14:17:14 +0000 |
|---|---|---|
| committer | nectar <nectar@FreeBSD.org> | 2002-09-18 14:17:14 +0000 |
| commit | 7458827ad1d35dab9a04225f963cd2a2531a1ae7 (patch) | |
| tree | d5143645a7cc49bb25a25e6aace23bb81dd25c85 /crypto/heimdal/doc/standardisation/draft-ietf-cat-kerberos-revisions-06.txt | |
| parent | 22fe384f6f4452e5def19ab22effb1b93a0a73ee (diff) | |
| download | FreeBSD-src-7458827ad1d35dab9a04225f963cd2a2531a1ae7.zip FreeBSD-src-7458827ad1d35dab9a04225f963cd2a2531a1ae7.tar.gz | |
These RFCs and internet-drafts are not really needed in the base
system, and I've not been importing them lately. cvs rm them now
so they can be cleaned out of the attic later.
Requested by: obrien
Diffstat (limited to 'crypto/heimdal/doc/standardisation/draft-ietf-cat-kerberos-revisions-06.txt')
| -rw-r--r-- | crypto/heimdal/doc/standardisation/draft-ietf-cat-kerberos-revisions-06.txt | 7301 |
1 files changed, 0 insertions, 7301 deletions
diff --git a/crypto/heimdal/doc/standardisation/draft-ietf-cat-kerberos-revisions-06.txt b/crypto/heimdal/doc/standardisation/draft-ietf-cat-kerberos-revisions-06.txt deleted file mode 100644 index ae79e8a..0000000 --- a/crypto/heimdal/doc/standardisation/draft-ietf-cat-kerberos-revisions-06.txt +++ /dev/null @@ -1,7301 +0,0 @@ -INTERNET-DRAFT Clifford Neuman - John Kohl - Theodore Ts'o - July 14, 2000 - Expires January 14, 2001 - -The Kerberos Network Authentication Service (V5) - - -draft-ietf-cat-kerberos-revisions-06.txt - -STATUS OF THIS MEMO - -This document is an Internet-Draft and is in full conformance with all -provisions of Section 10 of RFC 2026. Internet-Drafts are working documents -of the Internet Engineering Task Force (IETF), its areas, and its working -groups. Note that other groups may also distribute working documents as -Internet-Drafts. - -Internet-Drafts are draft documents valid for a maximum of six months and -may be updated, replaced, or obsoleted by other documents at any time. It -is inappropriate to use Internet-Drafts as reference material or to cite -them other than as "work in progress." - -The list of current Internet-Drafts can be accessed at -http://www.ietf.org/ietf/1id-abstracts.txt - -The list of Internet-Draft Shadow Directories can be accessed at -http://www.ietf.org/shadow.html. - -To learn the current status of any Internet-Draft, please check the -"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow -Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe), -ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). - -The distribution of this memo is unlimited. It is filed as -draft-ietf-cat-kerberos-revisions-06.txt, and expires January 14, 2001. -Please send comments to: krb-protocol@MIT.EDU - - This document is getting closer to a last call, but there are several - issues to be discussed. Some, but not all of these issues, are - highlighted in comments in the draft. We hope to resolve these issues - on the mailing list for the Kerberos working group, leading up to and - during the Pittsburgh IETF on a section by section basis, since this - is a long document, and it has been difficult to consider it as a - whole. Once sections are agreed to, it is out intent to issue the more - formal WG and IETF last calls. - -ABSTRACT - -This document provides an overview and specification of Version 5 of the -Kerberos protocol, and updates RFC1510 to clarify aspects of the protocol -and its intended use that require more detailed or clearer explanation than -was provided in RFC1510. This document is intended to provide a detailed -description of the protocol, suitable for implementation, together with -descriptions of the appropriate use of protocol messages and fields within -those messages. - -This document is not intended to describe Kerberos to the end user, system -administrator, or application developer. Higher level papers describing -Version 5 of the Kerberos system [NT94] and documenting version 4 [SNS88], -are available elsewhere. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -OVERVIEW - -This INTERNET-DRAFT describes the concepts and model upon which the -Kerberos network authentication system is based. It also specifies Version -5 of the Kerberos protocol. - -The motivations, goals, assumptions, and rationale behind most design -decisions are treated cursorily; they are more fully described in a paper -available in IEEE communications [NT94] and earlier in the Kerberos portion -of the Athena Technical Plan [MNSS87]. The protocols have been a proposed -standard and are being considered for advancement for draft standard -through the IETF standard process. Comments are encouraged on the -presentation, but only minor refinements to the protocol as implemented or -extensions that fit within current protocol framework will be considered at -this time. - -Requests for addition to an electronic mailing list for discussion of -Kerberos, kerberos@MIT.EDU, may be addressed to kerberos-request@MIT.EDU. -This mailing list is gatewayed onto the Usenet as the group -comp.protocols.kerberos. Requests for further information, including -documents and code availability, may be sent to info-kerberos@MIT.EDU. - -BACKGROUND - -The Kerberos model is based in part on Needham and Schroeder's trusted -third-party authentication protocol [NS78] and on modifications suggested -by Denning and Sacco [DS81]. The original design and implementation of -Kerberos Versions 1 through 4 was the work of two former Project Athena -staff members, Steve Miller of Digital Equipment Corporation and Clifford -Neuman (now at the Information Sciences Institute of the University of -Southern California), along with Jerome Saltzer, Technical Director of -Project Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many -other members of Project Athena have also contributed to the work on -Kerberos. - -Version 5 of the Kerberos protocol (described in this document) has evolved -from Version 4 based on new requirements and desires for features not -available in Version 4. The design of Version 5 of the Kerberos protocol -was led by Clifford Neuman and John Kohl with much input from the -community. The development of the MIT reference implementation was led at -MIT by John Kohl and Theodore T'so, with help and contributed code from -many others. Since RFC1510 was issued, extensions and revisions to the -protocol have been proposed by many individuals. Some of these proposals -are reflected in this document. Where such changes involved significant -effort, the document cites the contribution of the proposer. - -Reference implementations of both version 4 and version 5 of Kerberos are -publicly available and commercial implementations have been developed and -are widely used. Details on the differences between Kerberos Versions 4 and -5 can be found in [KNT92]. - -1. Introduction - -Kerberos provides a means of verifying the identities of principals, (e.g. -a workstation user or a network server) on an open (unprotected) network. -This is accomplished without relying on assertions by the host operating -system, without basing trust on host addresses, without requiring physical -security of all the hosts on the network, and under the assumption that -packets traveling along the network can be read, modified, and inserted at -will[1]. Kerberos performs authentication under these conditions as a -trusted third-party authentication service by using conventional (shared -secret key [2] cryptography. Kerberos extensions have been proposed and -implemented that provide for the use of public key cryptography during -certain phases of the authentication protocol. These extensions provide for - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -authentication of users registered with public key certification -authorities, and allow the system to provide certain benefits of public key -cryptography in situations where they are needed. - -The basic Kerberos authentication process proceeds as follows: A client -sends a request to the authentication server (AS) requesting 'credentials' -for a given server. The AS responds with these credentials, encrypted in -the client's key. The credentials consist of 1) a 'ticket' for the server -and 2) a temporary encryption key (often called a "session key"). The -client transmits the ticket (which contains the client's identity and a -copy of the session key, all encrypted in the server's key) to the server. -The session key (now shared by the client and server) is used to -authenticate the client, and may optionally be used to authenticate the -server. It may also be used to encrypt further communication between the -two parties or to exchange a separate sub-session key to be used to encrypt -further communication. - -Implementation of the basic protocol consists of one or more authentication -servers running on physically secure hosts. The authentication servers -maintain a database of principals (i.e., users and servers) and their -secret keys. Code libraries provide encryption and implement the Kerberos -protocol. In order to add authentication to its transactions, a typical -network application adds one or two calls to the Kerberos library directly -or through the Generic Security Services Application Programming Interface, -GSSAPI, described in separate document. These calls result in the -transmission of the necessary messages to achieve authentication. - -The Kerberos protocol consists of several sub-protocols (or exchanges). -There are two basic methods by which a client can ask a Kerberos server for -credentials. In the first approach, the client sends a cleartext request -for a ticket for the desired server to the AS. The reply is sent encrypted -in the client's secret key. Usually this request is for a ticket-granting -ticket (TGT) which can later be used with the ticket-granting server (TGS). -In the second method, the client sends a request to the TGS. The client -uses the TGT to authenticate itself to the TGS in the same manner as if it -were contacting any other application server that requires Kerberos -authentication. The reply is encrypted in the session key from the TGT. -Though the protocol specification describes the AS and the TGS as separate -servers, they are implemented in practice as different protocol entry -points within a single Kerberos server. - -Once obtained, credentials may be used to verify the identity of the -principals in a transaction, to ensure the integrity of messages exchanged -between them, or to preserve privacy of the messages. The application is -free to choose whatever protection may be necessary. - -To verify the identities of the principals in a transaction, the client -transmits the ticket to the application server. Since the ticket is sent -"in the clear" (parts of it are encrypted, but this encryption doesn't -thwart replay) and might be intercepted and reused by an attacker, -additional information is sent to prove that the message originated with -the principal to whom the ticket was issued. This information (called the -authenticator) is encrypted in the session key, and includes a timestamp. -The timestamp proves that the message was recently generated and is not a -replay. Encrypting the authenticator in the session key proves that it was -generated by a party possessing the session key. Since no one except the -requesting principal and the server know the session key (it is never sent -over the network in the clear) this guarantees the identity of the client. - -The integrity of the messages exchanged between principals can also be -guaranteed using the session key (passed in the ticket and contained in the -credentials). This approach provides detection of both replay attacks and -message stream modification attacks. It is accomplished by generating and -transmitting a collision-proof checksum (elsewhere called a hash or digest -function) of the client's message, keyed with the session key. Privacy and - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -integrity of the messages exchanged between principals can be secured by -encrypting the data to be passed using the session key contained in the -ticket or the subsession key found in the authenticator. - -The authentication exchanges mentioned above require read-only access to -the Kerberos database. Sometimes, however, the entries in the database must -be modified, such as when adding new principals or changing a principal's -key. This is done using a protocol between a client and a third Kerberos -server, the Kerberos Administration Server (KADM). There is also a protocol -for maintaining multiple copies of the Kerberos database. Neither of these -protocols are described in this document. - -1.1. Cross-Realm Operation - -The Kerberos protocol is designed to operate across organizational -boundaries. A client in one organization can be authenticated to a server -in another. Each organization wishing to run a Kerberos server establishes -its own 'realm'. The name of the realm in which a client is registered is -part of the client's name, and can be used by the end-service to decide -whether to honor a request. - -By establishing 'inter-realm' keys, the administrators of two realms can -allow a client authenticated in the local realm to prove its identity to -servers in other realms[3]. The exchange of inter-realm keys (a separate -key may be used for each direction) registers the ticket-granting service -of each realm as a principal in the other realm. A client is then able to -obtain a ticket-granting ticket for the remote realm's ticket-granting -service from its local realm. When that ticket-granting ticket is used, the -remote ticket-granting service uses the inter-realm key (which usually -differs from its own normal TGS key) to decrypt the ticket-granting ticket, -and is thus certain that it was issued by the client's own TGS. Tickets -issued by the remote ticket-granting service will indicate to the -end-service that the client was authenticated from another realm. - -A realm is said to communicate with another realm if the two realms share -an inter-realm key, or if the local realm shares an inter-realm key with an -intermediate realm that communicates with the remote realm. An -authentication path is the sequence of intermediate realms that are -transited in communicating from one realm to another. - -Realms are typically organized hierarchically. Each realm shares a key with -its parent and a different key with each child. If an inter-realm key is -not directly shared by two realms, the hierarchical organization allows an -authentication path to be easily constructed. If a hierarchical -organization is not used, it may be necessary to consult a database in -order to construct an authentication path between realms. - -Although realms are typically hierarchical, intermediate realms may be -bypassed to achieve cross-realm authentication through alternate -authentication paths (these might be established to make communication -between two realms more efficient). It is important for the end-service to -know which realms were transited when deciding how much faith to place in -the authentication process. To facilitate this decision, a field in each -ticket contains the names of the realms that were involved in -authenticating the client. - -The application server is ultimately responsible for accepting or rejecting -authentication and should check the transited field. The application server -may choose to rely on the KDC for the application server's realm to check -the transited field. The application server's KDC will set the -TRANSITED-POLICY-CHECKED flag in this case. The KDC's for intermediate -realms may also check the transited field as they issue -ticket-granting-tickets for other realms, but they are encouraged not to do -so. A client may request that the KDC's not check the transited field by -setting the DISABLE-TRANSITED-CHECK flag. KDC's are encouraged but not -required to honor this flag. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - [JBrezak] Should there be a section here on how clients determine what - realm a service is in? Something like: - - The client may not immediately know what realm a particular service - principal is in. There are 2 basic mechanisms that can be used to - determine the realm of a service. The first requires that the client - fully specify the service principal including the realm in the - Kerberos protocol request. If the Kerberos server for the specified - realm does not have a principal that exactly matches the service in - the request, the Kerberos server will return an error indicating that - the service principal was not found. Alternatively the client can make - a request providing just the service principal name and requesting - name canonicalization from the Kerberos server. The Kerberos server - will attempt to locate a service principal in its database that best - matches the request principal or provide a referral to another - Kerberos realm that may be contain the requested service principal. - -1.2. Authorization - -As an authentication service, Kerberos provides a means of verifying the -identity of principals on a network. Authentication is usually useful -primarily as a first step in the process of authorization, determining -whether a client may use a service, which objects the client is allowed to -access, and the type of access allowed for each. Kerberos does not, by -itself, provide authorization. Possession of a client ticket for a service -provides only for authentication of the client to that service, and in the -absence of a separate authorization procedure, it should not be considered -by an application as authorizing the use of that service. - -Such separate authorization methods may be implemented as application -specific access control functions and may be based on files such as the -application server, or on separately issued authorization credentials such -as those based on proxies [Neu93], or on other authorization services. -Separately authenticated authorization credentials may be embedded in a -tickets authorization data when encapsulated by the kdc-issued -authorization data element. - -Applications should not be modified to accept the mere issuance of a -service ticket by the Kerberos server (even by a modified Kerberos server) -as granting authority to use the service, since such applications may -become vulnerable to the bypass of this authorization check in an -environment if they interoperate with other KDCs or where other options for -application authentication (e.g. the PKTAPP proposal) are provided. - -1.3. Environmental assumptions - -Kerberos imposes a few assumptions on the environment in which it can -properly function: - - * 'Denial of service' attacks are not solved with Kerberos. There are - places in these protocols where an intruder can prevent an application - from participating in the proper authentication steps. Detection and - solution of such attacks (some of which can appear to be nnot-uncommon - 'normal' failure modes for the system) is usually best left to the - human administrators and users. - * Principals must keep their secret keys secret. If an intruder somehow - steals a principal's key, it will be able to masquerade as that - principal or impersonate any server to the legitimate principal. - * 'Password guessing' attacks are not solved by Kerberos. If a user - chooses a poor password, it is possible for an attacker to - successfully mount an offline dictionary attack by repeatedly - attempting to decrypt, with successive entries from a dictionary, - messages obtained which are encrypted under a key derived from the - user's password. - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - * Each host on the network must have a clock which is 'loosely - synchronized' to the time of the other hosts; this synchronization is - used to reduce the bookkeeping needs of application servers when they - do replay detection. The degree of "looseness" can be configured on a - per-server basis, but is typically on the order of 5 minutes. If the - clocks are synchronized over the network, the clock synchronization - protocol must itself be secured from network attackers. - * Principal identifiers are not recycled on a short-term basis. A - typical mode of access control will use access control lists (ACLs) to - grant permissions to particular principals. If a stale ACL entry - remains for a deleted principal and the principal identifier is - reused, the new principal will inherit rights specified in the stale - ACL entry. By not re-using principal identifiers, the danger of - inadvertent access is removed. - -1.4. Glossary of terms - -Below is a list of terms used throughout this document. - -Authentication - Verifying the claimed identity of a principal. -Authentication header - A record containing a Ticket and an Authenticator to be presented to a - server as part of the authentication process. -Authentication path - A sequence of intermediate realms transited in the authentication - process when communicating from one realm to another. -Authenticator - A record containing information that can be shown to have been - recently generated using the session key known only by the client and - server. -Authorization - The process of determining whether a client may use a service, which - objects the client is allowed to access, and the type of access - allowed for each. -Capability - A token that grants the bearer permission to access an object or - service. In Kerberos, this might be a ticket whose use is restricted - by the contents of the authorization data field, but which lists no - network addresses, together with the session key necessary to use the - ticket. -Ciphertext - The output of an encryption function. Encryption transforms plaintext - into ciphertext. -Client - A process that makes use of a network service on behalf of a user. - Note that in some cases a Server may itself be a client of some other - server (e.g. a print server may be a client of a file server). -Credentials - A ticket plus the secret session key necessary to successfully use - that ticket in an authentication exchange. -KDC - Key Distribution Center, a network service that supplies tickets and - temporary session keys; or an instance of that service or the host on - which it runs. The KDC services both initial ticket and - ticket-granting ticket requests. The initial ticket portion is - sometimes referred to as the Authentication Server (or service). The - ticket-granting ticket portion is sometimes referred to as the - ticket-granting server (or service). -Kerberos - Aside from the 3-headed dog guarding Hades, the name given to Project - Athena's authentication service, the protocol used by that service, or - the code used to implement the authentication service. -Plaintext - The input to an encryption function or the output of a decryption - function. Decryption transforms ciphertext into plaintext. - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -Principal - A uniquely named client or server instance that participates in a - network communication. -Principal identifier - The name used to uniquely identify each different principal. -Seal - To encipher a record containing several fields in such a way that the - fields cannot be individually replaced without either knowledge of the - encryption key or leaving evidence of tampering. -Secret key - An encryption key shared by a principal and the KDC, distributed - outside the bounds of the system, with a long lifetime. In the case of - a human user's principal, the secret key is derived from a password. -Server - A particular Principal which provides a resource to network clients. - The server is sometimes refered to as the Application Server. -Service - A resource provided to network clients; often provided by more than - one server (for example, remote file service). -Session key - A temporary encryption key used between two principals, with a - lifetime limited to the duration of a single login "session". -Sub-session key - A temporary encryption key used between two principals, selected and - exchanged by the principals using the session key, and with a lifetime - limited to the duration of a single association. -Ticket - A record that helps a client authenticate itself to a server; it - contains the client's identity, a session key, a timestamp, and other - information, all sealed using the server's secret key. It only serves - to authenticate a client when presented along with a fresh - Authenticator. - -2. Ticket flag uses and requests - -Each Kerberos ticket contains a set of flags which are used to indicate -various attributes of that ticket. Most flags may be requested by a client -when the ticket is obtained; some are automatically turned on and off by a -Kerberos server as required. The following sections explain what the -various flags mean, and gives examples of reasons to use such a flag. - -2.1. Initial and pre-authenticated tickets - -The INITIAL flag indicates that a ticket was issued using the AS protocol -and not issued based on a ticket-granting ticket. Application servers that -want to require the demonstrated knowledge of a client's secret key (e.g. a -password-changing program) can insist that this flag be set in any tickets -they accept, and thus be assured that the client's key was recently -presented to the application client. - -The PRE-AUTHENT and HW-AUTHENT flags provide addition information about the -initial authentication, regardless of whether the current ticket was issued -directly (in which case INITIAL will also be set) or issued on the basis of -a ticket-granting ticket (in which case the INITIAL flag is clear, but the -PRE-AUTHENT and HW-AUTHENT flags are carried forward from the -ticket-granting ticket). - -2.2. Invalid tickets - -The INVALID flag indicates that a ticket is invalid. Application servers -must reject tickets which have this flag set. A postdated ticket will -usually be issued in this form. Invalid tickets must be validated by the -KDC before use, by presenting them to the KDC in a TGS request with the -VALIDATE option specified. The KDC will only validate tickets after their -starttime has passed. The validation is required so that postdated tickets -which have been stolen before their starttime can be rendered permanently -invalid (through a hot-list mechanism) (see section 3.3.3.1). - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -2.3. Renewable tickets - -Applications may desire to hold tickets which can be valid for long periods -of time. However, this can expose their credentials to potential theft for -equally long periods, and those stolen credentials would be valid until the -expiration time of the ticket(s). Simply using short-lived tickets and -obtaining new ones periodically would require the client to have long-term -access to its secret key, an even greater risk. Renewable tickets can be -used to mitigate the consequences of theft. Renewable tickets have two -"expiration times": the first is when the current instance of the ticket -expires, and the second is the latest permissible value for an individual -expiration time. An application client must periodically (i.e. before it -expires) present a renewable ticket to the KDC, with the RENEW option set -in the KDC request. The KDC will issue a new ticket with a new session key -and a later expiration time. All other fields of the ticket are left -unmodified by the renewal process. When the latest permissible expiration -time arrives, the ticket expires permanently. At each renewal, the KDC may -consult a hot-list to determine if the ticket had been reported stolen -since its last renewal; it will refuse to renew such stolen tickets, and -thus the usable lifetime of stolen tickets is reduced. - -The RENEWABLE flag in a ticket is normally only interpreted by the -ticket-granting service (discussed below in section 3.3). It can usually be -ignored by application servers. However, some particularly careful -application servers may wish to disallow renewable tickets. - -If a renewable ticket is not renewed by its expiration time, the KDC will -not renew the ticket. The RENEWABLE flag is reset by default, but a client -may request it be set by setting the RENEWABLE option in the KRB_AS_REQ -message. If it is set, then the renew-till field in the ticket contains the -time after which the ticket may not be renewed. - -2.4. Postdated tickets - -Applications may occasionally need to obtain tickets for use much later, -e.g. a batch submission system would need tickets to be valid at the time -the batch job is serviced. However, it is dangerous to hold valid tickets -in a batch queue, since they will be on-line longer and more prone to -theft. Postdated tickets provide a way to obtain these tickets from the KDC -at job submission time, but to leave them "dormant" until they are -activated and validated by a further request of the KDC. If a ticket theft -were reported in the interim, the KDC would refuse to validate the ticket, -and the thief would be foiled. - -The MAY-POSTDATE flag in a ticket is normally only interpreted by the -ticket-granting service. It can be ignored by application servers. This -flag must be set in a ticket-granting ticket in order to issue a postdated -ticket based on the presented ticket. It is reset by default; it may be -requested by a client by setting the ALLOW-POSTDATE option in the -KRB_AS_REQ message. This flag does not allow a client to obtain a postdated -ticket-granting ticket; postdated ticket-granting tickets can only by -obtained by requesting the postdating in the KRB_AS_REQ message. The life -(endtime-starttime) of a postdated ticket will be the remaining life of the -ticket-granting ticket at the time of the request, unless the RENEWABLE -option is also set, in which case it can be the full life -(endtime-starttime) of the ticket-granting ticket. The KDC may limit how -far in the future a ticket may be postdated. - -The POSTDATED flag indicates that a ticket has been postdated. The -application server can check the authtime field in the ticket to see when -the original authentication occurred. Some services may choose to reject -postdated tickets, or they may only accept them within a certain period -after the original authentication. When the KDC issues a POSTDATED ticket, -it will also be marked as INVALID, so that the application client must -present the ticket to the KDC to be validated before use. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -2.5. Proxiable and proxy tickets - -At times it may be necessary for a principal to allow a service to perform -an operation on its behalf. The service must be able to take on the -identity of the client, but only for a particular purpose. A principal can -allow a service to take on the principal's identity for a particular -purpose by granting it a proxy. - -The process of granting a proxy using the proxy and proxiable flags is used -to provide credentials for use with specific services. Though conceptually -also a proxy, user's wishing to delegate their identity for ANY purpose -must use the ticket forwarding mechanism described in the next section to -forward a ticket granting ticket. - -The PROXIABLE flag in a ticket is normally only interpreted by the -ticket-granting service. It can be ignored by application servers. When -set, this flag tells the ticket-granting server that it is OK to issue a -new ticket (but not a ticket-granting ticket) with a different network -address based on this ticket. This flag is set if requested by the client -on initial authentication. By default, the client will request that it be -set when requesting a ticket granting ticket, and reset when requesting any -other ticket. - -This flag allows a client to pass a proxy to a server to perform a remote -request on its behalf, e.g. a print service client can give the print -server a proxy to access the client's files on a particular file server in -order to satisfy a print request. - -In order to complicate the use of stolen credentials, Kerberos tickets are -usually valid from only those network addresses specifically included in -the ticket[4]. When granting a proxy, the client must specify the new -network address from which the proxy is to be used, or indicate that the -proxy is to be issued for use from any address. - -The PROXY flag is set in a ticket by the TGS when it issues a proxy ticket. -Application servers may check this flag and at their option they may -require additional authentication from the agent presenting the proxy in -order to provide an audit trail. - -2.6. Forwardable tickets - -Authentication forwarding is an instance of a proxy where the service is -granted complete use of the client's identity. An example where it might be -used is when a user logs in to a remote system and wants authentication to -work from that system as if the login were local. - -The FORWARDABLE flag in a ticket is normally only interpreted by the -ticket-granting service. It can be ignored by application servers. The -FORWARDABLE flag has an interpretation similar to that of the PROXIABLE -flag, except ticket-granting tickets may also be issued with different -network addresses. This flag is reset by default, but users may request -that it be set by setting the FORWARDABLE option in the AS request when -they request their initial ticket- granting ticket. - -This flag allows for authentication forwarding without requiring the user -to enter a password again. If the flag is not set, then authentication -forwarding is not permitted, but the same result can still be achieved if -the user engages in the AS exchange specifying the requested network -addresses and supplies a password. - -The FORWARDED flag is set by the TGS when a client presents a ticket with -the FORWARDABLE flag set and requests a forwarded ticket by specifying the -FORWARDED KDC option and supplying a set of addresses for the new ticket. -It is also set in all tickets issued based on tickets with the FORWARDED -flag set. Application servers may choose to process FORWARDED tickets -differently than non-FORWARDED tickets. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -2.7 Name canonicalization [JBrezak] - -If a client does not have the full name information for a principal, it can -request that the Kerberos server attempt to lookup the name in its database -and return a canonical form of the requested principal or a referral to a -realm that has the requested principal in its namespace. Name -canonicalization allows a principal to have alternate names. Name -canonicalization must not be used to locate principal names supplied from -wildcards and is not a mechanism to be used to search a Kerberos database. - -The CANONICALIZE flag in a ticket request is used to indicate to the -Kerberos server that the client will accept an alternative name to the -principal in the request or a referral to another realm. Both the AS and -TGS must be able to interpret requests with this flag. - -By using this flag, the client can avoid extensive configuration needed to -map specific host names to a particular realm. - -2.8. Other KDC options - -There are two additional options which may be set in a client's request of -the KDC. The RENEWABLE-OK option indicates that the client will accept a -renewable ticket if a ticket with the requested life cannot otherwise be -provided. If a ticket with the requested life cannot be provided, then the -KDC may issue a renewable ticket with a renew-till equal to the the -requested endtime. The value of the renew-till field may still be adjusted -by site-determined limits or limits imposed by the individual principal or -server. - -The ENC-TKT-IN-SKEY option is honored only by the ticket-granting service. -It indicates that the ticket to be issued for the end server is to be -encrypted in the session key from the a additional second ticket-granting -ticket provided with the request. See section 3.3.3 for specific details. - -3. Message Exchanges - -The following sections describe the interactions between network clients -and servers and the messages involved in those exchanges. - -3.1. The Authentication Service Exchange - - Summary - Message direction Message type Section - 1. Client to Kerberos KRB_AS_REQ 5.4.1 - 2. Kerberos to client KRB_AS_REP or 5.4.2 - KRB_ERROR 5.9.1 - -The Authentication Service (AS) Exchange between the client and the -Kerberos Authentication Server is initiated by a client when it wishes to -obtain authentication credentials for a given server but currently holds no -credentials. In its basic form, the client's secret key is used for -encryption and decryption. This exchange is typically used at the -initiation of a login session to obtain credentials for a Ticket-Granting -Server which will subsequently be used to obtain credentials for other -servers (see section 3.3) without requiring further use of the client's -secret key. This exchange is also used to request credentials for services -which must not be mediated through the Ticket-Granting Service, but rather -require a principal's secret key, such as the password-changing service[5]. -This exchange does not by itself provide any assurance of the the identity -of the user[6]. - -The exchange consists of two messages: KRB_AS_REQ from the client to -Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for these -messages are described in sections 5.4.1, 5.4.2, and 5.9.1. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -In the request, the client sends (in cleartext) its own identity and the -identity of the server for which it is requesting credentials. The -response, KRB_AS_REP, contains a ticket for the client to present to the -server, and a session key that will be shared by the client and the server. -The session key and additional information are encrypted in the client's -secret key. The KRB_AS_REP message contains information which can be used -to detect replays, and to associate it with the message to which it -replies. Various errors can occur; these are indicated by an error response -(KRB_ERROR) instead of the KRB_AS_REP response. The error message is not -encrypted. The KRB_ERROR message contains information which can be used to -associate it with the message to which it replies. The lack of encryption -in the KRB_ERROR message precludes the ability to detect replays, -fabrications, or modifications of such messages. - -Without preautentication, the authentication server does not know whether -the client is actually the principal named in the request. It simply sends -a reply without knowing or caring whether they are the same. This is -acceptable because nobody but the principal whose identity was given in the -request will be able to use the reply. Its critical information is -encrypted in that principal's key. The initial request supports an optional -field that can be used to pass additional information that might be needed -for the initial exchange. This field may be used for preauthentication as -described in section [hl<>]. - -3.1.1. Generation of KRB_AS_REQ message - -The client may specify a number of options in the initial request. Among -these options are whether pre-authentication is to be performed; whether -the requested ticket is to be renewable, proxiable, or forwardable; whether -it should be postdated or allow postdating of derivative tickets; whether -the client requests name-canonicalization; and whether a renewable ticket -will be accepted in lieu of a non-renewable ticket if the requested ticket -expiration date cannot be satisfied by a non-renewable ticket (due to -configuration constraints; see section 4). See section A.1 for pseudocode. - -The client prepares the KRB_AS_REQ message and sends it to the KDC. - -3.1.2. Receipt of KRB_AS_REQ message - -If all goes well, processing the KRB_AS_REQ message will result in the -creation of a ticket for the client to present to the server. The format -for the ticket is described in section 5.3.1. The contents of the ticket -are determined as follows. - -3.1.3. Generation of KRB_AS_REP message - -The authentication server looks up the client and server principals named -in the KRB_AS_REQ in its database, extracting their respective keys. If -the requested client principal named in the request is not found in its -database, then an error message with a KDC_ERR_C_PRINCIPAL_UNKNOWN is -returned. If the request had the CANONICALIZE option set, then the AS can -attempt to lookup the client principal name in an alternate database, if it -is found an error message with a KDC_ERR_WRONG_REALM error code and the -cname and crealm in the error message must contain the true client -principal name and realm. - -If required, the server pre-authenticates the request, and if the -pre-authentication check fails, an error message with the code -KDC_ERR_PREAUTH_FAILED is returned. If the server cannot accommodate the -requested encryption type, an error message with code KDC_ERR_ETYPE_NOSUPP -is returned. Otherwise it generates a 'random' session key[7]. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -If there are multiple encryption keys registered for a client in the -Kerberos database (or if the key registered supports multiple encryption -types; e.g. DES3-CBC-SHA1 and DES3-CBC-SHA1-KD), then the etype field from -the AS request is used by the KDC to select the encryption method to be -used for encrypting the response to the client. If there is more than one -supported, strong encryption type in the etype list, the first valid etype -for which an encryption key is available is used. The encryption method -used to respond to a TGS request is taken from the keytype of the session -key found in the ticket granting ticket. - - JBrezak - the behavior of PW-SALT, and ETYPE-INFO should be explained - here; also about using keys that have different string-to-key - functions like AFSsalt - -When the etype field is present in a KDC request, whether an AS or TGS -request, the KDC will attempt to assign the type of the random session key -from the list of methods in the etype field. The KDC will select the -appropriate type using the list of methods provided together with -information from the Kerberos database indicating acceptable encryption -methods for the application server. The KDC will not issue tickets with a -weak session key encryption type. - -If the requested start time is absent, indicates a time in the past, or is -within the window of acceptable clock skew for the KDC and the POSTDATE -option has not been specified, then the start time of the ticket is set to -the authentication server's current time. If it indicates a time in the -future beyond the acceptable clock skew, but the POSTDATED option has not -been specified then the error KDC_ERR_CANNOT_POSTDATE is returned. -Otherwise the requested start time is checked against the policy of the -local realm (the administrator might decide to prohibit certain types or -ranges of postdated tickets), and if acceptable, the ticket's start time is -set as requested and the INVALID flag is set in the new ticket. The -postdated ticket must be validated before use by presenting it to the KDC -after the start time has been reached. - -The expiration time of the ticket will be set to the minimum of the -following: - - * The expiration time (endtime) requested in the KRB_AS_REQ message. - * The ticket's start time plus the maximum allowable lifetime associated - with the client principal (the authentication server's database - includes a maximum ticket lifetime field in each principal's record; - see section 4). - * The ticket's start time plus the maximum allowable lifetime associated - with the server principal. - * The ticket's start time plus the maximum lifetime set by the policy of - the local realm. - -If the requested expiration time minus the start time (as determined above) -is less than a site-determined minimum lifetime, an error message with code -KDC_ERR_NEVER_VALID is returned. If the requested expiration time for the -ticket exceeds what was determined as above, and if the 'RENEWABLE-OK' -option was requested, then the 'RENEWABLE' flag is set in the new ticket, -and the renew-till value is set as if the 'RENEWABLE' option were requested -(the field and option names are described fully in section 5.4.1). - -If the RENEWABLE option has been requested or if the RENEWABLE-OK option -has been set and a renewable ticket is to be issued, then the renew-till -field is set to the minimum of: - - * Its requested value. - * The start time of the ticket plus the minimum of the two maximum - renewable lifetimes associated with the principals' database entries. - * The start time of the ticket plus the maximum renewable lifetime set - by the policy of the local realm. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -The flags field of the new ticket will have the following options set if -they have been requested and if the policy of the local realm allows: -FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE. If the new -ticket is post-dated (the start time is in the future), its INVALID flag -will also be set. - -If all of the above succeed, the server formats a KRB_AS_REP message (see -section 5.4.2), copying the addresses in the request into the caddr of the -response, placing any required pre-authentication data into the padata of -the response, and encrypts the ciphertext part in the client's key using -the requested encryption method, and sends it to the client. See section -A.2 for pseudocode. - -3.1.4. Generation of KRB_ERROR message - -Several errors can occur, and the Authentication Server responds by -returning an error message, KRB_ERROR, to the client, with the error-code -and e-text fields set to appropriate values. The error message contents and -details are described in Section 5.9.1. - -3.1.5. Receipt of KRB_AS_REP message - -If the reply message type is KRB_AS_REP, then the client verifies that the -cname and crealm fields in the cleartext portion of the reply match what it -requested. If any padata fields are present, they may be used to derive the -proper secret key to decrypt the message. The client decrypts the encrypted -part of the response using its secret key, verifies that the nonce in the -encrypted part matches the nonce it supplied in its request (to detect -replays). It also verifies that the sname and srealm in the response match -those in the request (or are otherwise expected values), and that the host -address field is also correct. It then stores the ticket, session key, -start and expiration times, and other information for later use. The -key-expiration field from the encrypted part of the response may be checked -to notify the user of impending key expiration (the client program could -then suggest remedial action, such as a password change). See section A.3 -for pseudocode. - -Proper decryption of the KRB_AS_REP message is not sufficient to verify the -identity of the user; the user and an attacker could cooperate to generate -a KRB_AS_REP format message which decrypts properly but is not from the -proper KDC. If the host wishes to verify the identity of the user, it must -require the user to present application credentials which can be verified -using a securely-stored secret key for the host. If those credentials can -be verified, then the identity of the user can be assured. - -3.1.6. Receipt of KRB_ERROR message - -If the reply message type is KRB_ERROR, then the client interprets it as an -error and performs whatever application-specific tasks are necessary to -recover. If the client set the CANONICALIZE option and a -KDC_ERR_WRONG_REALM error was returned, the AS request should be retried to -the realm and client principal name specified in the error message crealm -and cname field respectively. - -3.2. The Client/Server Authentication Exchange - - Summary -Message direction Message type Section -Client to Application server KRB_AP_REQ 5.5.1 -[optional] Application server to client KRB_AP_REP or 5.5.2 - KRB_ERROR 5.9.1 - -The client/server authentication (CS) exchange is used by network -applications to authenticate the client to the server and vice versa. The -client must have already acquired credentials for the server using the AS -or TGS exchange. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -3.2.1. The KRB_AP_REQ message - -The KRB_AP_REQ contains authentication information which should be part of -the first message in an authenticated transaction. It contains a ticket, an -authenticator, and some additional bookkeeping information (see section -5.5.1 for the exact format). The ticket by itself is insufficient to -authenticate a client, since tickets are passed across the network in -cleartext[DS90], so the authenticator is used to prevent invalid replay of -tickets by proving to the server that the client knows the session key of -the ticket and thus is entitled to use the ticket. The KRB_AP_REQ message -is referred to elsewhere as the 'authentication header.' - -3.2.2. Generation of a KRB_AP_REQ message - -When a client wishes to initiate authentication to a server, it obtains -(either through a credentials cache, the AS exchange, or the TGS exchange) -a ticket and session key for the desired service. The client may re-use any -tickets it holds until they expire. To use a ticket the client constructs a -new Authenticator from the the system time, its name, and optionally an -application specific checksum, an initial sequence number to be used in -KRB_SAFE or KRB_PRIV messages, and/or a session subkey to be used in -negotiations for a session key unique to this particular session. -Authenticators may not be re-used and will be rejected if replayed to a -server[LGDSR87]. If a sequence number is to be included, it should be -randomly chosen so that even after many messages have been exchanged it is -not likely to collide with other sequence numbers in use. - -The client may indicate a requirement of mutual authentication or the use -of a session-key based ticket by setting the appropriate flag(s) in the -ap-options field of the message. - -The Authenticator is encrypted in the session key and combined with the -ticket to form the KRB_AP_REQ message which is then sent to the end server -along with any additional application-specific information. See section A.9 -for pseudocode. - -3.2.3. Receipt of KRB_AP_REQ message - -Authentication is based on the server's current time of day (clocks must be -loosely synchronized), the authenticator, and the ticket. Several errors -are possible. If an error occurs, the server is expected to reply to the -client with a KRB_ERROR message. This message may be encapsulated in the -application protocol if its 'raw' form is not acceptable to the protocol. -The format of error messages is described in section 5.9.1. - -The algorithm for verifying authentication information is as follows. If -the message type is not KRB_AP_REQ, the server returns the -KRB_AP_ERR_MSG_TYPE error. If the key version indicated by the Ticket in -the KRB_AP_REQ is not one the server can use (e.g., it indicates an old -key, and the server no longer possesses a copy of the old key), the -KRB_AP_ERR_BADKEYVER error is returned. If the USE-SESSION-KEY flag is set -in the ap-options field, it indicates to the server that the ticket is -encrypted in the session key from the server's ticket-granting ticket -rather than its secret key[10]. Since it is possible for the server to be -registered in multiple realms, with different keys in each, the srealm -field in the unencrypted portion of the ticket in the KRB_AP_REQ is used to -specify which secret key the server should use to decrypt that ticket. The -KRB_AP_ERR_NOKEY error code is returned if the server doesn't have the -proper key to decipher the ticket. - -The ticket is decrypted using the version of the server's key specified by -the ticket. If the decryption routines detect a modification of the ticket -(each encryption system must provide safeguards to detect modified -ciphertext; see section 6), the KRB_AP_ERR_BAD_INTEGRITY error is returned -(chances are good that different keys were used to encrypt and decrypt). - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -The authenticator is decrypted using the session key extracted from the -decrypted ticket. If decryption shows it to have been modified, the -KRB_AP_ERR_BAD_INTEGRITY error is returned. The name and realm of the -client from the ticket are compared against the same fields in the -authenticator. If they don't match, the KRB_AP_ERR_BADMATCH error is -returned (they might not match, for example, if the wrong session key was -used to encrypt the authenticator). The addresses in the ticket (if any) -are then searched for an address matching the operating-system reported -address of the client. If no match is found or the server insists on ticket -addresses but none are present in the ticket, the KRB_AP_ERR_BADADDR error -is returned. - -If the local (server) time and the client time in the authenticator differ -by more than the allowable clock skew (e.g., 5 minutes), the -KRB_AP_ERR_SKEW error is returned. If the server name, along with the -client name, time and microsecond fields from the Authenticator match any -recently-seen such tuples, the KRB_AP_ERR_REPEAT error is returned[11]. The -server must remember any authenticator presented within the allowable clock -skew, so that a replay attempt is guaranteed to fail. If a server loses -track of any authenticator presented within the allowable clock skew, it -must reject all requests until the clock skew interval has passed. This -assures that any lost or re-played authenticators will fall outside the -allowable clock skew and can no longer be successfully replayed (If this is -not done, an attacker could conceivably record the ticket and authenticator -sent over the network to a server, then disable the client's host, pose as -the disabled host, and replay the ticket and authenticator to subvert the -authentication.). If a sequence number is provided in the authenticator, -the server saves it for later use in processing KRB_SAFE and/or KRB_PRIV -messages. If a subkey is present, the server either saves it for later use -or uses it to help generate its own choice for a subkey to be returned in a -KRB_AP_REP message. - -The server computes the age of the ticket: local (server) time minus the -start time inside the Ticket. If the start time is later than the current -time by more than the allowable clock skew or if the INVALID flag is set in -the ticket, the KRB_AP_ERR_TKT_NYV error is returned. Otherwise, if the -current time is later than end time by more than the allowable clock skew, -the KRB_AP_ERR_TKT_EXPIRED error is returned. - -If all these checks succeed without an error, the server is assured that -the client possesses the credentials of the principal named in the ticket -and thus, the client has been authenticated to the server. See section A.10 -for pseudocode. - -Passing these checks provides only authentication of the named principal; -it does not imply authorization to use the named service. Applications must -make a separate authorization decisions based upon the authenticated name -of the user, the requested operation, local acces control information such -as that contained in a .k5login or .k5users file, and possibly a separate -distributed authorization service. - -3.2.4. Generation of a KRB_AP_REP message - -Typically, a client's request will include both the authentication -information and its initial request in the same message, and the server -need not explicitly reply to the KRB_AP_REQ. However, if mutual -authentication (not only authenticating the client to the server, but also -the server to the client) is being performed, the KRB_AP_REQ message will - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -have MUTUAL-REQUIRED set in its ap-options field, and a KRB_AP_REP message -is required in response. As with the error message, this message may be -encapsulated in the application protocol if its "raw" form is not -acceptable to the application's protocol. The timestamp and microsecond -field used in the reply must be the client's timestamp and microsecond -field (as provided in the authenticator)[12]. If a sequence number is to be -included, it should be randomly chosen as described above for the -authenticator. A subkey may be included if the server desires to negotiate -a different subkey. The KRB_AP_REP message is encrypted in the session key -extracted from the ticket. See section A.11 for pseudocode. - -3.2.5. Receipt of KRB_AP_REP message - -If a KRB_AP_REP message is returned, the client uses the session key from -the credentials obtained for the server[13] to decrypt the message, and -verifies that the timestamp and microsecond fields match those in the -Authenticator it sent to the server. If they match, then the client is -assured that the server is genuine. The sequence number and subkey (if -present) are retained for later use. See section A.12 for pseudocode. - -3.2.6. Using the encryption key - -After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and -server share an encryption key which can be used by the application. The -'true session key' to be used for KRB_PRIV, KRB_SAFE, or other -application-specific uses may be chosen by the application based on the -subkeys in the KRB_AP_REP message and the authenticator[14]. In some cases, -the use of this session key will be implicit in the protocol; in others the -method of use must be chosen from several alternatives. We leave the -protocol negotiations of how to use the key (e.g. selecting an encryption -or checksum type) to the application programmer; the Kerberos protocol does -not constrain the implementation options, but an example of how this might -be done follows. - -One way that an application may choose to negotiate a key to be used for -subequent integrity and privacy protection is for the client to propose a -key in the subkey field of the authenticator. The server can then choose a -key using the proposed key from the client as input, returning the new -subkey in the subkey field of the application reply. This key could then be -used for subsequent communication. To make this example more concrete, if -the encryption method in use required a 56 bit key, and for whatever -reason, one of the parties was prevented from using a key with more than 40 -unknown bits, this method would allow the the party which is prevented from -using more than 40 bits to either propose (if the client) an initial key -with a known quantity for 16 of those bits, or to mask 16 of the bits (if -the server) with the known quantity. The application implementor is warned, -however, that this is only an example, and that an analysis of the -particular crytosystem to be used, and the reasons for limiting the key -length, must be made before deciding whether it is acceptable to mask bits -of the key. - -With both the one-way and mutual authentication exchanges, the peers should -take care not to send sensitive information to each other without proper -assurances. In particular, applications that require privacy or integrity -should use the KRB_AP_REP response from the server to client to assure both -client and server of their peer's identity. If an application protocol -requires privacy of its messages, it can use the KRB_PRIV message (section -3.5). The KRB_SAFE message (section 3.4) can be used to assure integrity. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -3.3. The Ticket-Granting Service (TGS) Exchange - - Summary - Message direction Message type Section - 1. Client to Kerberos KRB_TGS_REQ 5.4.1 - 2. Kerberos to client KRB_TGS_REP or 5.4.2 - KRB_ERROR 5.9.1 - -The TGS exchange between a client and the Kerberos Ticket-Granting Server -is initiated by a client when it wishes to obtain authentication -credentials for a given server (which might be registered in a remote -realm), when it wishes to renew or validate an existing ticket, or when it -wishes to obtain a proxy ticket. In the first case, the client must already -have acquired a ticket for the Ticket-Granting Service using the AS -exchange (the ticket-granting ticket is usually obtained when a client -initially authenticates to the system, such as when a user logs in). The -message format for the TGS exchange is almost identical to that for the AS -exchange. The primary difference is that encryption and decryption in the -TGS exchange does not take place under the client's key. Instead, the -session key from the ticket-granting ticket or renewable ticket, or -sub-session key from an Authenticator is used. As is the case for all -application servers, expired tickets are not accepted by the TGS, so once a -renewable or ticket-granting ticket expires, the client must use a separate -exchange to obtain valid tickets. - -The TGS exchange consists of two messages: A request (KRB_TGS_REQ) from the -client to the Kerberos Ticket-Granting Server, and a reply (KRB_TGS_REP or -KRB_ERROR). The KRB_TGS_REQ message includes information authenticating the -client plus a request for credentials. The authentication information -consists of the authentication header (KRB_AP_REQ) which includes the -client's previously obtained ticket-granting, renewable, or invalid ticket. -In the ticket-granting ticket and proxy cases, the request may include one -or more of: a list of network addresses, a collection of typed -authorization data to be sealed in the ticket for authorization use by the -application server, or additional tickets (the use of which are described -later). The TGS reply (KRB_TGS_REP) contains the requested credentials, -encrypted in the session key from the ticket-granting ticket or renewable -ticket, or if present, in the sub-session key from the Authenticator (part -of the authentication header). The KRB_ERROR message contains an error code -and text explaining what went wrong. The KRB_ERROR message is not -encrypted. The KRB_TGS_REP message contains information which can be used -to detect replays, and to associate it with the message to which it -replies. The KRB_ERROR message also contains information which can be used -to associate it with the message to which it replies, but the lack of -encryption in the KRB_ERROR message precludes the ability to detect replays -or fabrications of such messages. - -3.3.1. Generation of KRB_TGS_REQ message - -Before sending a request to the ticket-granting service, the client must -determine in which realm the application server is registered[15], if it is -known. If the client does know the service principal name and realm and it -does not already possess a ticket-granting ticket for the appropriate -realm, then one must be obtained. This is first attempted by requesting a -ticket-granting ticket for the destination realm from a Kerberos server for -which the client does posess a ticket-granting ticket (using the -KRB_TGS_REQ message recursively). The Kerberos server may return a TGT for -the desired realm in which case one can proceed. - -If the client does not know the realm of the service or the true service -principal name, then the CANONICALIZE option must be used in the request. -This will cause the TGS to locate the service principal based on the target -service name in the ticket and return the service principal name in the -response. Alternatively, the Kerberos server may return a TGT for a realm -which is 'closer' to the desired realm (further along the standard - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -hierarchical path) or the realm that may contain the requested service -principal name in a request with the CANONCALIZE option set [JBrezak], in -which case this step must be repeated with a Kerberos server in the realm -specified in the returned TGT. If neither are returned, then the request -must be retried with a Kerberos server for a realm higher in the hierarchy. -This request will itself require a ticket-granting ticket for the higher -realm which must be obtained by recursively applying these directions. - -Once the client obtains a ticket-granting ticket for the appropriate realm, -it determines which Kerberos servers serve that realm, and contacts one. -The list might be obtained through a configuration file or network service -or it may be generated from the name of the realm; as long as the secret -keys exchanged by realms are kept secret, only denial of service results -from using a false Kerberos server. - -As in the AS exchange, the client may specify a number of options in the -KRB_TGS_REQ message. The client prepares the KRB_TGS_REQ message, providing -an authentication header as an element of the padata field, and including -the same fields as used in the KRB_AS_REQ message along with several -optional fields: the enc-authorization-data field for application server -use and additional tickets required by some options. - -In preparing the authentication header, the client can select a sub-session -key under which the response from the Kerberos server will be -encrypted[16]. If the sub-session key is not specified, the session key -from the ticket-granting ticket will be used. If the enc-authorization-data -is present, it must be encrypted in the sub-session key, if present, from -the authenticator portion of the authentication header, or if not present, -using the session key from the ticket-granting ticket. - -Once prepared, the message is sent to a Kerberos server for the destination -realm. See section A.5 for pseudocode. - -3.3.2. Receipt of KRB_TGS_REQ message - -The KRB_TGS_REQ message is processed in a manner similar to the KRB_AS_REQ -message, but there are many additional checks to be performed. First, the -Kerberos server must determine which server the accompanying ticket is for -and it must select the appropriate key to decrypt it. For a normal -KRB_TGS_REQ message, it will be for the ticket granting service, and the -TGS's key will be used. If the TGT was issued by another realm, then the -appropriate inter-realm key must be used. If the accompanying ticket is not -a ticket granting ticket for the current realm, but is for an application -server in the current realm, the RENEW, VALIDATE, or PROXY options are -specified in the request, and the server for which a ticket is requested is -the server named in the accompanying ticket, then the KDC will decrypt the -ticket in the authentication header using the key of the server for which -it was issued. If no ticket can be found in the padata field, the -KDC_ERR_PADATA_TYPE_NOSUPP error is returned. - -Once the accompanying ticket has been decrypted, the user-supplied checksum -in the Authenticator must be verified against the contents of the request, -and the message rejected if the checksums do not match (with an error code -of KRB_AP_ERR_MODIFIED) or if the checksum is not keyed or not -collision-proof (with an error code of KRB_AP_ERR_INAPP_CKSUM). If the -checksum type is not supported, the KDC_ERR_SUMTYPE_NOSUPP error is -returned. If the authorization-data are present, they are decrypted using -the sub-session key from the Authenticator. - -If any of the decryptions indicate failed integrity checks, the -KRB_AP_ERR_BAD_INTEGRITY error is returned. If the CANONICALIZE option is -set in the KRB_TGS_REQ, then the requested service name may not be the true -principal name or the service may not be in the TGS realm. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -3.3.3. Generation of KRB_TGS_REP message - -The KRB_TGS_REP message shares its format with the KRB_AS_REP -(KRB_KDC_REP), but with its type field set to KRB_TGS_REP. The detailed -specification is in section 5.4.2. - -The response will include a ticket for the requested server. The Kerberos -database is queried to retrieve the record for the requested server -(including the key with which the ticket will be encrypted). If the request -is for a ticket granting ticket for a remote realm, and if no key is shared -with the requested realm, then the Kerberos server will select the realm -"closest" to the requested realm with which it does share a key, and use -that realm instead. If the CANONICALIZE option is set, the TGS may return a -ticket containing the server name of the true service principal. If the -requested server cannot be found in the TGS database, then a TGT for -another trusted realm may be returned instead of a ticket for the service. -This TGT is a referral mechanism to cause the client to retry the request -to the realm of the TGT. These are the only cases where the response for -the KDC will be for a different server than that requested by the client. - -By default, the address field, the client's name and realm, the list of -transited realms, the time of initial authentication, the expiration time, -and the authorization data of the newly-issued ticket will be copied from -the ticket-granting ticket (TGT) or renewable ticket. If the transited -field needs to be updated, but the transited type is not supported, the -KDC_ERR_TRTYPE_NOSUPP error is returned. - -If the request specifies an endtime, then the endtime of the new ticket is -set to the minimum of (a) that request, (b) the endtime from the TGT, and -(c) the starttime of the TGT plus the minimum of the maximum life for the -application server and the maximum life for the local realm (the maximum -life for the requesting principal was already applied when the TGT was -issued). If the new ticket is to be a renewal, then the endtime above is -replaced by the minimum of (a) the value of the renew_till field of the -ticket and (b) the starttime for the new ticket plus the life -(endtime-starttime) of the old ticket. - -If the FORWARDED option has been requested, then the resulting ticket will -contain the addresses specified by the client. This option will only be -honored if the FORWARDABLE flag is set in the TGT. The PROXY option is -similar; the resulting ticket will contain the addresses specified by the -client. It will be honored only if the PROXIABLE flag in the TGT is set. -The PROXY option will not be honored on requests for additional -ticket-granting tickets. - -If the requested start time is absent, indicates a time in the past, or is -within the window of acceptable clock skew for the KDC and the POSTDATE -option has not been specified, then the start time of the ticket is set to -the authentication server's current time. If it indicates a time in the -future beyond the acceptable clock skew, but the POSTDATED option has not -been specified or the MAY-POSTDATE flag is not set in the TGT, then the -error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise, if the -ticket-granting ticket has the MAY-POSTDATE flag set, then the resulting -ticket will be postdated and the requested starttime is checked against the -policy of the local realm. If acceptable, the ticket's start time is set as -requested, and the INVALID flag is set. The postdated ticket must be -validated before use by presenting it to the KDC after the starttime has -been reached. However, in no case may the starttime, endtime, or renew-till -time of a newly-issued postdated ticket extend beyond the renew-till time -of the ticket-granting ticket. - -If the ENC-TKT-IN-SKEY option has been specified and an additional ticket -has been included in the request, the KDC will decrypt the additional -ticket using the key for the server to which the additional ticket was -issued and verify that it is a ticket-granting ticket. If the name of the - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -requested server is missing from the request, the name of the client in the -additional ticket will be used. Otherwise the name of the requested server -will be compared to the name of the client in the additional ticket and if -different, the request will be rejected. If the request succeeds, the -session key from the additional ticket will be used to encrypt the new -ticket that is issued instead of using the key of the server for which the -new ticket will be used[17]. - -If the name of the server in the ticket that is presented to the KDC as -part of the authentication header is not that of the ticket-granting server -itself, the server is registered in the realm of the KDC, and the RENEW -option is requested, then the KDC will verify that the RENEWABLE flag is -set in the ticket, that the INVALID flag is not set in the ticket, and that -the renew_till time is still in the future. If the VALIDATE option is -rqeuested, the KDC will check that the starttime has passed and the INVALID -flag is set. If the PROXY option is requested, then the KDC will check that -the PROXIABLE flag is set in the ticket. If the tests succeed, and the -ticket passes the hotlist check described in the next paragraph, the KDC -will issue the appropriate new ticket. - -3.3.3.1. Checking for revoked tickets - -Whenever a request is made to the ticket-granting server, the presented -ticket(s) is(are) checked against a hot-list of tickets which have been -canceled. This hot-list might be implemented by storing a range of issue -timestamps for 'suspect tickets'; if a presented ticket had an authtime in -that range, it would be rejected. In this way, a stolen ticket-granting -ticket or renewable ticket cannot be used to gain additional tickets -(renewals or otherwise) once the theft has been reported. Any normal ticket -obtained before it was reported stolen will still be valid (because they -require no interaction with the KDC), but only until their normal -expiration time. - -The ciphertext part of the response in the KRB_TGS_REP message is encrypted -in the sub-session key from the Authenticator, if present, or the session -key key from the ticket-granting ticket. It is not encrypted using the -client's secret key. Furthermore, the client's key's expiration date and -the key version number fields are left out since these values are stored -along with the client's database record, and that record is not needed to -satisfy a request based on a ticket-granting ticket. See section A.6 for -pseudocode. - -3.3.3.2. Encoding the transited field - -If the identity of the server in the TGT that is presented to the KDC as -part of the authentication header is that of the ticket-granting service, -but the TGT was issued from another realm, the KDC will look up the -inter-realm key shared with that realm and use that key to decrypt the -ticket. If the ticket is valid, then the KDC will honor the request, -subject to the constraints outlined above in the section describing the AS -exchange. The realm part of the client's identity will be taken from the -ticket-granting ticket. The name of the realm that issued the -ticket-granting ticket will be added to the transited field of the ticket -to be issued. This is accomplished by reading the transited field from the -ticket-granting ticket (which is treated as an unordered set of realm -names), adding the new realm to the set, then constructing and writing out -its encoded (shorthand) form (this may involve a rearrangement of the -existing encoding). - -Note that the ticket-granting service does not add the name of its own -realm. Instead, its responsibility is to add the name of the previous -realm. This prevents a malicious Kerberos server from intentionally leaving -out its own name (it could, however, omit other realms' names). - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -The names of neither the local realm nor the principal's realm are to be -included in the transited field. They appear elsewhere in the ticket and -both are known to have taken part in authenticating the principal. Since -the endpoints are not included, both local and single-hop inter-realm -authentication result in a transited field that is empty. - -Because the name of each realm transited is added to this field, it might -potentially be very long. To decrease the length of this field, its -contents are encoded. The initially supported encoding is optimized for the -normal case of inter-realm communication: a hierarchical arrangement of -realms using either domain or X.500 style realm names. This encoding -(called DOMAIN-X500-COMPRESS) is now described. - -Realm names in the transited field are separated by a ",". The ",", "\", -trailing "."s, and leading spaces (" ") are special characters, and if they -are part of a realm name, they must be quoted in the transited field by -preced- ing them with a "\". - -A realm name ending with a "." is interpreted as being prepended to the -previous realm. For example, we can encode traversal of EDU, MIT.EDU, -ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as: - - "EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.". - -Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were end-points, that -they would not be included in this field, and we would have: - - "EDU,MIT.,WASHINGTON.EDU" - -A realm name beginning with a "/" is interpreted as being appended to the -previous realm[18]. If it is to stand by itself, then it should be preceded -by a space (" "). For example, we can encode traversal of /COM/HP/APOLLO, -/COM/HP, /COM, and /COM/DEC as: - - "/COM,/HP,/APOLLO, /COM/DEC". - -Like the example above, if /COM/HP/APOLLO and /COM/DEC are endpoints, they -they would not be included in this field, and we would have: - - "/COM,/HP" - -A null subfield preceding or following a "," indicates that all realms -between the previous realm and the next realm have been traversed[19]. -Thus, "," means that all realms along the path between the client and the -server have been traversed. ",EDU, /COM," means that that all realms from -the client's realm up to EDU (in a domain style hierarchy) have been -traversed, and that everything from /COM down to the server's realm in an -X.500 style has also been traversed. This could occur if the EDU realm in -one hierarchy shares an inter-realm key directly with the /COM realm in -another hierarchy. - -3.3.4. Receipt of KRB_TGS_REP message - -When the KRB_TGS_REP is received by the client, it is processed in the same -manner as the KRB_AS_REP processing described above. The primary difference -is that the ciphertext part of the response must be decrypted using the -session key from the ticket-granting ticket rather than the client's secret -key. The server name returned in the reply is the true principal name of -the service. See section A.7 for pseudocode. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -3.4. The KRB_SAFE Exchange - -The KRB_SAFE message may be used by clients requiring the ability to detect -modifications of messages they exchange. It achieves this by including a -keyed collision-proof checksum of the user data and some control -information. The checksum is keyed with an encryption key (usually the last -key negotiated via subkeys, or the session key if no negotiation has -occured). - -3.4.1. Generation of a KRB_SAFE message - -When an application wishes to send a KRB_SAFE message, it collects its data -and the appropriate control information and computes a checksum over them. -The checksum algorithm should be a keyed one-way hash function (such as the -RSA- MD5-DES checksum algorithm specified in section 6.4.5, or the DES -MAC), generated using the sub-session key if present, or the session key. -Different algorithms may be selected by changing the checksum type in the -message. Unkeyed or non-collision-proof checksums are not suitable for this -use. - -The control information for the KRB_SAFE message includes both a timestamp -and a sequence number. The designer of an application using the KRB_SAFE -message must choose at least one of the two mechanisms. This choice should -be based on the needs of the application protocol. - -Sequence numbers are useful when all messages sent will be received by -one's peer. Connection state is presently required to maintain the session -key, so maintaining the next sequence number should not present an -additional problem. - -If the application protocol is expected to tolerate lost messages without -them being resent, the use of the timestamp is the appropriate replay -detection mechanism. Using timestamps is also the appropriate mechanism for -multi-cast protocols where all of one's peers share a common sub-session -key, but some messages will be sent to a subset of one's peers. - -After computing the checksum, the client then transmits the information and -checksum to the recipient in the message format specified in section 5.6.1. - -3.4.2. Receipt of KRB_SAFE message - -When an application receives a KRB_SAFE message, it verifies it as follows. -If any error occurs, an error code is reported for use by the application. - -The message is first checked by verifying that the protocol version and -type fields match the current version and KRB_SAFE, respectively. A -mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. -The application verifies that the checksum used is a collision-proof keyed -checksum, and if it is not, a KRB_AP_ERR_INAPP_CKSUM error is generated. If -the sender's address was included in the control information, the recipient -verifies that the operating system's report of the sender's address matches -the sender's address in the message, and (if a recipient address is -specified or the recipient requires an address) that one of the recipient's -addresses appears as the recipient's address in the message. A failed match -for either case generates a KRB_AP_ERR_BADADDR error. Then the timestamp -and usec and/or the sequence number fields are checked. If timestamp and -usec are expected and not present, or they are present but not current, the -KRB_AP_ERR_SKEW error is generated. If the server name, along with the -client name, time and microsecond fields from the Authenticator match any -recently-seen (sent or received[20] ) such tuples, the KRB_AP_ERR_REPEAT -error is generated. If an incorrect sequence number is included, or a -sequence number is expected but not present, the KRB_AP_ERR_BADORDER error -is generated. If neither a time-stamp and usec or a sequence number is -present, a KRB_AP_ERR_MODIFIED error is generated. Finally, the checksum is -computed over the data and control information, and if it doesn't match the -received checksum, a KRB_AP_ERR_MODIFIED error is generated. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -If all the checks succeed, the application is assured that the message was -generated by its peer and was not modi- fied in transit. - -3.5. The KRB_PRIV Exchange - -The KRB_PRIV message may be used by clients requiring confidentiality and -the ability to detect modifications of exchanged messages. It achieves this -by encrypting the messages and adding control information. - -3.5.1. Generation of a KRB_PRIV message - -When an application wishes to send a KRB_PRIV message, it collects its data -and the appropriate control information (specified in section 5.7.1) and -encrypts them under an encryption key (usually the last key negotiated via -subkeys, or the session key if no negotiation has occured). As part of the -control information, the client must choose to use either a timestamp or a -sequence number (or both); see the discussion in section 3.4.1 for -guidelines on which to use. After the user data and control information are -encrypted, the client transmits the ciphertext and some 'envelope' -information to the recipient. - -3.5.2. Receipt of KRB_PRIV message - -When an application receives a KRB_PRIV message, it verifies it as follows. -If any error occurs, an error code is reported for use by the application. - -The message is first checked by verifying that the protocol version and -type fields match the current version and KRB_PRIV, respectively. A -mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. -The application then decrypts the ciphertext and processes the resultant -plaintext. If decryption shows the data to have been modified, a -KRB_AP_ERR_BAD_INTEGRITY error is generated. If the sender's address was -included in the control information, the recipient verifies that the -operating system's report of the sender's address matches the sender's -address in the message, and (if a recipient address is specified or the -recipient requires an address) that one of the recipient's addresses -appears as the recipient's address in the message. A failed match for -either case generates a KRB_AP_ERR_BADADDR error. Then the timestamp and -usec and/or the sequence number fields are checked. If timestamp and usec -are expected and not present, or they are present but not current, the -KRB_AP_ERR_SKEW error is generated. If the server name, along with the -client name, time and microsecond fields from the Authenticator match any -recently-seen such tuples, the KRB_AP_ERR_REPEAT error is generated. If an -incorrect sequence number is included, or a sequence number is expected but -not present, the KRB_AP_ERR_BADORDER error is generated. If neither a -time-stamp and usec or a sequence number is present, a KRB_AP_ERR_MODIFIED -error is generated. - -If all the checks succeed, the application can assume the message was -generated by its peer, and was securely transmitted (without intruders able -to see the unencrypted contents). - -3.6. The KRB_CRED Exchange - -The KRB_CRED message may be used by clients requiring the ability to send -Kerberos credentials from one host to another. It achieves this by sending -the tickets together with encrypted data containing the session keys and -other information associated with the tickets. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -3.6.1. Generation of a KRB_CRED message - -When an application wishes to send a KRB_CRED message it first (using the -KRB_TGS exchange) obtains credentials to be sent to the remote host. It -then constructs a KRB_CRED message using the ticket or tickets so obtained, -placing the session key needed to use each ticket in the key field of the -corresponding KrbCredInfo sequence of the encrypted part of the the -KRB_CRED message. - -Other information associated with each ticket and obtained during the -KRB_TGS exchange is also placed in the corresponding KrbCredInfo sequence -in the encrypted part of the KRB_CRED message. The current time and, if -specifically required by the application the nonce, s-address, and -r-address fields, are placed in the encrypted part of the KRB_CRED message -which is then encrypted under an encryption key previosuly exchanged in the -KRB_AP exchange (usually the last key negotiated via subkeys, or the -session key if no negotiation has occured). - -3.6.2. Receipt of KRB_CRED message - -When an application receives a KRB_CRED message, it verifies it. If any -error occurs, an error code is reported for use by the application. The -message is verified by checking that the protocol version and type fields -match the current version and KRB_CRED, respectively. A mismatch generates -a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The application then -decrypts the ciphertext and processes the resultant plaintext. If -decryption shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY -error is generated. - -If present or required, the recipient verifies that the operating system's -report of the sender's address matches the sender's address in the message, -and that one of the recipient's addresses appears as the recipient's -address in the message. A failed match for either case generates a -KRB_AP_ERR_BADADDR error. The timestamp and usec fields (and the nonce -field if required) are checked next. If the timestamp and usec are not -present, or they are present but not current, the KRB_AP_ERR_SKEW error is -generated. - -If all the checks succeed, the application stores each of the new tickets -in its ticket cache together with the session key and other information in -the corresponding KrbCredInfo sequence from the encrypted part of the -KRB_CRED message. - -4. The Kerberos Database - -The Kerberos server must have access to a database containing the principal -identifiers and secret keys of principals to be authenticated[21]. - -4.1. Database contents - -A database entry should contain at least the following fields: - -Field Value - -name Principal's identifier -key Principal's secret key -p_kvno Principal's key version -max_life Maximum lifetime for Tickets -max_renewable_life Maximum total lifetime for renewable Tickets - -The name field is an encoding of the principal's identifier. The key field -contains an encryption key. This key is the principal's secret key. (The -key can be encrypted before storage under a Kerberos "master key" to -protect it in case the database is compromised but the master key is not. -In that case, an extra field must be added to indicate the master key - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -version used, see below.) The p_kvno field is the key version number of the -principal's secret key. The max_life field contains the maximum allowable -lifetime (endtime - starttime) for any Ticket issued for this principal. -The max_renewable_life field contains the maximum allowable total lifetime -for any renewable Ticket issued for this principal. (See section 3.1 for a -description of how these lifetimes are used in determining the lifetime of -a given Ticket.) - -A server may provide KDC service to several realms, as long as the database -representation provides a mechanism to distinguish between principal -records with identifiers which differ only in the realm name. - -When an application server's key changes, if the change is routine (i.e. -not the result of disclosure of the old key), the old key should be -retained by the server until all tickets that had been issued using that -key have expired. Because of this, it is possible for several keys to be -active for a single principal. Ciphertext encrypted in a principal's key is -always tagged with the version of the key that was used for encryption, to -help the recipient find the proper key for decryption. - -When more than one key is active for a particular principal, the principal -will have more than one record in the Kerberos database. The keys and key -version numbers will differ between the records (the rest of the fields may -or may not be the same). Whenever Kerberos issues a ticket, or responds to -a request for initial authentication, the most recent key (known by the -Kerberos server) will be used for encryption. This is the key with the -highest key version number. - -4.2. Additional fields - -Project Athena's KDC implementation uses additional fields in its database: - -Field Value - -K_kvno Kerberos' key version -expiration Expiration date for entry -attributes Bit field of attributes -mod_date Timestamp of last modification -mod_name Modifying principal's identifier - -The K_kvno field indicates the key version of the Kerberos master key under -which the principal's secret key is encrypted. - -After an entry's expiration date has passed, the KDC will return an error -to any client attempting to gain tickets as or for the principal. (A -database may want to maintain two expiration dates: one for the principal, -and one for the principal's current key. This allows password aging to work -independently of the principal's expiration date. However, due to the -limited space in the responses, the KDC must combine the key expiration and -principal expiration date into a single value called 'key_exp', which is -used as a hint to the user to take administrative action.) - -The attributes field is a bitfield used to govern the operations involving -the principal. This field might be useful in conjunction with user -registration procedures, for site-specific policy implementations (Project -Athena currently uses it for their user registration process controlled by -the system-wide database service, Moira [LGDSR87]), to identify whether a -principal can play the role of a client or server or both, to note whether -a server is appropriate trusted to recieve credentials delegated by a -client, or to identify the 'string to key' conversion algorithm used for a -principal's key[22]. Other bits are used to indicate that certain ticket -options should not be allowed in tickets encrypted under a principal's key -(one bit each): Disallow issuing postdated tickets, disallow issuing -forwardable tickets, disallow issuing tickets based on TGT authentication, -disallow issuing renewable tickets, disallow issuing proxiable tickets, and -disallow issuing tickets for which the principal is the server. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -The mod_date field contains the time of last modification of the entry, and -the mod_name field contains the name of the principal which last modified -the entry. - -4.3. Frequently Changing Fields - -Some KDC implementations may wish to maintain the last time that a request -was made by a particular principal. Information that might be maintained -includes the time of the last request, the time of the last request for a -ticket-granting ticket, the time of the last use of a ticket-granting -ticket, or other times. This information can then be returned to the user -in the last-req field (see section 5.2). - -Other frequently changing information that can be maintained is the latest -expiration time for any tickets that have been issued using each key. This -field would be used to indicate how long old keys must remain valid to -allow the continued use of outstanding tickets. - -4.4. Site Constants - -The KDC implementation should have the following configurable constants or -options, to allow an administrator to make and enforce policy decisions: - - * The minimum supported lifetime (used to determine whether the - KDC_ERR_NEVER_VALID error should be returned). This constant should - reflect reasonable expectations of round-trip time to the KDC, - encryption/decryption time, and processing time by the client and - target server, and it should allow for a minimum 'useful' lifetime. - * The maximum allowable total (renewable) lifetime of a ticket - (renew_till - starttime). - * The maximum allowable lifetime of a ticket (endtime - starttime). - * Whether to allow the issue of tickets with empty address fields - (including the ability to specify that such tickets may only be issued - if the request specifies some authorization_data). - * Whether proxiable, forwardable, renewable or post-datable tickets are - to be issued. - -5. Message Specifications - -The following sections describe the exact contents and encoding of protocol -messages and objects. The ASN.1 base definitions are presented in the first -subsection. The remaining subsections specify the protocol objects (tickets -and authenticators) and messages. Specification of encryption and checksum -techniques, and the fields related to them, appear in section 6. - -Optional field in ASN.1 sequences - -For optional integer value and date fields in ASN.1 sequences where a -default value has been specified, certain default values will not be -allowed in the encoding because these values will always be represented -through defaulting by the absence of the optional field. For example, one -will not send a microsecond zero value because one must make sure that -there is only one way to encode this value. - -Additional fields in ASN.1 sequences - -Implementations receiving Kerberos messages with additional fields present -in ASN.1 sequences should carry the those fields through, unmodified, when -the message is forwarded. Implementations should not drop such fields if -the sequence is reencoded. - -5.1. ASN.1 Distinguished Encoding Representation - -All uses of ASN.1 in Kerberos shall use the Distinguished Encoding -Representation of the data elements as described in the X.509 -specification, section 8.7 [X509-88]. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -5.2. ASN.1 Base Definitions - -The following ASN.1 base definitions are used in the rest of this section. -Note that since the underscore character (_) is not permitted in ASN.1 -names, the hyphen (-) is used in its place for the purposes of ASN.1 names. - -Realm ::= GeneralString -PrincipalName ::= SEQUENCE { - name-type[0] INTEGER, - name-string[1] SEQUENCE OF GeneralString -} - -Kerberos realms are encoded as GeneralStrings. Realms shall not contain a -character with the code 0 (the ASCII NUL). Most realms will usually consist -of several components separated by periods (.), in the style of Internet -Domain Names, or separated by slashes (/) in the style of X.500 names. -Acceptable forms for realm names are specified in section 7. A -PrincipalName is a typed sequence of components consisting of the following -sub-fields: - -name-type - This field specifies the type of name that follows. Pre-defined values - for this field are specified in section 7.2. The name-type should be - treated as a hint. Ignoring the name type, no two names can be the - same (i.e. at least one of the components, or the realm, must be - different). This constraint may be eliminated in the future. -name-string - This field encodes a sequence of components that form a name, each - component encoded as a GeneralString. Taken together, a PrincipalName - and a Realm form a principal identifier. Most PrincipalNames will have - only a few components (typically one or two). - -KerberosTime ::= GeneralizedTime - -- Specifying UTC time zone (Z) - -The timestamps used in Kerberos are encoded as GeneralizedTimes. An -encoding shall specify the UTC time zone (Z) and shall not include any -fractional portions of the seconds. It further shall not include any -separators. Example: The only valid format for UTC time 6 minutes, 27 -seconds after 9 pm on 6 November 1985 is 19851106210627Z. - -HostAddress ::= SEQUENCE { - addr-type[0] INTEGER, - address[1] OCTET STRING -} - -HostAddresses ::= SEQUENCE OF HostAddress - -The host adddress encodings consists of two fields: - -addr-type - This field specifies the type of address that follows. Pre-defined - values for this field are specified in section 8.1. -address - This field encodes a single address of type addr-type. - -The two forms differ slightly. HostAddress contains exactly one address; -HostAddresses contains a sequence of possibly many addresses. - -AuthorizationData ::= SEQUENCE OF SEQUENCE { - ad-type[0] INTEGER, - ad-data[1] OCTET STRING -} - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -ad-data - This field contains authorization data to be interpreted according to - the value of the corresponding ad-type field. -ad-type - This field specifies the format for the ad-data subfield. All negative - values are reserved for local use. Non-negative values are reserved - for registered use. - -Each sequence of type and data is refered to as an authorization element. -Elements may be application specific, however, there is a common set of -recursive elements that should be understood by all implementations. These -elements contain other elements embedded within them, and the -interpretation of the encapsulating element determines which of the -embedded elements must be interpreted, and which may be ignored. -Definitions for these common elements may be found in Appendix B. - -TicketExtensions ::= SEQUENCE OF SEQUENCE { - te-type[0] INTEGER, - te-data[1] OCTET STRING -} - - - -te-data - This field contains opaque data that must be caried with the ticket to - support extensions to the Kerberos protocol including but not limited - to some forms of inter-realm key exchange and plaintext authorization - data. See appendix C for some common uses of this field. -te-type - This field specifies the format for the te-data subfield. All negative - values are reserved for local use. Non-negative values are reserved - for registered use. - -APOptions ::= BIT STRING - -- reserved(0), - -- use-session-key(1), - -- mutual-required(2) - -TicketFlags ::= BIT STRING - -- reserved(0), - -- forwardable(1), - -- forwarded(2), - -- proxiable(3), - -- proxy(4), - -- may-postdate(5), - -- postdated(6), - -- invalid(7), - -- renewable(8), - -- initial(9), - -- pre-authent(10), - -- hw-authent(11), - -- transited-policy-checked(12), - -- ok-as-delegate(13) - -KDCOptions ::= BIT STRING io - -- reserved(0), - -- forwardable(1), - -- forwarded(2), - -- proxiable(3), - -- proxy(4), - -- allow-postdate(5), - -- postdated(6), - -- unused7(7), - -- renewable(8), - -- unused9(9), - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - -- unused10(10), - -- unused11(11), - -- unused12(12), - -- unused13(13), - -- requestanonymous(14), - -- canonicalize(15), - -- disable-transited-check(26), - -- renewable-ok(27), - -- enc-tkt-in-skey(28), - -- renew(30), - -- validate(31) - -ASN.1 Bit strings have a length and a value. When used in Kerberos for the -APOptions, TicketFlags, and KDCOptions, the length of the bit string on -generated values should be the smallest number of bits needed to include -the highest order bit that is set (1), but in no case less than 32 bits. -The ASN.1 representation of the bit strings uses unnamed bits, with the -meaning of the individual bits defined by the comments in the specification -above. Implementations should accept values of bit strings of any length -and treat the value of flags corresponding to bits beyond the end of the -bit string as if the bit were reset (0). Comparison of bit strings of -different length should treat the smaller string as if it were padded with -zeros beyond the high order bits to the length of the longer string[23]. - -LastReq ::= SEQUENCE OF SEQUENCE { - lr-type[0] INTEGER, - lr-value[1] KerberosTime -} - -lr-type - This field indicates how the following lr-value field is to be - interpreted. Negative values indicate that the information pertains - only to the responding server. Non-negative values pertain to all - servers for the realm. If the lr-type field is zero (0), then no - information is conveyed by the lr-value subfield. If the absolute - value of the lr-type field is one (1), then the lr-value subfield is - the time of last initial request for a TGT. If it is two (2), then the - lr-value subfield is the time of last initial request. If it is three - (3), then the lr-value subfield is the time of issue for the newest - ticket-granting ticket used. If it is four (4), then the lr-value - subfield is the time of the last renewal. If it is five (5), then the - lr-value subfield is the time of last request (of any type). If it is - (6), then the lr-value subfield is the time when the password will - expire. -lr-value - This field contains the time of the last request. the time must be - interpreted according to the contents of the accompanying lr-type - subfield. - -See section 6 for the definitions of Checksum, ChecksumType, EncryptedData, -EncryptionKey, EncryptionType, and KeyType. - -5.3. Tickets and Authenticators - -This section describes the format and encryption parameters for tickets and -authenticators. When a ticket or authenticator is included in a protocol -message it is treated as an opaque object. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -5.3.1. Tickets - -A ticket is a record that helps a client authenticate to a service. A -Ticket contains the following information: - -Ticket ::= [APPLICATION 1] SEQUENCE { - tkt-vno[0] INTEGER, - realm[1] Realm, - sname[2] PrincipalName, - enc-part[3] EncryptedData, - extensions[4] TicketExtensions OPTIONAL -} - --- Encrypted part of ticket -EncTicketPart ::= [APPLICATION 3] SEQUENCE { - flags[0] TicketFlags, - key[1] EncryptionKey, - crealm[2] Realm, - cname[3] PrincipalName, - transited[4] TransitedEncoding, - authtime[5] KerberosTime, - starttime[6] KerberosTime OPTIONAL, - endtime[7] KerberosTime, - renew-till[8] KerberosTime OPTIONAL, - caddr[9] HostAddresses OPTIONAL, - authorization-data[10] AuthorizationData OPTIONAL -} --- encoded Transited field -TransitedEncoding ::= SEQUENCE { - tr-type[0] INTEGER, -- must be -registered - contents[1] OCTET STRING -} - -The encoding of EncTicketPart is encrypted in the key shared by Kerberos -and the end server (the server's secret key). See section 6 for the format -of the ciphertext. - -tkt-vno - This field specifies the version number for the ticket format. This - document describes version number 5. -realm - This field specifies the realm that issued a ticket. It also serves to - identify the realm part of the server's principal identifier. Since a - Kerberos server can only issue tickets for servers within its realm, - the two will always be identical. -sname - This field specifies all components of the name part of the server's - identity, including those parts that identify a specific instance of a - service. -enc-part - This field holds the encrypted encoding of the EncTicketPart sequence. -extensions - This optional field contains a sequence of extentions that may be used - to carry information that must be carried with the ticket to support - several extensions, including but not limited to plaintext - authorization data, tokens for exchanging inter-realm keys, and other - information that must be associated with a ticket for use by the - application server. See Appendix C for definitions of some common - extensions. - - Note that some older versions of Kerberos did not support this field. - Because this is an optional field it will not break older clients, but - older clients might strip this field from the ticket before sending it - to the application server. This limits the usefulness of this ticket - field to environments where the ticket will not be parsed and - reconstructed by these older Kerberos clients. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - If it is known that the client will strip this field from the ticket, - as an interim measure the KDC may append this field to the end of the - enc-part of the ticket and append a traler indicating the lenght of - the appended extensions field. (this paragraph is open for discussion, - including the form of the traler). -flags - This field indicates which of various options were used or requested - when the ticket was issued. It is a bit-field, where the selected - options are indicated by the bit being set (1), and the unselected - options and reserved fields being reset (0). Bit 0 is the most - significant bit. The encoding of the bits is specified in section 5.2. - The flags are described in more detail above in section 2. The - meanings of the flags are: - - Bit(s) Name Description - - 0 RESERVED - Reserved for future expansion of this - field. - - 1 FORWARDABLE - The FORWARDABLE flag is normally only - interpreted by the TGS, and can be - ignored by end servers. When set, this - flag tells the ticket-granting server - that it is OK to issue a new ticket- - granting ticket with a different network - address based on the presented ticket. - - 2 FORWARDED - When set, this flag indicates that the - ticket has either been forwarded or was - issued based on authentication involving - a forwarded ticket-granting ticket. - - 3 PROXIABLE - The PROXIABLE flag is normally only - interpreted by the TGS, and can be - ignored by end servers. The PROXIABLE - flag has an interpretation identical to - that of the FORWARDABLE flag, except - that the PROXIABLE flag tells the - ticket-granting server that only non- - ticket-granting tickets may be issued - with different network addresses. - - 4 PROXY - When set, this flag indicates that a - ticket is a proxy. - - 5 MAY-POSTDATE - The MAY-POSTDATE flag is normally only - interpreted by the TGS, and can be - ignored by end servers. This flag tells - the ticket-granting server that a post- - dated ticket may be issued based on this - ticket-granting ticket. - - 6 POSTDATED - This flag indicates that this ticket has - been postdated. The end-service can - check the authtime field to see when the - original authentication occurred. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - 7 INVALID - This flag indicates that a ticket is - invalid, and it must be validated by the - KDC before use. Application servers - must reject tickets which have this flag - set. - - 8 RENEWABLE - The RENEWABLE flag is normally only - interpreted by the TGS, and can usually - be ignored by end servers (some particu- - larly careful servers may wish to disal- - low renewable tickets). A renewable - ticket can be used to obtain a replace- - ment ticket that expires at a later - date. - - 9 INITIAL - This flag indicates that this ticket was - issued using the AS protocol, and not - issued based on a ticket-granting - ticket. - - 10 PRE-AUTHENT - This flag indicates that during initial - authentication, the client was authenti- - cated by the KDC before a ticket was - issued. The strength of the pre- - authentication method is not indicated, - but is acceptable to the KDC. - - 11 HW-AUTHENT - This flag indicates that the protocol - employed for initial authentication - required the use of hardware expected to - be possessed solely by the named client. - The hardware authentication method is - selected by the KDC and the strength of - the method is not indicated. - - 12 TRANSITED This flag indicates that the KDC for the - POLICY-CHECKED realm has checked the transited field - against a realm defined policy for - trusted certifiers. If this flag is - reset (0), then the application server - must check the transited field itself, - and if unable to do so it must reject - the authentication. If the flag is set - (1) then the application server may skip - its own validation of the transited - field, relying on the validation - performed by the KDC. At its option the - application server may still apply its - own validation based on a separate - policy for acceptance. - - 13 OK-AS-DELEGATE This flag indicates that the server (not - the client) specified in the ticket has - been determined by policy of the realm - to be a suitable recipient of - delegation. A client can use the - presence of this flag to help it make a - decision whether to delegate credentials - (either grant a proxy or a forwarded - ticket granting ticket) to this server. - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - The client is free to ignore the value - of this flag. When setting this flag, - an administrator should consider the - Security and placement of the server on - which the service will run, as well as - whether the service requires the use of - delegated credentials. - - 14 ANONYMOUS - This flag indicates that the principal - named in the ticket is a generic princi- - pal for the realm and does not identify - the individual using the ticket. The - purpose of the ticket is only to - securely distribute a session key, and - not to identify the user. Subsequent - requests using the same ticket and ses- - sion may be considered as originating - from the same user, but requests with - the same username but a different ticket - are likely to originate from different - users. - - 15-31 RESERVED - Reserved for future use. - -key - This field exists in the ticket and the KDC response and is used to - pass the session key from Kerberos to the application server and the - client. The field's encoding is described in section 6.2. -crealm - This field contains the name of the realm in which the client is - registered and in which initial authentication took place. -cname - This field contains the name part of the client's principal - identifier. -transited - This field lists the names of the Kerberos realms that took part in - authenticating the user to whom this ticket was issued. It does not - specify the order in which the realms were transited. See section - 3.3.3.2 for details on how this field encodes the traversed realms. - When the names of CA's are to be embedded inthe transited field (as - specified for some extentions to the protocol), the X.500 names of the - CA's should be mapped into items in the transited field using the - mapping defined by RFC2253. -authtime - This field indicates the time of initial authentication for the named - principal. It is the time of issue for the original ticket on which - this ticket is based. It is included in the ticket to provide - additional information to the end service, and to provide the - necessary information for implementation of a `hot list' service at - the KDC. An end service that is particularly paranoid could refuse to - accept tickets for which the initial authentication occurred "too far" - in the past. This field is also returned as part of the response from - the KDC. When returned as part of the response to initial - authentication (KRB_AS_REP), this is the current time on the Kerberos - server[24]. -starttime - This field in the ticket specifies the time after which the ticket is - valid. Together with endtime, this field specifies the life of the - ticket. If it is absent from the ticket, its value should be treated - as that of the authtime field. - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -endtime - This field contains the time after which the ticket will not be - honored (its expiration time). Note that individual services may place - their own limits on the life of a ticket and may reject tickets which - have not yet expired. As such, this is really an upper bound on the - expiration time for the ticket. -renew-till - This field is only present in tickets that have the RENEWABLE flag set - in the flags field. It indicates the maximum endtime that may be - included in a renewal. It can be thought of as the absolute expiration - time for the ticket, including all renewals. -caddr - This field in a ticket contains zero (if omitted) or more (if present) - host addresses. These are the addresses from which the ticket can be - used. If there are no addresses, the ticket can be used from any - location. The decision by the KDC to issue or by the end server to - accept zero-address tickets is a policy decision and is left to the - Kerberos and end-service administrators; they may refuse to issue or - accept such tickets. The suggested and default policy, however, is - that such tickets will only be issued or accepted when additional - information that can be used to restrict the use of the ticket is - included in the authorization_data field. Such a ticket is a - capability. - - Network addresses are included in the ticket to make it harder for an - attacker to use stolen credentials. Because the session key is not - sent over the network in cleartext, credentials can't be stolen simply - by listening to the network; an attacker has to gain access to the - session key (perhaps through operating system security breaches or a - careless user's unattended session) to make use of stolen tickets. - - It is important to note that the network address from which a - connection is received cannot be reliably determined. Even if it could - be, an attacker who has compromised the client's workstation could use - the credentials from there. Including the network addresses only makes - it more difficult, not impossible, for an attacker to walk off with - stolen credentials and then use them from a "safe" location. -authorization-data - The authorization-data field is used to pass authorization data from - the principal on whose behalf a ticket was issued to the application - service. If no authorization data is included, this field will be left - out. Experience has shown that the name of this field is confusing, - and that a better name for this field would be restrictions. - Unfortunately, it is not possible to change the name of this field at - this time. - - This field contains restrictions on any authority obtained on the - basis of authentication using the ticket. It is possible for any - principal in posession of credentials to add entries to the - authorization data field since these entries further restrict what can - be done with the ticket. Such additions can be made by specifying the - additional entries when a new ticket is obtained during the TGS - exchange, or they may be added during chained delegation using the - authorization data field of the authenticator. - - Because entries may be added to this field by the holder of - credentials, except when an entry is separately authenticated by - encapulation in the kdc-issued element, it is not allowable for the - presence of an entry in the authorization data field of a ticket to - amplify the priveleges one would obtain from using a ticket. - - The data in this field may be specific to the end service; the field - will contain the names of service specific objects, and the rights to - those objects. The format for this field is described in section 5.2. - Although Kerberos is not concerned with the format of the contents of - the sub-fields, it does carry type information (ad-type). - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - By using the authorization_data field, a principal is able to issue a - proxy that is valid for a specific purpose. For example, a client - wishing to print a file can obtain a file server proxy to be passed to - the print server. By specifying the name of the file in the - authorization_data field, the file server knows that the print server - can only use the client's rights when accessing the particular file to - be printed. - - A separate service providing authorization or certifying group - membership may be built using the authorization-data field. In this - case, the entity granting authorization (not the authorized entity), - may obtain a ticket in its own name (e.g. the ticket is issued in the - name of a privelege server), and this entity adds restrictions on its - own authority and delegates the restricted authority through a proxy - to the client. The client would then present this authorization - credential to the application server separately from the - authentication exchange. Alternatively, such authorization credentials - may be embedded in the ticket authenticating the authorized entity, - when the authorization is separately authenticated using the - kdc-issued authorization data element (see B.4). - - Similarly, if one specifies the authorization-data field of a proxy - and leaves the host addresses blank, the resulting ticket and session - key can be treated as a capability. See [Neu93] for some suggested - uses of this field. - - The authorization-data field is optional and does not have to be - included in a ticket. - -5.3.2. Authenticators - -An authenticator is a record sent with a ticket to a server to certify the -client's knowledge of the encryption key in the ticket, to help the server -detect replays, and to help choose a "true session key" to use with the -particular session. The encoding is encrypted in the ticket's session key -shared by the client and the server: - --- Unencrypted authenticator -Authenticator ::= [APPLICATION 2] SEQUENCE { - authenticator-vno[0] INTEGER, - crealm[1] Realm, - cname[2] PrincipalName, - cksum[3] Checksum OPTIONAL, - cusec[4] INTEGER, - ctime[5] KerberosTime, - subkey[6] EncryptionKey OPTIONAL, - seq-number[7] INTEGER OPTIONAL, - authorization-data[8] AuthorizationData OPTIONAL -} - - -authenticator-vno - This field specifies the version number for the format of the - authenticator. This document specifies version 5. -crealm and cname - These fields are the same as those described for the ticket in section - 5.3.1. -cksum - This field contains a checksum of the the applica- tion data that - accompanies the KRB_AP_REQ. -cusec - This field contains the microsecond part of the client's timestamp. - Its value (before encryption) ranges from 0 to 999999. It often - appears along with ctime. The two fields are used together to specify - a reasonably accurate timestamp. - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -ctime - This field contains the current time on the client's host. -subkey - This field contains the client's choice for an encryption key which is - to be used to protect this specific application session. Unless an - application specifies otherwise, if this field is left out the session - key from the ticket will be used. -seq-number - This optional field includes the initial sequence number to be used by - the KRB_PRIV or KRB_SAFE messages when sequence numbers are used to - detect replays (It may also be used by application specific messages). - When included in the authenticator this field specifies the initial - sequence number for messages from the client to the server. When - included in the AP-REP message, the initial sequence number is that - for messages from the server to the client. When used in KRB_PRIV or - KRB_SAFE messages, it is incremented by one after each message is - sent. Sequence numbers fall in the range of 0 through 2^32 - 1 and - wrap to zero following the value 2^32 - 1. - - For sequence numbers to adequately support the detection of replays - they should be non-repeating, even across connection boundaries. The - initial sequence number should be random and uniformly distributed - across the full space of possible sequence numbers, so that it cannot - be guessed by an attacker and so that it and the successive sequence - numbers do not repeat other sequences. -authorization-data - This field is the same as described for the ticket in section 5.3.1. - It is optional and will only appear when additional restrictions are - to be placed on the use of a ticket, beyond those carried in the - ticket itself. - -5.4. Specifications for the AS and TGS exchanges - -This section specifies the format of the messages used in the exchange -between the client and the Kerberos server. The format of possible error -messages appears in section 5.9.1. - -5.4.1. KRB_KDC_REQ definition - -The KRB_KDC_REQ message has no type of its own. Instead, its type is one of -KRB_AS_REQ or KRB_TGS_REQ depending on whether the request is for an -initial ticket or an additional ticket. In either case, the message is sent -from the client to the Authentication Server to request credentials for a -service. - -The message fields are: - -AS-REQ ::= [APPLICATION 10] KDC-REQ -TGS-REQ ::= [APPLICATION 12] KDC-REQ - -KDC-REQ ::= SEQUENCE { - pvno[1] INTEGER, - msg-type[2] INTEGER, - padata[3] SEQUENCE OF PA-DATA OPTIONAL, - req-body[4] KDC-REQ-BODY -} - -PA-DATA ::= SEQUENCE { - padata-type[1] INTEGER, - padata-value[2] OCTET STRING, - -- might be encoded AP-REQ -} - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -KDC-REQ-BODY ::= SEQUENCE { - kdc-options[0] KDCOptions, - cname[1] PrincipalName OPTIONAL, - -- Used only in AS-REQ - realm[2] Realm, -- Server's realm - -- Also client's in AS-REQ - sname[3] PrincipalName OPTIONAL, - from[4] KerberosTime OPTIONAL, - till[5] KerberosTime OPTIONAL, - rtime[6] KerberosTime OPTIONAL, - nonce[7] INTEGER, - etype[8] SEQUENCE OF INTEGER, - -- EncryptionType, - -- in preference order - addresses[9] HostAddresses OPTIONAL, - enc-authorization-data[10] EncryptedData OPTIONAL, - -- Encrypted AuthorizationData - -- encoding - additional-tickets[11] SEQUENCE OF Ticket OPTIONAL -} - -The fields in this message are: - -pvno - This field is included in each message, and specifies the protocol - version number. This document specifies protocol version 5. -msg-type - This field indicates the type of a protocol message. It will almost - always be the same as the application identifier associated with a - message. It is included to make the identifier more readily accessible - to the application. For the KDC-REQ message, this type will be - KRB_AS_REQ or KRB_TGS_REQ. -padata - The padata (pre-authentication data) field contains a sequence of - authentication information which may be needed before credentials can - be issued or decrypted. In the case of requests for additional tickets - (KRB_TGS_REQ), this field will include an element with padata-type of - PA-TGS-REQ and data of an authentication header (ticket-granting - ticket and authenticator). The checksum in the authenticator (which - must be collision-proof) is to be computed over the KDC-REQ-BODY - encoding. In most requests for initial authentication (KRB_AS_REQ) and - most replies (KDC-REP), the padata field will be left out. - - This field may also contain information needed by certain extensions - to the Kerberos protocol. For example, it might be used to initially - verify the identity of a client before any response is returned. When - this field is used to authenticate or pre-authenticate a request, it - should contain a keyed checksum over the KDC-REQ-BODY to bind the - pre-authentication data to rest of the request. The KDC, as a matter - of policy, may decide whether to honor a KDC-REQ which includes any - pre-authentication data that does not contain the checksum field. - PA-ENC-TIMESTAMP defines a pre-authentication data type that is used - for authenticating a client by way of an encrypted timestamp. This is - accomplished with a padata field with padata-type equal to - PA-ENC-TIMESTAMP and padata-value defined as follows (query: the - checksum is new in this definition. If the optional field will break - things we can keep the old PA-ENC-TS-ENC, and define a new alternate - form that includes the checksum). : - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - padata-type ::= PA-ENC-TIMESTAMP - padata-value ::= EncryptedData -- PA-ENC-TS-ENC - - PA-ENC-TS-ENC ::= SEQUENCE { - patimestamp[0] KerberosTime, -- client's time - pausec[1] INTEGER OPTIONAL, - pachecksum[2] checksum OPTIONAL - -- keyed checksum of -KDC-REQ-BODY - } - - with patimestamp containing the client's time and pausec containing - the microseconds which may be omitted if a client will not generate - more than one request per second. The ciphertext (padata-value) - consists of the PA-ENC-TS-ENC sequence, encrypted using the client's - secret key. - - [use-specified-kvno item is here for discussion and may be removed] It - may also be used by the client to specify the version of a key that is - being used for accompanying preauthentication, and/or which should be - used to encrypt the reply from the KDC. - - PA-USE-SPECIFIED-KVNO ::= Integer - - The KDC should only accept and abide by the value of the - use-specified-kvno preauthentication data field when the specified key - is still valid and until use of a new key is confirmed. This situation - is likely to occur primarily during the period during which an updated - key is propagating to other KDC's in a realm. - - The padata field can also contain information needed to help the KDC - or the client select the key needed for generating or decrypting the - response. This form of the padata is useful for supporting the use of - certain token cards with Kerberos. The details of such extensions are - specified in separate documents. See [Pat92] for additional uses of - this field. -padata-type - The padata-type element of the padata field indicates the way that the - padata-value element is to be interpreted. Negative values of - padata-type are reserved for unregistered use; non-negative values are - used for a registered interpretation of the element type. -req-body - This field is a placeholder delimiting the extent of the remaining - fields. If a checksum is to be calculated over the request, it is - calculated over an encoding of the KDC-REQ-BODY sequence which is - enclosed within the req-body field. -kdc-options - This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests to the - KDC and indicates the flags that the client wants set on the tickets - as well as other information that is to modify the behavior of the - KDC. Where appropriate, the name of an option may be the same as the - flag that is set by that option. Although in most case, the bit in the - options field will be the same as that in the flags field, this is not - guaranteed, so it is not acceptable to simply copy the options field - to the flags field. There are various checks that must be made before - honoring an option anyway. - - The kdc_options field is a bit-field, where the selected options are - indicated by the bit being set (1), and the unselected options and - reserved fields being reset (0). The encoding of the bits is specified - in section 5.2. The options are described in more detail above in - section 2. The meanings of the options are: - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - Bit(s) Name Description - 0 RESERVED - Reserved for future expansion of -this - field. - - 1 FORWARDABLE - The FORWARDABLE option indicates -that - the ticket to be issued is to have -its - forwardable flag set. It may only -be - set on the initial request, or in a -sub- - sequent request if the -ticket-granting - ticket on which it is based is also -for- - wardable. - - 2 FORWARDED - The FORWARDED option is only -specified - in a request to the -ticket-granting - server and will only be honored if -the - ticket-granting ticket in the -request - has its FORWARDABLE bit set. -This - option indicates that this is a -request - for forwarding. The address(es) of -the - host from which the resulting ticket -is - to be valid are included in -the - addresses field of the request. - - 3 PROXIABLE - The PROXIABLE option indicates that -the - ticket to be issued is to have its -prox- - iable flag set. It may only be set -on - the initial request, or in a -subsequent - request if the ticket-granting ticket -on - which it is based is also proxiable. - - 4 PROXY - The PROXY option indicates that this -is - a request for a proxy. This option -will - only be honored if the -ticket-granting - ticket in the request has its -PROXIABLE - bit set. The address(es) of the -host - from which the resulting ticket is to -be - valid are included in the -addresses - field of the request. - - 5 ALLOW-POSTDATE - The ALLOW-POSTDATE option indicates -that - the ticket to be issued is to have -its - MAY-POSTDATE flag set. It may only -be - set on the initial request, or in a -sub- - sequent request if the -ticket-granting - ticket on which it is based also has -its - MAY-POSTDATE flag set. - - 6 POSTDATED - The POSTDATED option indicates that -this - is a request for a postdated -ticket. - This option will only be honored if -the - ticket-granting ticket on which it -is - based has its MAY-POSTDATE flag -set. - The resulting ticket will also have -its - INVALID flag set, and that flag may -be - reset by a subsequent request to the -KDC - after the starttime in the ticket -has - been reached. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - 7 UNUSED - This option is presently unused. - - 8 RENEWABLE - The RENEWABLE option indicates that -the - ticket to be issued is to have -its - RENEWABLE flag set. It may only be -set - on the initial request, or when -the - ticket-granting ticket on which -the - request is based is also renewable. -If - this option is requested, then the -rtime - field in the request contains -the - desired absolute expiration time for -the - ticket. - - 9 RESERVED - Reserved for PK-Cross - - 10-13 UNUSED - These options are presently unused. - - 14 REQUEST-ANONYMOUS - The REQUEST-ANONYMOUS option -indicates - that the ticket to be issued is not -to - identify the user to which it -was - issued. Instead, the principal -identif- - ier is to be generic, as specified -by - the policy of the realm (e.g. -usually - anonymous@realm). The purpose of -the - ticket is only to securely distribute -a - session key, and not to identify -the - user. The ANONYMOUS flag on the -ticket - to be returned should be set. If -the - local realms policy does not -permit - anonymous credentials, the request is -to - be rejected. - - 15 CANONICALIZE - The CANONICALIZE option indicates that - the client will accept the return of a - true server name instead of the name - specified in the request. In addition - the client will be able to process - any TGT referrals that will direct - the client to another realm to locate - the requested server. If a KDC does - not support name- canonicalization, - the option is ignored and the - appropriate - KDC_ERR_C_PRINCIPAL_UNKNOWN or - KDC_ERR_S_PRINCIPAL_UNKNOWN error is - returned. [JBrezak] - - 16-25 RESERVED - Reserved for future use. - - 26 DISABLE-TRANSITED-CHECK - By default the KDC will check the - transited field of a ticket-granting- - ticket against the policy of the local - realm before it will issue derivative - tickets based on the ticket granting - ticket. If this flag is set in the - request, checking of the transited -field - is disabled. Tickets issued without -the - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - performance of this check will be -noted - by the reset (0) value of the - TRANSITED-POLICY-CHECKED flag, - indicating to the application server - that the tranisted field must be -checked - locally. KDC's are encouraged but not - required to honor the - DISABLE-TRANSITED-CHECK option. - - 27 RENEWABLE-OK - The RENEWABLE-OK option indicates that -a - renewable ticket will be acceptable if -a - ticket with the requested life -cannot - otherwise be provided. If a ticket -with - the requested life cannot be -provided, - then a renewable ticket may be -issued - with a renew-till equal to the -the - requested endtime. The value of -the - renew-till field may still be limited -by - local limits, or limits selected by -the - individual principal or server. - - 28 ENC-TKT-IN-SKEY - This option is used only by the -ticket- - granting service. The -ENC-TKT-IN-SKEY - option indicates that the ticket for -the - end server is to be encrypted in -the - session key from the additional -ticket- - granting ticket provided. - - 29 RESERVED - Reserved for future use. - - 30 RENEW - This option is used only by the -ticket- - granting service. The RENEW -option - indicates that the present request -is - for a renewal. The ticket provided -is - encrypted in the secret key for -the - server on which it is valid. -This - option will only be honored if -the - ticket to be renewed has its -RENEWABLE - flag set and if the time in its -renew- - till field has not passed. The -ticket - to be renewed is passed in the -padata - field as part of the -authentication - header. - - 31 VALIDATE - This option is used only by the -ticket- - granting service. The VALIDATE -option - indicates that the request is to -vali- - date a postdated ticket. It will -only - be honored if the ticket presented -is - postdated, presently has its -INVALID - flag set, and would be otherwise -usable - at this time. A ticket cannot be -vali- - dated before its starttime. The -ticket - presented for validation is encrypted -in - the key of the server for which it -is - valid and is passed in the padata -field - as part of the authentication header. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -cname and sname - These fields are the same as those described for the ticket in section - 5.3.1. sname may only be absent when the ENC-TKT-IN-SKEY option is - specified. If absent, the name of the server is taken from the name of - the client in the ticket passed as additional-tickets. -enc-authorization-data - The enc-authorization-data, if present (and it can only be present in - the TGS_REQ form), is an encoding of the desired authorization-data - encrypted under the sub-session key if present in the Authenticator, - or alternatively from the session key in the ticket-granting ticket, - both from the padata field in the KRB_AP_REQ. -realm - This field specifies the realm part of the server's principal - identifier. In the AS exchange, this is also the realm part of the - client's principal identifier. If the CANONICALIZE option is set, the - realm is used as a hint to the KDC for its database lookup. -from - This field is included in the KRB_AS_REQ and KRB_TGS_REQ ticket - requests when the requested ticket is to be postdated. It specifies - the desired start time for the requested ticket. If this field is - omitted then the KDC should use the current time instead. -till - This field contains the expiration date requested by the client in a - ticket request. It is optional and if omitted the requested ticket is - to have the maximum endtime permitted according to KDC policy for the - parties to the authentication exchange as limited by expiration date - of the ticket granting ticket or other preauthentication credentials. -rtime - This field is the requested renew-till time sent from a client to the - KDC in a ticket request. It is optional. -nonce - This field is part of the KDC request and response. It it intended to - hold a random number generated by the client. If the same number is - included in the encrypted response from the KDC, it provides evidence - that the response is fresh and has not been replayed by an attacker. - Nonces must never be re-used. Ideally, it should be generated - randomly, but if the correct time is known, it may suffice[25]. -etype - This field specifies the desired encryption algorithm to be used in - the response. -addresses - This field is included in the initial request for tickets, and - optionally included in requests for additional tickets from the - ticket-granting server. It specifies the addresses from which the - requested ticket is to be valid. Normally it includes the addresses - for the client's host. If a proxy is requested, this field will - contain other addresses. The contents of this field are usually copied - by the KDC into the caddr field of the resulting ticket. -additional-tickets - Additional tickets may be optionally included in a request to the - ticket-granting server. If the ENC-TKT-IN-SKEY option has been - specified, then the session key from the additional ticket will be - used in place of the server's key to encrypt the new ticket. When he - ENC-TKT-IN-SKEY option is used for user-to-user authentication, this - addional ticket may be a TGT issued by the local realm or an - inter-realm TGT issued for the current KDC's realm by a remote KDC. If - more than one option which requires additional tickets has been - specified, then the additional tickets are used in the order specified - by the ordering of the options bits (see kdc-options, above). - -The application code will be either ten (10) or twelve (12) depending on -whether the request is for an initial ticket (AS-REQ) or for an additional -ticket (TGS-REQ). - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -The optional fields (addresses, authorization-data and additional-tickets) -are only included if necessary to perform the operation specified in the -kdc-options field. - -It should be noted that in KRB_TGS_REQ, the protocol version number appears -twice and two different message types appear: the KRB_TGS_REQ message -contains these fields as does the authentication header (KRB_AP_REQ) that -is passed in the padata field. - -5.4.2. KRB_KDC_REP definition - -The KRB_KDC_REP message format is used for the reply from the KDC for -either an initial (AS) request or a subsequent (TGS) request. There is no -message type for KRB_KDC_REP. Instead, the type will be either KRB_AS_REP -or KRB_TGS_REP. The key used to encrypt the ciphertext part of the reply -depends on the message type. For KRB_AS_REP, the ciphertext is encrypted in -the client's secret key, and the client's key version number is included in -the key version number for the encrypted data. For KRB_TGS_REP, the -ciphertext is encrypted in the sub-session key from the Authenticator, or -if absent, the session key from the ticket-granting ticket used in the -request. In that case, no version number will be present in the -EncryptedData sequence. - -The KRB_KDC_REP message contains the following fields: - -AS-REP ::= [APPLICATION 11] KDC-REP -TGS-REP ::= [APPLICATION 13] KDC-REP - -KDC-REP ::= SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, - padata[2] SEQUENCE OF PA-DATA OPTIONAL, - crealm[3] Realm, - cname[4] PrincipalName, - ticket[5] Ticket, - enc-part[6] EncryptedData -} - -EncASRepPart ::= [APPLICATION 25[27]] EncKDCRepPart -EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart - -EncKDCRepPart ::= SEQUENCE { - key[0] EncryptionKey, - last-req[1] LastReq, - nonce[2] INTEGER, - key-expiration[3] KerberosTime OPTIONAL, - flags[4] TicketFlags, - authtime[5] KerberosTime, - starttime[6] KerberosTime OPTIONAL, - endtime[7] KerberosTime, - renew-till[8] KerberosTime OPTIONAL, - srealm[9] Realm, - sname[10] PrincipalName, - caddr[11] HostAddresses OPTIONAL -} - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is either - KRB_AS_REP or KRB_TGS_REP. - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -padata - This field is described in detail in section 5.4.1. One possible use - for this field is to encode an alternate "mix-in" string to be used - with a string-to-key algorithm (such as is described in section - 6.3.2). This ability is useful to ease transitions if a realm name - needs to change (e.g. when a company is acquired); in such a case all - existing password-derived entries in the KDC database would be flagged - as needing a special mix-in string until the next password change. -crealm, cname, srealm and sname - These fields are the same as those described for the ticket in section - 5.3.1. -ticket - The newly-issued ticket, from section 5.3.1. -enc-part - This field is a place holder for the ciphertext and related - information that forms the encrypted part of a message. The - description of the encrypted part of the message follows each - appearance of this field. The encrypted part is encoded as described - in section 6.1. -key - This field is the same as described for the ticket in section 5.3.1. -last-req - This field is returned by the KDC and specifies the time(s) of the - last request by a principal. Depending on what information is - available, this might be the last time that a request for a - ticket-granting ticket was made, or the last time that a request based - on a ticket-granting ticket was successful. It also might cover all - servers for a realm, or just the particular server. Some - implementations may display this information to the user to aid in - discovering unauthorized use of one's identity. It is similar in - spirit to the last login time displayed when logging into timesharing - systems. -nonce - This field is described above in section 5.4.1. -key-expiration - The key-expiration field is part of the response from the KDC and - specifies the time that the client's secret key is due to expire. The - expiration might be the result of password aging or an account - expiration. This field will usually be left out of the TGS reply since - the response to the TGS request is encrypted in a session key and no - client information need be retrieved from the KDC database. It is up - to the application client (usually the login program) to take - appropriate action (such as notifying the user) if the expiration time - is imminent. -flags, authtime, starttime, endtime, renew-till and caddr - These fields are duplicates of those found in the encrypted portion of - the attached ticket (see section 5.3.1), provided so the client may - verify they match the intended request and to assist in proper ticket - caching. If the message is of type KRB_TGS_REP, the caddr field will - only be filled in if the request was for a proxy or forwarded ticket, - or if the user is substituting a subset of the addresses from the - ticket granting ticket. If the client-requested addresses are not - present or not used, then the addresses contained in the ticket will - be the same as those included in the ticket-granting ticket. - -5.5. Client/Server (CS) message specifications - -This section specifies the format of the messages used for the -authentication of the client to the application server. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -5.5.1. KRB_AP_REQ definition - -The KRB_AP_REQ message contains the Kerberos protocol version number, the -message type KRB_AP_REQ, an options field to indicate any options in use, -and the ticket and authenticator themselves. The KRB_AP_REQ message is -often referred to as the 'authentication header'. - -AP-REQ ::= [APPLICATION 14] SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, - ap-options[2] APOptions, - ticket[3] Ticket, - authenticator[4] EncryptedData -} - -APOptions ::= BIT STRING { - reserved(0), - use-session-key(1), - mutual-required(2) -} - - - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is - KRB_AP_REQ. -ap-options - This field appears in the application request (KRB_AP_REQ) and affects - the way the request is processed. It is a bit-field, where the - selected options are indicated by the bit being set (1), and the - unselected options and reserved fields being reset (0). The encoding - of the bits is specified in section 5.2. The meanings of the options - are: - - Bit(s) Name Description - - 0 RESERVED - Reserved for future expansion of this - field. - - 1 USE-SESSION-KEY - The USE-SESSION-KEY option indicates - that the ticket the client is presenting - to a server is encrypted in the session - key from the server's ticket-granting - ticket. When this option is not speci- - fied, the ticket is encrypted in the - server's secret key. - - 2 MUTUAL-REQUIRED - The MUTUAL-REQUIRED option tells the - server that the client requires mutual - authentication, and that it must respond - with a KRB_AP_REP message. - - 3-31 RESERVED - Reserved for future use. - -ticket - This field is a ticket authenticating the client to the server. -authenticator - This contains the authenticator, which includes the client's choice of - a subkey. Its encoding is described in section 5.3.2. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -5.5.2. KRB_AP_REP definition - -The KRB_AP_REP message contains the Kerberos protocol version number, the -message type, and an encrypted time- stamp. The message is sent in in -response to an application request (KRB_AP_REQ) where the mutual -authentication option has been selected in the ap-options field. - -AP-REP ::= [APPLICATION 15] SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, - enc-part[2] EncryptedData -} - -EncAPRepPart ::= [APPLICATION 27[29]] SEQUENCE { - ctime[0] KerberosTime, - cusec[1] INTEGER, - subkey[2] EncryptionKey OPTIONAL, - seq-number[3] INTEGER OPTIONAL -} - -The encoded EncAPRepPart is encrypted in the shared session key of the -ticket. The optional subkey field can be used in an application-arranged -negotiation to choose a per association session key. - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is - KRB_AP_REP. -enc-part - This field is described above in section 5.4.2. -ctime - This field contains the current time on the client's host. -cusec - This field contains the microsecond part of the client's timestamp. -subkey - This field contains an encryption key which is to be used to protect - this specific application session. See section 3.2.6 for specifics on - how this field is used to negotiate a key. Unless an application - specifies otherwise, if this field is left out, the sub-session key - from the authenticator, or if also left out, the session key from the - ticket will be used. - -5.5.3. Error message reply - -If an error occurs while processing the application request, the KRB_ERROR -message will be sent in response. See section 5.9.1 for the format of the -error message. The cname and crealm fields may be left out if the server -cannot determine their appropriate values from the corresponding KRB_AP_REQ -message. If the authenticator was decipherable, the ctime and cusec fields -will contain the values from it. - -5.6. KRB_SAFE message specification - -This section specifies the format of a message that can be used by either -side (client or server) of an application to send a tamper-proof message to -its peer. It presumes that a session key has previously been exchanged (for -example, by using the KRB_AP_REQ/KRB_AP_REP messages). - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -5.6.1. KRB_SAFE definition - -The KRB_SAFE message contains user data along with a collision-proof -checksum keyed with the last encryption key negotiated via subkeys, or the -session key if no negotiation has occured. The message fields are: - -KRB-SAFE ::= [APPLICATION 20] SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, - safe-body[2] KRB-SAFE-BODY, - cksum[3] Checksum -} - -KRB-SAFE-BODY ::= SEQUENCE { - user-data[0] OCTET STRING, - timestamp[1] KerberosTime OPTIONAL, - usec[2] INTEGER OPTIONAL, - seq-number[3] INTEGER OPTIONAL, - s-address[4] HostAddress OPTIONAL, - r-address[5] HostAddress OPTIONAL -} - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is - KRB_SAFE. -safe-body - This field is a placeholder for the body of the KRB-SAFE message. -cksum - This field contains the checksum of the application data. Checksum - details are described in section 6.4. The checksum is computed over - the encoding of the KRB-SAFE sequence. First, the cksum is zeroed and - the checksum is computed over the encoding of the KRB-SAFE sequence, - then the checksum is set to the result of that computation, and - finally the KRB-SAFE sequence is encoded again. -user-data - This field is part of the KRB_SAFE and KRB_PRIV messages and contain - the application specific data that is being passed from the sender to - the recipient. -timestamp - This field is part of the KRB_SAFE and KRB_PRIV messages. Its contents - are the current time as known by the sender of the message. By - checking the timestamp, the recipient of the message is able to make - sure that it was recently generated, and is not a replay. -usec - This field is part of the KRB_SAFE and KRB_PRIV headers. It contains - the microsecond part of the timestamp. -seq-number - This field is described above in section 5.3.2. -s-address - This field specifies the address in use by the sender of the message. - It may be omitted if not required by the application protocol. The - application designer considering omission of this field is warned, - that the inclusion of this address prevents some kinds of replay - attacks (e.g., reflection attacks) and that it is only acceptable to - omit this address if there is sufficient information in the integrity - protected part of the application message for the recipient to - unambiguously determine if it was the intended recipient. -r-address - This field specifies the address in use by the recipient of the - message. It may be omitted for some uses (such as broadcast - protocols), but the recipient may arbitrarily reject such messages. - This field along with s-address can be used to help detect messages - which have been incorrectly or maliciously delivered to the wrong - recipient. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -5.7. KRB_PRIV message specification - -This section specifies the format of a message that can be used by either -side (client or server) of an application to securely and privately send a -message to its peer. It presumes that a session key has previously been -exchanged (for example, by using the KRB_AP_REQ/KRB_AP_REP messages). - -5.7.1. KRB_PRIV definition - -The KRB_PRIV message contains user data encrypted in the Session Key. The -message fields are: - -KRB-PRIV ::= [APPLICATION 21] SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, - enc-part[3] EncryptedData -} - -EncKrbPrivPart ::= [APPLICATION 28[31]] SEQUENCE { - user-data[0] OCTET STRING, - timestamp[1] KerberosTime OPTIONAL, - usec[2] INTEGER OPTIONAL, - seq-number[3] INTEGER OPTIONAL, - s-address[4] HostAddress OPTIONAL, -- sender's -addr - r-address[5] HostAddress OPTIONAL -- recip's -addr -} - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is - KRB_PRIV. -enc-part - This field holds an encoding of the EncKrbPrivPart sequence encrypted - under the session key[32]. This encrypted encoding is used for the - enc-part field of the KRB-PRIV message. See section 6 for the format - of the ciphertext. -user-data, timestamp, usec, s-address and r-address - These fields are described above in section 5.6.1. -seq-number - This field is described above in section 5.3.2. - -5.8. KRB_CRED message specification - -This section specifies the format of a message that can be used to send -Kerberos credentials from one principal to another. It is presented here to -encourage a common mechanism to be used by applications when forwarding -tickets or providing proxies to subordinate servers. It presumes that a -session key has already been exchanged perhaps by using the -KRB_AP_REQ/KRB_AP_REP messages. - -5.8.1. KRB_CRED definition - -The KRB_CRED message contains a sequence of tickets to be sent and -information needed to use the tickets, including the session key from each. -The information needed to use the tickets is encrypted under an encryption -key previously exchanged or transferred alongside the KRB_CRED message. The -message fields are: - -KRB-CRED ::= [APPLICATION 22] SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, -- KRB_CRED - tickets[2] SEQUENCE OF Ticket, - enc-part[3] EncryptedData -} - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -EncKrbCredPart ::= [APPLICATION 29] SEQUENCE { - ticket-info[0] SEQUENCE OF KrbCredInfo, - nonce[1] INTEGER OPTIONAL, - timestamp[2] KerberosTime OPTIONAL, - usec[3] INTEGER OPTIONAL, - s-address[4] HostAddress OPTIONAL, - r-address[5] HostAddress OPTIONAL -} - -KrbCredInfo ::= SEQUENCE { - key[0] EncryptionKey, - prealm[1] Realm OPTIONAL, - pname[2] PrincipalName OPTIONAL, - flags[3] TicketFlags OPTIONAL, - authtime[4] KerberosTime OPTIONAL, - starttime[5] KerberosTime OPTIONAL, - endtime[6] KerberosTime OPTIONAL - renew-till[7] KerberosTime OPTIONAL, - srealm[8] Realm OPTIONAL, - sname[9] PrincipalName OPTIONAL, - caddr[10] HostAddresses OPTIONAL -} - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is - KRB_CRED. -tickets - These are the tickets obtained from the KDC specifically for use by - the intended recipient. Successive tickets are paired with the - corresponding KrbCredInfo sequence from the enc-part of the KRB-CRED - message. -enc-part - This field holds an encoding of the EncKrbCredPart sequence encrypted - under the session key shared between the sender and the intended - recipient. This encrypted encoding is used for the enc-part field of - the KRB-CRED message. See section 6 for the format of the ciphertext. -nonce - If practical, an application may require the inclusion of a nonce - generated by the recipient of the message. If the same value is - included as the nonce in the message, it provides evidence that the - message is fresh and has not been replayed by an attacker. A nonce - must never be re-used; it should be generated randomly by the - recipient of the message and provided to the sender of the message in - an application specific manner. -timestamp and usec - These fields specify the time that the KRB-CRED message was generated. - The time is used to provide assurance that the message is fresh. -s-address and r-address - These fields are described above in section 5.6.1. They are used - optionally to provide additional assurance of the integrity of the - KRB-CRED message. -key - This field exists in the corresponding ticket passed by the KRB-CRED - message and is used to pass the session key from the sender to the - intended recipient. The field's encoding is described in section 6.2. - -The following fields are optional. If present, they can be associated with -the credentials in the remote ticket file. If left out, then it is assumed -that the recipient of the credentials already knows their value. - -prealm and pname - The name and realm of the delegated principal identity. -flags, authtime, starttime, endtime, renew-till, srealm, sname, and caddr - These fields contain the values of the correspond- ing fields from the - ticket found in the ticket field. Descriptions of the fields are - identical to the descriptions in the KDC-REP message. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -5.9. Error message specification - -This section specifies the format for the KRB_ERROR message. The fields -included in the message are intended to return as much information as -possible about an error. It is not expected that all the information -required by the fields will be available for all types of errors. If the -appropriate information is not available when the message is composed, the -corresponding field will be left out of the message. - -Note that since the KRB_ERROR message is only optionally integrity -protected, it is quite possible for an intruder to synthesize or modify -such a message. In particular, this means that unless appropriate integrity -protection mechanisms have been applied to the KRB_ERROR message, the -client should not use any fields in this message for security-critical -purposes, such as setting a system clock or generating a fresh -authenticator. The message can be useful, however, for advising a user on -the reason for some failure. - -5.9.1. KRB_ERROR definition - -The KRB_ERROR message consists of the following fields: - -KRB-ERROR ::= [APPLICATION 30] SEQUENCE { - pvno[0] INTEGER, - msg-type[1] INTEGER, - ctime[2] KerberosTime OPTIONAL, - cusec[3] INTEGER OPTIONAL, - stime[4] KerberosTime, - susec[5] INTEGER, - error-code[6] INTEGER, - crealm[7] Realm OPTIONAL, - cname[8] PrincipalName OPTIONAL, - realm[9] Realm, -- Correct realm - sname[10] PrincipalName, -- Correct name - e-text[11] GeneralString OPTIONAL, - e-data[12] OCTET STRING OPTIONAL, - e-cksum[13] Checksum OPTIONAL, -} - - - -pvno and msg-type - These fields are described above in section 5.4.1. msg-type is - KRB_ERROR. -ctime - This field is described above in section 5.4.1. -cusec - This field is described above in section 5.5.2. -stime - This field contains the current time on the server. It is of type - KerberosTime. -susec - This field contains the microsecond part of the server's timestamp. - Its value ranges from 0 to 999999. It appears along with stime. The - two fields are used in conjunction to specify a reasonably accurate - timestamp. -error-code - This field contains the error code returned by Kerberos or the server - when a request fails. To interpret the value of this field see the - list of error codes in section 8. Implementations are encouraged to - provide for national language support in the display of error - messages. -crealm, cname, srealm and sname - These fields are described above in section 5.3.1. - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - -e-text - This field contains additional text to help explain the error code - associated with the failed request (for example, it might include a - principal name which was unknown). -e-data - This field contains additional data about the error for use by the - application to help it recover from or handle the error. If present, - this field will contain the encoding of a sequence of TypedData - (TYPED-DATA below), unless the errorcode is KDC_ERR_PREAUTH_REQUIRED, - in which case it will contain the encoding of a sequence of of padata - fields (METHOD-DATA below), each corresponding to an acceptable - pre-authentication method and optionally containing data for the - method: - - TYPED-DATA ::= SEQUENCE of TypeData - METHOD-DATA ::= SEQUENCE of PA-DATA - - TypedData ::= SEQUENCE { - data-type[0] INTEGER, - data-value[1] OCTET STRING OPTIONAL - } - - Note that e-data-types have been reserved for all PA data types - defined prior to July 1999. For the KDC_ERR_PREAUTH_REQUIRED message, - when using new PA data types defined in July 1999 or later, the - METHOD-DATA sequence must itself be encapsulated in an TypedData - element of type TD-PADATA. All new implementations interpreting the - METHOD-DATA field for the KDC_ERR_PREAUTH_REQUIRED message must accept - a type of TD-PADATA, extract the typed data field and interpret the - use any elements encapsulated in the TD-PADATA elements as if they - were present in the METHOD-DATA sequence. -e-cksum - This field contains an optional checksum for the KRB-ERROR message. - The checksum is calculated over the Kerberos ASN.1 encoding of the - KRB-ERROR message with the checksum absent. The checksum is then added - to the KRB-ERROR structure and the message is re-encoded. The Checksum - should be calculated using the session key from the ticket granting - ticket or service ticket, where available. If the error is in response - to a TGS or AP request, the checksum should be calculated uing the the - session key from the client's ticket. If the error is in response to - an AS request, then the checksum should be calulated using the - client's secret key ONLY if there has been suitable preauthentication - to prove knowledge of the secret key by the client[33]. If a checksum - can not be computed because the key to be used is not available, no - checksum will be included. - - 6. Encryption and Checksum Specifications - - The Kerberos protocols described in this document are designed to use - stream encryption ciphers, which can be simulated using commonly - available block encryption ciphers, such as the Data Encryption - Standard [DES77], and triple DES variants, in conjunction with block - chaining and checksum methods [DESM80]. Encryption is used to prove - the identities of the network entities participating in message - exchanges. The Key Distribution Center for each realm is trusted by - all principals registered in that realm to store a secret key in - confidence. Proof of knowledge of this secret key is used to verify - the authenticity of a principal. - - The KDC uses the principal's secret key (in the AS exchange) or a - shared session key (in the TGS exchange) to encrypt responses to - ticket requests; the ability to obtain the secret key or session key - implies the knowledge of the appropriate keys and the identity of the - KDC. The ability of a principal to decrypt the KDC response and - present a Ticket and a properly formed Authenticator (generated with - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - the session key from the KDC response) to a service verifies the - identity of the principal; likewise the ability of the service to - extract the session key from the Ticket and prove its knowledge - thereof in a response verifies the identity of the service. - - The Kerberos protocols generally assume that the encryption used is - secure from cryptanalysis; however, in some cases, the order of fields - in the encrypted portions of messages are arranged to minimize the - effects of poorly chosen keys. It is still important to choose good - keys. If keys are derived from user-typed passwords, those passwords - need to be well chosen to make brute force attacks more difficult. - Poorly chosen keys still make easy targets for intruders. - - The following sections specify the encryption and checksum mechanisms - currently defined for Kerberos. The encodings, chaining, and padding - requirements for each are described. For encryption methods, it is - often desirable to place random information (often referred to as a - confounder) at the start of the message. The requirements for a - confounder are specified with each encryption mechanism. - - Some encryption systems use a block-chaining method to improve the the - security characteristics of the ciphertext. However, these chaining - methods often don't provide an integrity check upon decryption. Such - systems (such as DES in CBC mode) must be augmented with a checksum of - the plain-text which can be verified at decryption and used to detect - any tampering or damage. Such checksums should be good at detecting - burst errors in the input. If any damage is detected, the decryption - routine is expected to return an error indicating the failure of an - integrity check. Each encryption type is expected to provide and - verify an appropriate checksum. The specification of each encryption - method sets out its checksum requirements. - - Finally, where a key is to be derived from a user's password, an - algorithm for converting the password to a key of the appropriate type - is included. It is desirable for the string to key function to be - one-way, and for the mapping to be different in different realms. This - is important because users who are registered in more than one realm - will often use the same password in each, and it is desirable that an - attacker compromising the Kerberos server in one realm not obtain or - derive the user's key in another. - - For an discussion of the integrity characteristics of the candidate - encryption and checksum methods considered for Kerberos, the reader is - referred to [SG92]. - - 6.1. Encryption Specifications - - The following ASN.1 definition describes all encrypted messages. The - enc-part field which appears in the unencrypted part of messages in - section 5 is a sequence consisting of an encryption type, an optional - key version number, and the ciphertext. - - EncryptedData ::= SEQUENCE { - etype[0] INTEGER, -- EncryptionType - kvno[1] INTEGER OPTIONAL, - cipher[2] OCTET STRING -- ciphertext - } - - - - etype - This field identifies which encryption algorithm was used to - encipher the cipher. Detailed specifications for selected - encryption types appear later in this section. - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - kvno - This field contains the version number of the key under which - data is encrypted. It is only present in messages encrypted under - long lasting keys, such as principals' secret keys. - cipher - This field contains the enciphered text, encoded as an OCTET - STRING. - The cipher field is generated by applying the specified encryption - algorithm to data composed of the message and algorithm-specific - inputs. Encryption mechanisms defined for use with Kerberos must take - sufficient measures to guarantee the integrity of the plaintext, and - we recommend they also take measures to protect against precomputed - dictionary attacks. If the encryption algorithm is not itself capable - of doing so, the protections can often be enhanced by adding a - checksum and a confounder. - - The suggested format for the data to be encrypted includes a - confounder, a checksum, the encoded plaintext, and any necessary - padding. The msg-seq field contains the part of the protocol message - described in section 5 which is to be encrypted. The confounder, - checksum, and padding are all untagged and untyped, and their length - is exactly sufficient to hold the appropriate item. The type and - length is implicit and specified by the particular encryption type - being used (etype). The format for the data to be encrypted for some - methods is described in the following diagram, but other methods may - deviate from this layour - so long as the definition of the method - defines the layout actually in use. - - +-----------+----------+-------------+-----+ - |confounder | check | msg-seq | pad | - +-----------+----------+-------------+-----+ - - The format cannot be described in ASN.1, but for those who prefer an - ASN.1-like notation: - - CipherText ::= ENCRYPTED SEQUENCE { - confounder[0] UNTAGGED[35] OCTET STRING(conf_length) -OPTIONAL, - check[1] UNTAGGED OCTET STRING(checksum_length) -OPTIONAL, - msg-seq[2] MsgSequence, - pad UNTAGGED OCTET STRING(pad_length) OPTIONAL - } - - One generates a random confounder of the appropriate length, placing - it in confounder; zeroes out check; calculates the appropriate - checksum over confounder, check, and msg-seq, placing the result in - check; adds the necessary padding; then encrypts using the specified - encryption type and the appropriate key. - - Unless otherwise specified, a definition of an encryption algorithm - that specifies a checksum, a length for the confounder field, or an - octet boundary for padding uses this ciphertext format[36]. Those - fields which are not specified will be omitted. - - In the interest of allowing all implementations using a particular - encryption type to communicate with all others using that type, the - specification of an encryption type defines any checksum that is - needed as part of the encryption process. If an alternative checksum - is to be used, a new encryption type must be defined. - - Some cryptosystems require additional information beyond the key and - the data to be encrypted. For example, DES, when used in - cipher-block-chaining mode, requires an initialization vector. If - required, the description for each encryption type must specify the - source of such additional information. 6.2. Encryption Keys - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - The sequence below shows the encoding of an encryption key: - - EncryptionKey ::= SEQUENCE { - keytype[0] INTEGER, - keyvalue[1] OCTET STRING - } - - keytype - This field specifies the type of encryption that is to be - performed using the key that follows in the keyvalue field. It - will always correspond to the etype to be used to generate or - decode the EncryptedData. In cases when multiple algorithms use a - common kind of key (e.g., if the encryption algorithm uses an - alternate checksum algorithm for an integrity check, or a - different chaining mechanism), the keytype provides information - needed to determine which algorithm is to be used. - keyvalue - This field contains the key itself, encoded as an octet string. - All negative values for the encryption key type are reserved for local - use. All non-negative values are reserved for officially assigned type - fields and interpreta- tions. - - 6.3. Encryption Systems - - 6.3.1. The NULL Encryption System (null) - - If no encryption is in use, the encryption system is said to be the - NULL encryption system. In the NULL encryption system there is no - checksum, confounder or padding. The ciphertext is simply the - plaintext. The NULL Key is used by the null encryption system and is - zero octets in length, with keytype zero (0). - - 6.3.2. DES in CBC mode with a CRC-32 checksum (des-cbc-crc) - - The des-cbc-crc encryption mode encrypts information under the Data - Encryption Standard [DES77] using the cipher block chaining mode - [DESM80]. A CRC-32 checksum (described in ISO 3309 [ISO3309]) is - applied to the confounder and message sequence (msg-seq) and placed in - the cksum field. DES blocks are 8 bytes. As a result, the data to be - encrypted (the concatenation of confounder, checksum, and message) - must be padded to an 8 byte boundary before encryption. The details of - the encryption of this data are identical to those for the des-cbc-md5 - encryption mode. - - Note that, since the CRC-32 checksum is not collision-proof, an - attacker could use a probabilistic chosen-plaintext attack to generate - a valid message even if a confounder is used [SG92]. The use of - collision-proof checksums is recommended for environments where such - attacks represent a significant threat. The use of the CRC-32 as the - checksum for ticket or authenticator is no longer mandated as an - interoperability requirement for Kerberos Version 5 Specification 1 - (See section 9.1 for specific details). - - 6.3.3. DES in CBC mode with an MD4 checksum (des-cbc-md4) - - The des-cbc-md4 encryption mode encrypts information under the Data - Encryption Standard [DES77] using the cipher block chaining mode - [DESM80]. An MD4 checksum (described in [MD492]) is applied to the - confounder and message sequence (msg-seq) and placed in the cksum - field. DES blocks are 8 bytes. As a result, the data to be encrypted - (the concatenation of confounder, checksum, and message) must be - padded to an 8 byte boundary before encryption. The details of the - encryption of this data are identical to those for the des-cbc-md5 - encryption mode. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - 6.3.4. DES in CBC mode with an MD5 checksum (des-cbc-md5) - - The des-cbc-md5 encryption mode encrypts information under the Data - Encryption Standard [DES77] using the cipher block chaining mode - [DESM80]. An MD5 checksum (described in [MD5-92].) is applied to the - confounder and message sequence (msg-seq) and placed in the cksum - field. DES blocks are 8 bytes. As a result, the data to be encrypted - (the concatenation of confounder, checksum, and message) must be - padded to an 8 byte boundary before encryption. - - Plaintext and DES ciphtertext are encoded as blocks of 8 octets which - are concatenated to make the 64-bit inputs for the DES algorithms. The - first octet supplies the 8 most significant bits (with the octet's - MSbit used as the DES input block's MSbit, etc.), the second octet the - next 8 bits, ..., and the eighth octet supplies the 8 least - significant bits. - - Encryption under DES using cipher block chaining requires an - additional input in the form of an initialization vector. Unless - otherwise specified, zero should be used as the initialization vector. - Kerberos' use of DES requires an 8 octet confounder. - - The DES specifications identify some 'weak' and 'semi-weak' keys; - those keys shall not be used for encrypting messages for use in - Kerberos. Additionally, because of the way that keys are derived for - the encryption of checksums, keys shall not be used that yield 'weak' - or 'semi-weak' keys when eXclusive-ORed with the hexadecimal constant - F0F0F0F0F0F0F0F0. - - A DES key is 8 octets of data, with keytype one (1). This consists of - 56 bits of key, and 8 parity bits (one per octet). The key is encoded - as a series of 8 octets written in MSB-first order. The bits within - the key are also encoded in MSB order. For example, if the encryption - key is (B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8) - where B1,B2,...,B56 are the key bits in MSB order, and P1,P2,...,P8 - are the parity bits, the first octet of the key would be - B1,B2,...,B7,P1 (with B1 as the MSbit). [See the FIPS 81 introduction - for reference.] - - String to key transformation - - To generate a DES key from a text string (password), a "salt" is - concatenated to the text string, and then padded with ASCII nulls to - an 8 byte boundary. This "salt" is normally the realm and each - component of the principal's name appended. However, sometimes - different salts are used --- for example, when a realm is renamed, or - if a user changes her username, or for compatibility with Kerberos V4 - (whose string-to-key algorithm uses a null string for the salt). This - string is then fan-folded and eXclusive-ORed with itself to form an 8 - byte DES key. Before eXclusive-ORing a block, every byte is shifted - one bit to the left to leave the lowest bit zero. The key is the - "corrected" by correcting the parity on the key, and if the key - matches a 'weak' or 'semi-weak' key as described in the DES - specification, it is eXclusive-ORed with the constant - 00000000000000F0. This key is then used to generate a DES CBC checksum - on the initial string (with the salt appended). The result of the CBC - checksum is the "corrected" as described above to form the result - which is return as the key. Pseudocode follows: - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - name_to_default_salt(realm, name) { - s = realm - for(each component in name) { - s = s + component; - } - return s; - } - - key_correction(key) { - fixparity(key); - if (is_weak_key_key(key)) - key = key XOR 0xF0; - return(key); - } - - string_to_key(string,salt) { - - odd = 1; - s = string + salt; - tempkey = NULL; - pad(s); /* with nulls to 8 byte boundary */ - for(8byteblock in s) { - if(odd == 0) { - odd = 1; - reverse(8byteblock) - } - else odd = 0; - left shift every byte in 8byteblock one bit; - tempkey = tempkey XOR 8byteblock; - } - tempkey = key_correction(tempkey); - key = key_correction(DES-CBC-check(s,tempkey)); - return(key); - } - - 6.3.5. Triple DES with HMAC-SHA1 Kerberos Encryption Type with and - without Key Derivation [Original draft by Marc Horowitz, revisions by - David Miller] - - There are still a few pieces of this specification to be included - by falue, rather than by reference. This will be done before the - Pittsburgh IETF. - This encryption type is based on the Triple DES cryptosystem, the - HMAC-SHA1 [Krawczyk96] message authentication algorithm, and key - derivation for Kerberos V5 [HorowitzB96]. Key derivation may or may - not be used in conjunction with the use of Triple DES keys. - - Algorithm Identifiers - - The des3-cbc-hmac-sha1 encryption type has been assigned the value 7. - The des3-cbc-hmac-sha1-kd encryption type, specifying the key - derivation variant of the encryption type, has been assigned the value - 16. The hmac-sha1-des3 checksum type has been assigned the value 13. - The hmac-sha1-des3-kd checksum type, specifying the key derivation - variant of the checksum, has been assigned the value 12. - - Triple DES Key Production - - The EncryptionKey value is 24 octets long. The 7 most significant bits - of each octet contain key bits, and the least significant bit is the - inverse of the xor of the key bits. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - For the purposes of key derivation, the block size is 64 bits, and the - key size is 168 bits. The 168 bits output by key derivation are - converted to an EncryptionKey value as follows. First, the 168 bits - are divided into three groups of 56 bits, which are expanded - individually into 64 bits as follows: - - 1 2 3 4 5 6 7 p - 9 10 11 12 13 14 15 p - 17 18 19 20 21 22 23 p - 25 26 27 28 29 30 31 p - 33 34 35 36 37 38 39 p - 41 42 43 44 45 46 47 p - 49 50 51 52 53 54 55 p - 56 48 40 32 24 16 8 p - - The "p" bits are parity bits computed over the data bits. The output - of the three expansions are concatenated to form the EncryptionKey - value. - - When the HMAC-SHA1 of a string is computed, the key is used in the - EncryptedKey form. - - The string-to-key function is used to tranform UNICODE passwords into - DES3 keys. The DES3 string-to-key function relies on the "N-fold" - algorithm, which is detailed in [9]. The description of the N-fold - algorithm in that document is as follows: - o To n-fold a number X, replicate the input value to a length that - is the least common multiple of n and the length of X. Before - each repetition, the input is rotated to the right by 13 bit - positions. The successive n-bit chunks are added together using - 1's-complement addition (that is, addition with end-around carry) - to yield an n-bit result" - o The n-fold algorithm, as with DES string-to-key, is applied to - the password string concatenated with a salt value. The salt - value is derived in the same was as for the DES string-to-key - algorithm. For 3-key triple DES then, the operation will involve - a 168-fold of the input password string. The remainder of the - string-to-key function for DES3 is shown here in pseudocode: - - DES3string-to-key(passwordString, key) - - salt = name_to_default_salt(realm, name) - s = passwordString + salt - tmpKey1 = 168-fold(s) - parityFix(tmpKey1); - if not weakKey(tmpKey1) - /* - * Encrypt temp key in itself with a - * zero initialization vector - * - * Function signature is DES3encrypt(plain, key, iv) - * with cipher as the return value - */ - tmpKey2 = DES3encrypt(tmpKey1, tmpKey1, zeroIvec) - /* - * Encrypt resultant temp key in itself with third component - * of first temp key as initialization vector - */ - key = DES3encrypt(tmpKey2, tmpKey1, tmpKey1[2]) - parityFix(key) - if not weakKey(key) - return SUCCESS - else - return FAILURE - else - return FAILURE - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - The weakKey function above is the same weakKey function used with DES - keys, but applied to each of the three single DES keys that comprise - the triple DES key. - - The lengths of UNICODE encoded character strings include the trailing - terminator character (0). - - Encryption Types des3-cbc-hmac-sha1 and des3-cbc-hmac-sha1-kd - - EncryptedData using this type must be generated as described in - [Horowitz96]. The encryption algorithm is Triple DES in Outer-CBC - mode. The checksum algorithm is HMAC-SHA1. If the key derivation - variant of the encryption type is used, encryption key values are - modified according to the method under the Key Derivation section - below. - - Unless otherwise specified, a zero IV must be used. - - If the length of the input data is not a multiple of the block size, - zero octets must be used to pad the plaintext to the next eight-octet - boundary. The counfounder must be eight random octets (one block). - - Checksum Types hmac-sha1-des3 and hmac-sha1-des3-kd - - Checksums using this type must be generated as described in - [Horowitz96]. The keyed hash algorithm is HMAC-SHA1. If the key - derivation variant of the checksum type is used, checksum key values - are modified according to the method under the Key Derivation section - below. - - Key Derivation - - In the Kerberos protocol, cryptographic keys are used in a number of - places. In order to minimize the effect of compromising a key, it is - desirable to use a different key for each of these places. Key - derivation [Horowitz96] can be used to construct different keys for - each operation from the keys transported on the network. For this to - be possible, a small change to the specification is necessary. - - This section specifies a profile for the use of key derivation - [Horowitz96] with Kerberos. For each place where a key is used, a - ``key usage'' must is specified for that purpose. The key, key usage, - and encryption/checksum type together describe the transformation from - plaintext to ciphertext, or plaintext to checksum. - - Key Usage Values - - This is a complete list of places keys are used in the kerberos - protocol, with key usage values and RFC 1510 section numbers: - - 1. AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with the - client key (section 5.4.1) - 2. AS-REP Ticket and TGS-REP Ticket (includes tgs session key or - application session key), encrypted with the service key - (section 5.4.2) - 3. AS-REP encrypted part (includes tgs session key or application - session key), encrypted with the client key (section 5.4.2) - 4. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the tgs - session key (section 5.4.1) - 5. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the tgs - authenticator subkey (section 5.4.1) - 6. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum, keyed - with the tgs session key (sections 5.3.2, 5.4.1) - 7. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes tgs - authenticator subkey), encrypted with the tgs session key - (section 5.3.2) - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - 8. TGS-REP encrypted part (includes application session key), - encrypted with the tgs session key (section 5.4.2) - 9. TGS-REP encrypted part (includes application session key), - encrypted with the tgs authenticator subkey (section 5.4.2) - 10. AP-REQ Authenticator cksum, keyed with the application session - key (section 5.3.2) - 11. AP-REQ Authenticator (includes application authenticator - subkey), encrypted with the application session key (section - 5.3.2) - 12. AP-REP encrypted part (includes application session subkey), - encrypted with the application session key (section 5.5.2) - 13. KRB-PRIV encrypted part, encrypted with a key chosen by the - application (section 5.7.1) - 14. KRB-CRED encrypted part, encrypted with a key chosen by the - application (section 5.6.1) - 15. KRB-SAVE cksum, keyed with a key chosen by the application - (section 5.8.1) - 18. KRB-ERROR checksum (e-cksum in section 5.9.1) - 19. AD-KDCIssued checksum (ad-checksum in appendix B.1) - 20. Checksum for Mandatory Ticket Extensions (appendix B.6) - 21. Checksum in Authorization Data in Ticket Extensions (appendix B.7) - - Key usage values between 1024 and 2047 (inclusive) are reserved for - application use. Applications should use even values for encryption - and odd values for checksums within this range. - - A few of these key usages need a little clarification. A service which - receives an AP-REQ has no way to know if the enclosed Ticket was part - of an AS-REP or TGS-REP. Therefore, key usage 2 must always be used - for generating a Ticket, whether it is in response to an AS- REQ or - TGS-REQ. - - There might exist other documents which define protocols in terms of - the RFC1510 encryption types or checksum types. Such documents would - not know about key usages. In order that these documents continue to - be meaningful until they are updated, key usages 1024 and 1025 must be - used to derive keys for encryption and checksums, respectively. New - protocols defined in terms of the Kerberos encryption and checksum - types should use their own key usages. Key usages may be registered - with IANA to avoid conflicts. Key usages must be unsigned 32 bit - integers. Zero is not permitted. - - Defining Cryptosystems Using Key Derivation - - Kerberos requires that the ciphertext component of EncryptedData be - tamper-resistant as well as confidential. This implies encryption and - integrity functions, which must each use their own separate keys. So, - for each key usage, two keys must be generated, one for encryption - (Ke), and one for integrity (Ki): - - Ke = DK(protocol key, key usage | 0xAA) - Ki = DK(protocol key, key usage | 0x55) - - where the protocol key is from the EncryptionKey from the wire - protocol, and the key usage is represented as a 32 bit integer in - network byte order. The ciphertest must be generated from the - plaintext as follows: - - ciphertext = E(Ke, confounder | plaintext | padding) | - H(Ki, confounder | plaintext | padding) - - The confounder and padding are specific to the encryption algorithm E. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - When generating a checksum only, there is no need for a confounder or - padding. Again, a new key (Kc) must be used. Checksums must be - generated from the plaintext as follows: - - Kc = DK(protocol key, key usage | 0x99) - MAC = H(Kc, plaintext) - - Note that each enctype is described by an encryption algorithm E and a - keyed hash algorithm H, and each checksum type is described by a keyed - hash algorithm H. HMAC, with an appropriate hash, is required for use - as H. - - Key Derivation from Passwords - - The well-known constant for password key derivation must be the byte - string {0x6b 0x65 0x72 0x62 0x65 0x72 0x6f 0x73}. These values - correspond to the ASCII encoding for the string "kerberos". - - 6.4. Checksums - - The following is the ASN.1 definition used for a checksum: - - Checksum ::= SEQUENCE { - cksumtype[0] INTEGER, - checksum[1] OCTET STRING - } - - cksumtype - This field indicates the algorithm used to generate the - accompanying checksum. - checksum - This field contains the checksum itself, encoded as an octet - string. - Detailed specification of selected checksum types appear later in this - section. Negative values for the checksum type are reserved for local - use. All non-negative values are reserved for officially assigned type - fields and interpretations. - - Checksums used by Kerberos can be classified by two properties: - whether they are collision-proof, and whether they are keyed. It is - infeasible to find two plaintexts which generate the same checksum - value for a collision-proof checksum. A key is required to perturb or - initialize the algorithm in a keyed checksum. To prevent - message-stream modification by an active attacker, unkeyed checksums - should only be used when the checksum and message will be subsequently - encrypted (e.g. the checksums defined as part of the encryption - algorithms covered earlier in this section). - - Collision-proof checksums can be made tamper-proof if the checksum - value is encrypted before inclusion in a message. In such cases, the - composition of the checksum and the encryption algorithm must be - considered a separate checksum algorithm (e.g. RSA-MD5 encrypted using - DES is a new checksum algorithm of type RSA-MD5-DES). For most keyed - checksums, as well as for the encrypted forms of unkeyed - collision-proof checksums, Kerberos prepends a confounder before the - checksum is calculated. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - 6.4.1. The CRC-32 Checksum (crc32) - - The CRC-32 checksum calculates a checksum based on a cyclic redundancy - check as described in ISO 3309 [ISO3309]. The resulting checksum is - four (4) octets in length. The CRC-32 is neither keyed nor - collision-proof. The use of this checksum is not recommended. An - attacker using a probabilistic chosen-plaintext attack as described in - [SG92] might be able to generate an alternative message that satisfies - the checksum. The use of collision-proof checksums is recommended for - environments where such attacks represent a significant threat. - - 6.4.2. The RSA MD4 Checksum (rsa-md4) - - The RSA-MD4 checksum calculates a checksum using the RSA MD4 algorithm - [MD4-92]. The algorithm takes as input an input message of arbitrary - length and produces as output a 128-bit (16 octet) checksum. RSA-MD4 - is believed to be collision-proof. - - 6.4.3. RSA MD4 Cryptographic Checksum Using DES (rsa-md4-des) - - The RSA-MD4-DES checksum calculates a keyed collision-proof checksum - by prepending an 8 octet confounder before the text, applying the RSA - MD4 checksum algorithm, and encrypting the confounder and the checksum - using DES in cipher-block-chaining (CBC) mode using a variant of the - key, where the variant is computed by eXclusive-ORing the key with the - constant F0F0F0F0F0F0F0F0[39]. The initialization vector should be - zero. The resulting checksum is 24 octets long (8 octets of which are - redundant). This checksum is tamper-proof and believed to be - collision-proof. - - The DES specifications identify some weak keys' and 'semi-weak keys'; - those keys shall not be used for generating RSA-MD4 checksums for use - in Kerberos. - - The format for the checksum is described in the follow- ing diagram: - - -+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | des-cbc(confounder + rsa-md4(confounder+msg),key=var(key),iv=0) -| - -+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - - The format cannot be described in ASN.1, but for those who prefer an - ASN.1-like notation: - - rsa-md4-des-checksum ::= ENCRYPTED UNTAGGED SEQUENCE { - confounder[0] UNTAGGED OCTET STRING(8), - check[1] UNTAGGED OCTET STRING(16) - } - - 6.4.4. The RSA MD5 Checksum (rsa-md5) - - The RSA-MD5 checksum calculates a checksum using the RSA MD5 - algorithm. [MD5-92]. The algorithm takes as input an input message of - arbitrary length and produces as output a 128-bit (16 octet) checksum. - RSA-MD5 is believed to be collision-proof. - - 6.4.5. RSA MD5 Cryptographic Checksum Using DES (rsa-md5-des) - - The RSA-MD5-DES checksum calculates a keyed collision-proof checksum - by prepending an 8 octet confounder before the text, applying the RSA - MD5 checksum algorithm, and encrypting the confounder and the checksum - using DES in cipher-block-chaining (CBC) mode using a variant of the - key, where the variant is computed by eXclusive-ORing the key with the - hexadecimal constant F0F0F0F0F0F0F0F0. The initialization vector - should be zero. The resulting checksum is 24 octets long (8 octets of - which are redundant). This checksum is tamper-proof and believed to be - collision-proof. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - The DES specifications identify some 'weak keys' and 'semi-weak keys'; - those keys shall not be used for encrypting RSA-MD5 checksums for use - in Kerberos. - - The format for the checksum is described in the following diagram: - - -+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - | des-cbc(confounder + rsa-md5(confounder+msg),key=var(key),iv=0) -| - -+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ - - The format cannot be described in ASN.1, but for those who prefer an - ASN.1-like notation: - - rsa-md5-des-checksum ::= ENCRYPTED UNTAGGED SEQUENCE { - confounder[0] UNTAGGED OCTET STRING(8), - check[1] UNTAGGED OCTET STRING(16) - } - - 6.4.6. DES cipher-block chained checksum (des-mac) - - The DES-MAC checksum is computed by prepending an 8 octet confounder - to the plaintext, performing a DES CBC-mode encryption on the result - using the key and an initialization vector of zero, taking the last - block of the ciphertext, prepending the same confounder and encrypting - the pair using DES in cipher-block-chaining (CBC) mode using a a - variant of the key, where the variant is computed by eXclusive-ORing - the key with the hexadecimal constant F0F0F0F0F0F0F0F0. The - initialization vector should be zero. The resulting checksum is 128 - bits (16 octets) long, 64 bits of which are redundant. This checksum - is tamper-proof and collision-proof. - - The format for the checksum is described in the following diagram: - - -+--+--+--+--+--+--+--+--+-----+-----+-----+-----+-----+-----+-----+-----+ - | des-cbc(confounder + des-mac(conf+msg,iv=0,key),key=var(key),iv=0) -| - -+--+--+--+--+--+--+--+--+-----+-----+-----+-----+-----+-----+-----+-----+ - - The format cannot be described in ASN.1, but for those who prefer an - ASN.1-like notation: - - des-mac-checksum ::= ENCRYPTED UNTAGGED SEQUENCE { - confounder[0] UNTAGGED OCTET STRING(8), - check[1] UNTAGGED OCTET STRING(8) - } - - The DES specifications identify some 'weak' and 'semi-weak' keys; - those keys shall not be used for generating DES-MAC checksums for use - in Kerberos, nor shall a key be used whose variant is 'weak' or - 'semi-weak'. - - 6.4.7. RSA MD4 Cryptographic Checksum Using DES alternative - (rsa-md4-des-k) - - The RSA-MD4-DES-K checksum calculates a keyed collision-proof checksum - by applying the RSA MD4 checksum algorithm and encrypting the results - using DES in cipher-block-chaining (CBC) mode using a DES key as both - key and initialization vector. The resulting checksum is 16 octets - long. This checksum is tamper-proof and believed to be - collision-proof. Note that this checksum type is the old method for - encoding the RSA-MD4-DES checksum and it is no longer recommended. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - 6.4.8. DES cipher-block chained checksum alternative (des-mac-k) - - The DES-MAC-K checksum is computed by performing a DES CBC-mode - encryption of the plaintext, and using the last block of the - ciphertext as the checksum value. It is keyed with an encryption key - and an initialization vector; any uses which do not specify an - additional initialization vector will use the key as both key and - initialization vector. The resulting checksum is 64 bits (8 octets) - long. This checksum is tamper-proof and collision-proof. Note that - this checksum type is the old method for encoding the DES-MAC checksum - and it is no longer recommended. The DES specifications identify some - 'weak keys' and 'semi-weak keys'; those keys shall not be used for - generating DES-MAC checksums for use in Kerberos. - - 7. Naming Constraints - - 7.1. Realm Names - - Although realm names are encoded as GeneralStrings and although a - realm can technically select any name it chooses, interoperability - across realm boundaries requires agreement on how realm names are to - be assigned, and what information they imply. - - To enforce these conventions, each realm must conform to the - conventions itself, and it must require that any realms with which - inter-realm keys are shared also conform to the conventions and - require the same from its neighbors. - - Kerberos realm names are case sensitive. Realm names that differ only - in the case of the characters are not equivalent. There are presently - four styles of realm names: domain, X500, other, and reserved. - Examples of each style follow: - - domain: ATHENA.MIT.EDU (example) - X500: C=US/O=OSF (example) - other: NAMETYPE:rest/of.name=without-restrictions (example) - reserved: reserved, but will not conflict with above - - Domain names must look like domain names: they consist of components - separated by periods (.) and they contain neither colons (:) nor - slashes (/). Though domain names themselves are case insensitive, in - order for realms to match, the case must match as well. When - establishing a new realm name based on an internet domain name it is - recommended by convention that the characters be converted to upper - case. - - X.500 names contain an equal (=) and cannot contain a colon (:) before - the equal. The realm names for X.500 names will be string - representations of the names with components separated by slashes. - Leading and trailing slashes will not be included. - - Names that fall into the other category must begin with a prefix that - contains no equal (=) or period (.) and the prefix must be followed by - a colon (:) and the rest of the name. All prefixes must be assigned - before they may be used. Presently none are assigned. - - The reserved category includes strings which do not fall into the - first three categories. All names in this category are reserved. It is - unlikely that names will be assigned to this category unless there is - a very strong argument for not using the 'other' category. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - These rules guarantee that there will be no conflicts between the - various name styles. The following additional constraints apply to the - assignment of realm names in the domain and X.500 categories: the name - of a realm for the domain or X.500 formats must either be used by the - organization owning (to whom it was assigned) an Internet domain name - or X.500 name, or in the case that no such names are registered, - authority to use a realm name may be derived from the authority of the - parent realm. For example, if there is no domain name for E40.MIT.EDU, - then the administrator of the MIT.EDU realm can authorize the creation - of a realm with that name. - - This is acceptable because the organization to which the parent is - assigned is presumably the organization authorized to assign names to - its children in the X.500 and domain name systems as well. If the - parent assigns a realm name without also registering it in the domain - name or X.500 hierarchy, it is the parent's responsibility to make - sure that there will not in the future exists a name identical to the - realm name of the child unless it is assigned to the same entity as - the realm name. - - 7.2. Principal Names - - As was the case for realm names, conventions are needed to ensure that - all agree on what information is implied by a principal name. The - name-type field that is part of the principal name indicates the kind - of information implied by the name. The name-type should be treated as - a hint. Ignoring the name type, no two names can be the same (i.e. at - least one of the components, or the realm, must be different). The - following name types are defined: - - name-type value meaning - - NT-UNKNOWN 0 Name type not known - NT-PRINCIPAL 1 General principal name (e.g. username, DCE -principal) - NT-SRV-INST 2 Service and other unique instance (krbtgt) - NT-SRV-HST 3 Service with host name as instance (telnet, rcmds) - NT-SRV-XHST 4 Service with slash-separated host name components - NT-UID 5 Unique ID - NT-X500-PRINCIPAL 6 Encoded X.509 Distingished name [RFC 1779] - NT-SMTP-NAME 7 Name in form of SMTP email name (e.g. -user@foo.com) - - When a name implies no information other than its uniqueness at a - particular time the name type PRINCIPAL should be used. The principal - name type should be used for users, and it might also be used for a - unique server. If the name is a unique machine generated ID that is - guaranteed never to be reassigned then the name type of UID should be - used (note that it is generally a bad idea to reassign names of any - type since stale entries might remain in access control lists). - - If the first component of a name identifies a service and the - remaining components identify an instance of the service in a server - specified manner, then the name type of SRV-INST should be used. An - example of this name type is the Kerberos ticket-granting service - whose name has a first component of krbtgt and a second component - identifying the realm for which the ticket is valid. - - If instance is a single component following the service name and the - instance identifies the host on which the server is running, then the - name type SRV-HST should be used. This type is typically used for - Internet services such as telnet and the Berkeley R commands. If the - separate components of the host name appear as successive components - following the name of the service, then the name type SRV-XHST should - be used. This type might be used to identify servers on hosts with - X.500 names where the slash (/) might otherwise be ambiguous. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - A name type of NT-X500-PRINCIPAL should be used when a name from an - X.509 certificiate is translated into a Kerberos name. The encoding of - the X.509 name as a Kerberos principal shall conform to the encoding - rules specified in RFC 2253. - - A name type of SMTP allows a name to be of a form that resembles a - SMTP email name. This name type can be used in conjunction with - name-canonicalization to allow a free-form of username to be specified - as a client name and allow the KDC to determine the Kerberos principal - name for the requested name. [JBrezak] - - A name type of UNKNOWN should be used when the form of the name is not - known. When comparing names, a name of type UNKNOWN will match - principals authenticated with names of any type. A principal - authenticated with a name of type UNKNOWN, however, will only match - other names of type UNKNOWN. - - Names of any type with an initial component of 'krbtgt' are reserved - for the Kerberos ticket granting service. See section 8.2.3 for the - form of such names. - - 7.2.1. Name of server principals - - The principal identifier for a server on a host will generally be - composed of two parts: (1) the realm of the KDC with which the server - is registered, and (2) a two-component name of type NT-SRV-HST if the - host name is an Internet domain name or a multi-component name of type - NT-SRV-XHST if the name of the host is of a form such as X.500 that - allows slash (/) separators. The first component of the two- or - multi-component name will identify the service and the latter - components will identify the host. Where the name of the host is not - case sensitive (for example, with Internet domain names) the name of - the host must be lower case. If specified by the application protocol - for services such as telnet and the Berkeley R commands which run with - system privileges, the first component may be the string 'host' - instead of a service specific identifier. When a host has an official - name and one or more aliases, the official name of the host must be - used when constructing the name of the server principal. - - 8. Constants and other defined values - - 8.1. Host address types - - All negative values for the host address type are reserved for local - use. All non-negative values are reserved for officially assigned type - fields and interpretations. - - The values of the types for the following addresses are chosen to - match the defined address family constants in the Berkeley Standard - Distributions of Unix. They can be found in with symbolic names AF_xxx - (where xxx is an abbreviation of the address family name). - - Internet (IPv4) Addresses - - Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded in - MSB order. The type of IPv4 addresses is two (2). - - Internet (IPv6) Addresses [Westerlund] - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - IPv6 addresses are 128-bit (16-octet) quantities, encoded in MSB - order. The type of IPv6 addresses is twenty-four (24). [RFC1883] - [RFC1884]. The following addresses (see [RFC1884]) MUST not appear in - any Kerberos packet: - o the Unspecified Address - o the Loopback Address - o Link-Local addresses - IPv4-mapped IPv6 addresses MUST be represented as addresses of type 2. - - CHAOSnet addresses - - CHAOSnet addresses are 16-bit (2-octet) quantities, encoded in MSB - order. The type of CHAOSnet addresses is five (5). - - ISO addresses - - ISO addresses are variable-length. The type of ISO addresses is seven - (7). - - Xerox Network Services (XNS) addresses - - XNS addresses are 48-bit (6-octet) quantities, encoded in MSB order. - The type of XNS addresses is six (6). - - AppleTalk Datagram Delivery Protocol (DDP) addresses - - AppleTalk DDP addresses consist of an 8-bit node number and a 16-bit - network number. The first octet of the address is the node number; the - remaining two octets encode the network number in MSB order. The type - of AppleTalk DDP addresses is sixteen (16). - - DECnet Phase IV addresses - - DECnet Phase IV addresses are 16-bit addresses, encoded in LSB order. - The type of DECnet Phase IV addresses is twelve (12). - - Netbios addresses - - Netbios addresses are 16-octet addresses typically composed of 1 to 15 - characters, trailing blank (ascii char 20) filled, with a 16th octet - of 0x0. The type of Netbios addresses is 20 (0x14). - - 8.2. KDC messages - - 8.2.1. UDP/IP transport - - When contacting a Kerberos server (KDC) for a KRB_KDC_REQ request - using UDP IP transport, the client shall send a UDP datagram - containing only an encoding of the request to port 88 (decimal) at the - KDC's IP address; the KDC will respond with a reply datagram - containing only an encoding of the reply message (either a KRB_ERROR - or a KRB_KDC_REP) to the sending port at the sender's IP address. - Kerberos servers supporting IP transport must accept UDP requests on - port 88 (decimal). The response to a request made through UDP/IP - transport must also use UDP/IP transport. - - 8.2.2. TCP/IP transport [Westerlund,Danielsson] - - Kerberos servers (KDC's) should accept TCP requests on port 88 - (decimal) and clients should support the sending of TCP requests on - port 88 (decimal). When the KRB_KDC_REQ message is sent to the KDC - over a TCP stream, a new connection will be established for each - authentication exchange (request and response). The KRB_KDC_REP or - KRB_ERROR message will be returned to the client on the same TCP - stream that was established for the request. The response to a request - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - made through TCP/IP transport must also use TCP/IP transport. - Implementors should note that some extentions to the Kerberos protocol - will not work if any implementation not supporting the TCP transport - is involved (client or KDC). Implementors are strongly urged to - support the TCP transport on both the client and server and are - advised that the current notation of "should" support will likely - change in the future to must support. The KDC may close the TCP stream - after sending a response, but may leave the stream open if it expects - a followup - in which case it may close the stream at any time if - resource constratints or other factors make it desirable to do so. - Care must be taken in managing TCP/IP connections with the KDC to - prevent denial of service attacks based on the number of TCP/IP - connections with the KDC that remain open. If multiple exchanges with - the KDC are needed for certain forms of preauthentication, multiple - TCP connections may be required. A client may close the stream after - receiving response, and should close the stream if it does not expect - to send followup messages. The client must be prepared to have the - stream closed by the KDC at anytime, in which case it must simply - connect again when it is ready to send subsequent messages. - - The first four octets of the TCP stream used to transmit the request - request will encode in network byte order the length of the request - (KRB_KDC_REQ), and the length will be followed by the request itself. - The response will similarly be preceeded by a 4 octet encoding in - network byte order of the length of the KRB_KDC_REP or the KRB_ERROR - message and will be followed by the KRB_KDC_REP or the KRB_ERROR - response. If the sign bit is set on the integer represented by the - first 4 octets, then the next 4 octets will be read, extending the - length of the field by another 4 octets (less the sign bit which is - reserved for future expansion). - - 8.2.3. OSI transport - - During authentication of an OSI client to an OSI server, the mutual - authentication of an OSI server to an OSI client, the transfer of - credentials from an OSI client to an OSI server, or during exchange of - private or integrity checked messages, Kerberos protocol messages may - be treated as opaque objects and the type of the authentication - mechanism will be: - - OBJECT IDENTIFIER ::= {iso (1), org(3), dod(6),internet(1), -security(5),kerberosv5(2)} - - Depending on the situation, the opaque object will be an - authentication header (KRB_AP_REQ), an authentication reply - (KRB_AP_REP), a safe message (KRB_SAFE), a private message (KRB_PRIV), - or a credentials message (KRB_CRED). The opaque data contains an - application code as specified in the ASN.1 description for each - message. The application code may be used by Kerberos to determine the - message type. - - 8.2.3. Name of the TGS - - The principal identifier of the ticket-granting service shall be - composed of three parts: (1) the realm of the KDC issuing the TGS - ticket (2) a two-part name of type NT-SRV-INST, with the first part - "krbtgt" and the second part the name of the realm which will accept - the ticket-granting ticket. For example, a ticket-granting ticket - issued by the ATHENA.MIT.EDU realm to be used to get tickets from the - ATHENA.MIT.EDU KDC has a principal identifier of "ATHENA.MIT.EDU" - (realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A ticket-granting ticket - issued by the ATHENA.MIT.EDU realm to be used to get tickets from the - MIT.EDU realm has a principal identifier of "ATHENA.MIT.EDU" (realm), - ("krbtgt", "MIT.EDU") (name). - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - 8.3. Protocol constants and associated values - - The following tables list constants used in the protocol and defines - their meanings. Ranges are specified in the "specification" section - that limit the values of constants for which values are defined here. - This allows implementations to make assumptions about the maximum - values that will be received for these constants. Implementation - receiving values outside the range specified in the "specification" - section may reject the request, but they must recover cleanly. - - Encryption type etype value block size minimum pad confounder -size - NULL 0 1 0 0 - des-cbc-crc 1 8 4 8 - des-cbc-md4 2 8 0 8 - des-cbc-md5 3 8 0 8 - reserved 4 - des3-cbc-md5 5 8 0 8 - reserved 6 - des3-cbc-sha1 7 8 0 8 - dsaWithSHA1-CmsOID 9 -(pkinit) - md5WithRSAEncryption-CmsOID 10 -(pkinit) - sha1WithRSAEncryption-CmsOID 11 -(pkinit) - rc2CBC-EnvOID 12 -(pkinit) - rsaEncryption-EnvOID 13 (pkinit from PKCS#1 -v1.5) - rsaES-OAEP-ENV-OID 14 (pkinit from PKCS#1 -v2.0) - des-ede3-cbc-Env-OID 15 -(pkinit) - des3-cbc-sha1-kd 16 (Tom -Yu) - rc4-hmac 23 -(swift) - rc4-hmac-exp 24 -(swift) - - reserved 0x8003 - - Checksum type sumtype value checksum size - CRC32 1 4 - rsa-md4 2 16 - rsa-md4-des 3 24 - des-mac 4 16 - des-mac-k 5 8 - rsa-md4-des-k 6 16 (drop rsa ?) - rsa-md5 7 16 (drop rsa ?) - rsa-md5-des 8 24 (drop rsa ?) - rsa-md5-des3 9 24 (drop rsa ?) - hmac-sha1-des3-kd 12 20 - hmac-sha1-des3 13 20 - sha1 (unkeyed) 14 20 - - padata type padata-type value - - PA-TGS-REQ 1 - PA-ENC-TIMESTAMP 2 - PA-PW-SALT 3 - reserved 4 - PA-ENC-UNIX-TIME 5 (depricated) - PA-SANDIA-SECUREID 6 - PA-SESAME 7 - PA-OSF-DCE 8 - PA-CYBERSAFE-SECUREID 9 - PA-AFS3-SALT 10 - PA-ETYPE-INFO 11 - PA-SAM-CHALLENGE 12 (sam/otp) - PA-SAM-RESPONSE 13 (sam/otp) - PA-PK-AS-REQ 14 (pkinit) - PA-PK-AS-REP 15 (pkinit) - PA-USE-SPECIFIED-KVNO 20 - PA-SAM-REDIRECT 21 (sam/otp) - PA-GET-FROM-TYPED-DATA 22 - PA-SAM-ETYPE-INFO 23 (sam/otp) - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - data-type value form of typed-data - - reserved 1-21 - TD-PADATA 22 - TD-PKINIT-CMS-CERTIFICATES 101 CertificateSet from CMS - TD-KRB-PRINCIPAL 102 - TD-KRB-REALM 103 - TD-TRUSTED-CERTIFIERS 104 - TD-CERTIFICATE-INDEX 105 - TD-APP-DEFINED-ERROR 106 - - authorization data type ad-type value - AD-IF-RELEVANT 1 - AD-INTENDED-FOR-SERVER 2 - AD-INTENDED-FOR-APPLICATION-CLASS 3 - AD-KDC-ISSUED 4 - AD-OR 5 - AD-MANDATORY-TICKET-EXTENSIONS 6 - AD-IN-TICKET-EXTENSIONS 7 - reserved values 8-63 - OSF-DCE 64 - SESAME 65 - AD-OSF-DCE-PKI-CERTID 66 (hemsath@us.ibm.com) - AD-WIN200-PAC 128 -(jbrezak@exchange.microsoft.com) - - Ticket Extension Types - - TE-TYPE-NULL 0 Null ticket extension - TE-TYPE-EXTERNAL-ADATA 1 Integrity protected authorization -data - reserved 2 TE-TYPE-PKCROSS-KDC - TE-TYPE-PKCROSS-CLIENT 3 PKCROSS cross realm key ticket - TE-TYPE-CYBERSAFE-EXT 4 Assigned to CyberSafe Corp - reserved 5 TE-TYPE-DEST-HOST - - alternate authentication type method-type value - reserved values 0-63 - ATT-CHALLENGE-RESPONSE 64 - - transited encoding type tr-type value - DOMAIN-X500-COMPRESS 1 - reserved values all others - - Label Value Meaning or MIT code - - pvno 5 current Kerberos protocol version number - - message types - - KRB_AS_REQ 10 Request for initial authentication - KRB_AS_REP 11 Response to KRB_AS_REQ request - KRB_TGS_REQ 12 Request for authentication based on TGT - KRB_TGS_REP 13 Response to KRB_TGS_REQ request - KRB_AP_REQ 14 application request to server - KRB_AP_REP 15 Response to KRB_AP_REQ_MUTUAL - KRB_SAFE 20 Safe (checksummed) application message - KRB_PRIV 21 Private (encrypted) application message - KRB_CRED 22 Private (encrypted) message to forward -credentials - KRB_ERROR 30 Error response - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - name types - - KRB_NT_UNKNOWN 0 Name type not known - KRB_NT_PRINCIPAL 1 Just the name of the principal as in DCE, or -for users - KRB_NT_SRV_INST 2 Service and other unique instance (krbtgt) - KRB_NT_SRV_HST 3 Service with host name as instance (telnet, -rcommands) - KRB_NT_SRV_XHST 4 Service with host as remaining components - KRB_NT_UID 5 Unique ID - KRB_NT_X500_PRINCIPAL 6 Encoded X.509 Distingished name [RFC 2253] - - error codes - - KDC_ERR_NONE 0 No error - KDC_ERR_NAME_EXP 1 Client's entry in database has -expired - KDC_ERR_SERVICE_EXP 2 Server's entry in database has -expired - KDC_ERR_BAD_PVNO 3 Requested protocol version number -not supported - KDC_ERR_C_OLD_MAST_KVNO 4 Client's key encrypted in old -master key - KDC_ERR_S_OLD_MAST_KVNO 5 Server's key encrypted in old -master key - KDC_ERR_C_PRINCIPAL_UNKNOWN 6 Client not found in Kerberos -database - KDC_ERR_S_PRINCIPAL_UNKNOWN 7 Server not found in Kerberos -database - KDC_ERR_PRINCIPAL_NOT_UNIQUE 8 Multiple principal entries in -database - KDC_ERR_NULL_KEY 9 The client or server has a null key - KDC_ERR_CANNOT_POSTDATE 10 Ticket not eligible for postdating - KDC_ERR_NEVER_VALID 11 Requested start time is later than -end time - KDC_ERR_POLICY 12 KDC policy rejects request - KDC_ERR_BADOPTION 13 KDC cannot accommodate requested -option - KDC_ERR_ETYPE_NOSUPP 14 KDC has no support for encryption -type - KDC_ERR_SUMTYPE_NOSUPP 15 KDC has no support for checksum -type - KDC_ERR_PADATA_TYPE_NOSUPP 16 KDC has no support for padata type - KDC_ERR_TRTYPE_NOSUPP 17 KDC has no support for transited -type - KDC_ERR_CLIENT_REVOKED 18 Clients credentials have been -revoked - KDC_ERR_SERVICE_REVOKED 19 Credentials for server have been -revoked - KDC_ERR_TGT_REVOKED 20 TGT has been revoked - KDC_ERR_CLIENT_NOTYET 21 Client not yet valid - try again -later - KDC_ERR_SERVICE_NOTYET 22 Server not yet valid - try again -later - KDC_ERR_KEY_EXPIRED 23 Password has expired - change -password to reset - KDC_ERR_PREAUTH_FAILED 24 Pre-authentication information was -invalid - KDC_ERR_PREAUTH_REQUIRED 25 Additional -pre-authenticationrequired [40] - KDC_ERR_SERVER_NOMATCH 26 Requested server and ticket don't -match - KDC_ERR_MUST_USE_USER2USER 27 Server principal valid for -user2user only - KDC_ERR_PATH_NOT_ACCPETED 28 KDC Policy rejects transited path - KDC_ERR_SVC_UNAVAILABLE 29 A service is not available - KRB_AP_ERR_BAD_INTEGRITY 31 Integrity check on decrypted field -failed - KRB_AP_ERR_TKT_EXPIRED 32 Ticket expired - KRB_AP_ERR_TKT_NYV 33 Ticket not yet valid - KRB_AP_ERR_REPEAT 34 Request is a replay - KRB_AP_ERR_NOT_US 35 The ticket isn't for us - KRB_AP_ERR_BADMATCH 36 Ticket and authenticator don't -match - KRB_AP_ERR_SKEW 37 Clock skew too great - KRB_AP_ERR_BADADDR 38 Incorrect net address - KRB_AP_ERR_BADVERSION 39 Protocol version mismatch - KRB_AP_ERR_MSG_TYPE 40 Invalid msg type - KRB_AP_ERR_MODIFIED 41 Message stream modified - KRB_AP_ERR_BADORDER 42 Message out of order - KRB_AP_ERR_BADKEYVER 44 Specified version of key is not -available - KRB_AP_ERR_NOKEY 45 Service key not available - KRB_AP_ERR_MUT_FAIL 46 Mutual authentication failed - KRB_AP_ERR_BADDIRECTION 47 Incorrect message direction - KRB_AP_ERR_METHOD 48 Alternative authentication method -required - KRB_AP_ERR_BADSEQ 49 Incorrect sequence number in -message - KRB_AP_ERR_INAPP_CKSUM 50 Inappropriate type of checksum in -message - KRB_AP_PATH_NOT_ACCEPTED 51 Policy rejects transited path - KRB_ERR_RESPONSE_TOO_BIG 52 Response too big for UDP, retry -with TCP - KRB_ERR_GENERIC 60 Generic error (description in -e-text) - KRB_ERR_FIELD_TOOLONG 61 Field is too long for this -implementation - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - KDC_ERROR_CLIENT_NOT_TRUSTED 62 (pkinit) - KDC_ERROR_KDC_NOT_TRUSTED 63 (pkinit) - KDC_ERROR_INVALID_SIG 64 (pkinit) - KDC_ERR_KEY_TOO_WEAK 65 (pkinit) - KDC_ERR_CERTIFICATE_MISMATCH 66 (pkinit) - KRB_AP_ERR_NO_TGT 67 (user-to-user) - KDC_ERR_WRONG_REALM 68 (user-to-user) - KRB_AP_ERR_USER_TO_USER_REQUIRED 69 (user-to-user) - KDC_ERR_CANT_VERIFY_CERTIFICATE 70 (pkinit) - KDC_ERR_INVALID_CERTIFICATE 71 (pkinit) - KDC_ERR_REVOKED_CERTIFICATE 72 (pkinit) - KDC_ERR_REVOCATION_STATUS_UNKNOWN 73 (pkinit) - KDC_ERR_REVOCATION_STATUS_UNAVAILABLE 74 (pkinit) - KDC_ERR_CLIENT_NAME_MISMATCH 75 (pkinit) - KDC_ERR_KDC_NAME_MISMATCH 76 (pkinit) - - 9. Interoperability requirements - - Version 5 of the Kerberos protocol supports a myriad of options. Among - these are multiple encryption and checksum types, alternative encoding - schemes for the transited field, optional mechanisms for - pre-authentication, the handling of tickets with no addresses, options - for mutual authentication, user to user authentication, support for - proxies, forwarding, postdating, and renewing tickets, the format of - realm names, and the handling of authorization data. - - In order to ensure the interoperability of realms, it is necessary to - define a minimal configuration which must be supported by all - implementations. This minimal configuration is subject to change as - technology does. For example, if at some later date it is discovered - that one of the required encryption or checksum algorithms is not - secure, it will be replaced. - - 9.1. Specification 2 - - This section defines the second specification of these options. - Implementations which are configured in this way can be said to - support Kerberos Version 5 Specification 2 (5.1). Specification 1 - (depricated) may be found in RFC1510. - - Transport - - TCP/IP and UDP/IP transport must be supported by KDCs claiming - conformance to specification 2. Kerberos clients claiming conformance - to specification 2 must support UDP/IP transport for messages with the - KDC and should support TCP/IP transport. - - Encryption and checksum methods - - The following encryption and checksum mechanisms must be supported. - Implementations may support other mechanisms as well, but the - additional mechanisms may only be used when communicating with - principals known to also support them: This list is to be determined. - - Encryption: DES-CBC-MD5, one triple des variant (tbd) - Checksums: CRC-32, DES-MAC, DES-MAC-K, and DES-MD5 (tbd) - - Realm Names - - All implementations must understand hierarchical realms in both the - Internet Domain and the X.500 style. When a ticket granting ticket for - an unknown realm is requested, the KDC must be able to determine the - names of the intermediate realms between the KDCs realm and the - requested realm. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - Transited field encoding - - DOMAIN-X500-COMPRESS (described in section 3.3.3.2) must be supported. - Alternative encodings may be supported, but they may be used only when - that encoding is supported by ALL intermediate realms. - - Pre-authentication methods - - The TGS-REQ method must be supported. The TGS-REQ method is not used - on the initial request. The PA-ENC-TIMESTAMP method must be supported - by clients but whether it is enabled by default may be determined on a - realm by realm basis. If not used in the initial request and the error - KDC_ERR_PREAUTH_REQUIRED is returned specifying PA-ENC-TIMESTAMP as an - acceptable method, the client should retry the initial request using - the PA-ENC-TIMESTAMP preauthentication method. Servers need not - support the PA-ENC-TIMESTAMP method, but if not supported the server - should ignore the presence of PA-ENC-TIMESTAMP pre-authentication in a - request. - - Mutual authentication - - Mutual authentication (via the KRB_AP_REP message) must be supported. - - Ticket addresses and flags - - All KDC's must pass on tickets that carry no addresses (i.e. if a TGT - contains no addresses, the KDC will return derivative tickets), but - each realm may set its own policy for issuing such tickets, and each - application server will set its own policy with respect to accepting - them. - - Proxies and forwarded tickets must be supported. Individual realms and - application servers can set their own policy on when such tickets will - be accepted. - - All implementations must recognize renewable and postdated tickets, - but need not actually implement them. If these options are not - supported, the starttime and endtime in the ticket shall specify a - ticket's entire useful life. When a postdated ticket is decoded by a - server, all implementations shall make the presence of the postdated - flag visible to the calling server. - - User-to-user authentication - - Support for user to user authentication (via the ENC-TKT-IN-SKEY KDC - option) must be provided by implementations, but individual realms may - decide as a matter of policy to reject such requests on a - per-principal or realm-wide basis. - - Authorization data - - Implementations must pass all authorization data subfields from - ticket-granting tickets to any derivative tickets unless directed to - suppress a subfield as part of the definition of that registered - subfield type (it is never incorrect to pass on a subfield, and no - registered subfield types presently specify suppression at the KDC). - - Implementations must make the contents of any authorization data - subfields available to the server when a ticket is used. - Implementations are not required to allow clients to specify the - contents of the authorization data fields. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - Constant ranges - - All protocol constants are constrained to 32 bit (signed) values - unless further constrained by the protocol definition. This limit is - provided to allow implementations to make assumptions about the - maximum values that will be received for these constants. - Implementation receiving values outside this range may reject the - request, but they must recover cleanly. - - 9.2. Recommended KDC values - - Following is a list of recommended values for a KDC implementation, - based on the list of suggested configuration constants (see section - 4.4). - - minimum lifetime 5 minutes - maximum renewable lifetime 1 week - maximum ticket lifetime 1 day - empty addresses only when suitable restrictions appear - in authorization data - proxiable, etc. Allowed. - - 10. REFERENCES - - [NT94] B. Clifford Neuman and Theodore Y. Ts'o, "An Authenti- - cation Service for Computer Networks," IEEE Communica- - tions Magazine, Vol. 32(9), pp. 33-38 (September 1994). - - [MNSS87] S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H. - Saltzer, Section E.2.1: Kerberos Authentication and - Authorization System, M.I.T. Project Athena, Cambridge, - Massachusetts (December 21, 1987). - - [SNS88] J. G. Steiner, B. C. Neuman, and J. I. Schiller, "Ker- - beros: An Authentication Service for Open Network Sys- - tems," pp. 191-202 in Usenix Conference Proceedings, - Dallas, Texas (February, 1988). - - [NS78] Roger M. Needham and Michael D. Schroeder, "Using - Encryption for Authentication in Large Networks of Com- - puters," Communications of the ACM, Vol. 21(12), - pp. 993-999 (December, 1978). - - [DS81] Dorothy E. Denning and Giovanni Maria Sacco, "Time- - stamps in Key Distribution Protocols," Communications - of the ACM, Vol. 24(8), pp. 533-536 (August 1981). - - [KNT92] John T. Kohl, B. Clifford Neuman, and Theodore Y. Ts'o, - "The Evolution of the Kerberos Authentication Service," - in an IEEE Computer Society Text soon to be published - (June 1992). - - [Neu93] B. Clifford Neuman, "Proxy-Based Authorization and - Accounting for Distributed Systems," in Proceedings of - the 13th International Conference on Distributed Com- - puting Systems, Pittsburgh, PA (May, 1993). - - [DS90] Don Davis and Ralph Swick, "Workstation Services and - Kerberos Authentication at Project Athena," Technical - Memorandum TM-424, MIT Laboratory for Computer Science - (February 1990). - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - [LGDSR87] P. J. Levine, M. R. Gretzinger, J. M. Diaz, W. E. Som- - merfeld, and K. Raeburn, Section E.1: Service Manage- - ment System, M.I.T. Project Athena, Cambridge, Mas- - sachusetts (1987). - - [X509-88] CCITT, Recommendation X.509: The Directory Authentica- - tion Framework, December 1988. - - [Pat92]. J. Pato, Using Pre-Authentication to Avoid Password - Guessing Attacks, Open Software Foundation DCE Request - for Comments 26 (December 1992). - - [DES77] National Bureau of Standards, U.S. Department of Com- - merce, "Data Encryption Standard," Federal Information - Processing Standards Publication 46, Washington, DC - (1977). - - [DESM80] National Bureau of Standards, U.S. Department of Com- - merce, "DES Modes of Operation," Federal Information - Processing Standards Publication 81, Springfield, VA - (December 1980). - - [SG92] Stuart G. Stubblebine and Virgil D. Gligor, "On Message - Integrity in Cryptographic Protocols," in Proceedings - of the IEEE Symposium on Research in Security and - Privacy, Oakland, California (May 1992). - - [IS3309] International Organization for Standardization, "ISO - Information Processing Systems - Data Communication - - High-Level Data Link Control Procedure - Frame Struc- - ture," IS 3309 (October 1984). 3rd Edition. - - [MD4-92] R. Rivest, "The MD4 Message Digest Algorithm," RFC - 1320, MIT Laboratory for Computer Science (April - 1992). - - [MD5-92] R. Rivest, "The MD5 Message Digest Algorithm," RFC - 1321, MIT Laboratory for Computer Science (April - 1992). - - [KBC96] H. Krawczyk, M. Bellare, and R. Canetti, "HMAC: Keyed- - Hashing for Message Authentication," Working Draft - draft-ietf-ipsec-hmac-md5-01.txt, (August 1996). - - [Horowitz96] Horowitz, M., "Key Derivation for Authentication, - Integrity, and Privacy", -draft-horowitz-key-derivation-02.txt, - August 1998. - - [HorowitzB96] Horowitz, M., "Key Derivation for Kerberos V5", draft- - horowitz-kerb-key-derivation-01.txt, September 1998. - - [Krawczyk96] Krawczyk, H., Bellare, and M., Canetti, R., "HMAC: - Keyed-Hashing for Message Authentication", -draft-ietf-ipsec-hmac- - md5-01.txt, August, 1996. - - A. Pseudo-code for protocol processing - - This appendix provides pseudo-code describing how the messages are to - be constructed and interpreted by clients and servers. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - A.1. KRB_AS_REQ generation - - request.pvno := protocol version; /* pvno = 5 */ - request.msg-type := message type; /* type = KRB_AS_REQ */ - - if(pa_enc_timestamp_required) then - request.padata.padata-type = PA-ENC-TIMESTAMP; - get system_time; - padata-body.patimestamp,pausec = system_time; - encrypt padata-body into request.padata.padata-value - using client.key; /* derived from password */ - endif - - body.kdc-options := users's preferences; - body.cname := user's name; - body.realm := user's realm; - body.sname := service's name; /* usually "krbtgt", -"localrealm" */ - if (body.kdc-options.POSTDATED is set) then - body.from := requested starting time; - else - omit body.from; - endif - body.till := requested end time; - if (body.kdc-options.RENEWABLE is set) then - body.rtime := requested final renewal time; - endif - body.nonce := random_nonce(); - body.etype := requested etypes; - if (user supplied addresses) then - body.addresses := user's addresses; - else - omit body.addresses; - endif - omit body.enc-authorization-data; - request.req-body := body; - - kerberos := lookup(name of local kerberos server (or servers)); - send(packet,kerberos); - - wait(for response); - if (timed_out) then - retry or use alternate server; - endif - - A.2. KRB_AS_REQ verification and KRB_AS_REP generation - - decode message into req; - - client := lookup(req.cname,req.realm); - server := lookup(req.sname,req.realm); - - get system_time; - kdc_time := system_time.seconds; - - if (!client) then - /* no client in Database */ - error_out(KDC_ERR_C_PRINCIPAL_UNKNOWN); - endif - if (!server) then - /* no server in Database */ - error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN); - endif - - if(client.pa_enc_timestamp_required and - pa_enc_timestamp not present) then - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - error_out(KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP)); - endif - - if(pa_enc_timestamp present) then - decrypt req.padata-value into decrypted_enc_timestamp - using client.key; - using auth_hdr.authenticator.subkey; - if (decrypt_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - if(decrypted_enc_timestamp is not within allowable -skew) then - error_out(KDC_ERR_PREAUTH_FAILED); - endif - if(decrypted_enc_timestamp and usec is replay) - error_out(KDC_ERR_PREAUTH_FAILED); - endif - add decrypted_enc_timestamp and usec to replay cache; - endif - - use_etype := first supported etype in req.etypes; - - if (no support for req.etypes) then - error_out(KDC_ERR_ETYPE_NOSUPP); - endif - - new_tkt.vno := ticket version; /* = 5 */ - new_tkt.sname := req.sname; - new_tkt.srealm := req.srealm; - reset all flags in new_tkt.flags; - - /* It should be noted that local policy may affect the */ - /* processing of any of these flags. For example, some */ - /* realms may refuse to issue renewable tickets */ - - if (req.kdc-options.FORWARDABLE is set) then - set new_tkt.flags.FORWARDABLE; - endif - if (req.kdc-options.PROXIABLE is set) then - set new_tkt.flags.PROXIABLE; - endif - - if (req.kdc-options.ALLOW-POSTDATE is set) then - set new_tkt.flags.MAY-POSTDATE; - endif - if ((req.kdc-options.RENEW is set) or - (req.kdc-options.VALIDATE is set) or - (req.kdc-options.PROXY is set) or - (req.kdc-options.FORWARDED is set) or - (req.kdc-options.ENC-TKT-IN-SKEY is set)) then - error_out(KDC_ERR_BADOPTION); - endif - - new_tkt.session := random_session_key(); - new_tkt.cname := req.cname; - new_tkt.crealm := req.crealm; - new_tkt.transited := empty_transited_field(); - - new_tkt.authtime := kdc_time; - - if (req.kdc-options.POSTDATED is set) then - if (against_postdate_policy(req.from)) then - error_out(KDC_ERR_POLICY); - endif - set new_tkt.flags.POSTDATED; - set new_tkt.flags.INVALID; - new_tkt.starttime := req.from; - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - else - omit new_tkt.starttime; /* treated as authtime when omitted -*/ - endif - if (req.till = 0) then - till := infinity; - else - till := req.till; - endif - - new_tkt.endtime := min(till, - new_tkt.starttime+client.max_life, - new_tkt.starttime+server.max_life, - new_tkt.starttime+max_life_for_realm); - - if ((req.kdc-options.RENEWABLE-OK is set) and - (new_tkt.endtime < req.till)) then - /* we set the RENEWABLE option for later processing */ - set req.kdc-options.RENEWABLE; - req.rtime := req.till; - endif - - if (req.rtime = 0) then - rtime := infinity; - else - rtime := req.rtime; - endif - - if (req.kdc-options.RENEWABLE is set) then - set new_tkt.flags.RENEWABLE; - new_tkt.renew-till := min(rtime, - -new_tkt.starttime+client.max_rlife, - -new_tkt.starttime+server.max_rlife, - -new_tkt.starttime+max_rlife_for_realm); - else - omit new_tkt.renew-till; /* only present if RENEWABLE -*/ - endif - - if (req.addresses) then - new_tkt.caddr := req.addresses; - else - omit new_tkt.caddr; - endif - - new_tkt.authorization_data := empty_authorization_data(); - - encode to-be-encrypted part of ticket into OCTET STRING; - new_tkt.enc-part := encrypt OCTET STRING - using etype_for_key(server.key), server.key, -server.p_kvno; - - /* Start processing the response */ - - resp.pvno := 5; - resp.msg-type := KRB_AS_REP; - resp.cname := req.cname; - resp.crealm := req.realm; - resp.ticket := new_tkt; - - resp.key := new_tkt.session; - resp.last-req := fetch_last_request_info(client); - resp.nonce := req.nonce; - resp.key-expiration := client.expiration; - resp.flags := new_tkt.flags; - - resp.authtime := new_tkt.authtime; - resp.starttime := new_tkt.starttime; - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - resp.endtime := new_tkt.endtime; - - if (new_tkt.flags.RENEWABLE) then - resp.renew-till := new_tkt.renew-till; - endif - - resp.realm := new_tkt.realm; - resp.sname := new_tkt.sname; - - resp.caddr := new_tkt.caddr; - - encode body of reply into OCTET STRING; - - resp.enc-part := encrypt OCTET STRING - using use_etype, client.key, client.p_kvno; - send(resp); - - A.3. KRB_AS_REP verification - - decode response into resp; - - if (resp.msg-type = KRB_ERROR) then - if(error = KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP)) -then - set pa_enc_timestamp_required; - goto KRB_AS_REQ; - endif - process_error(resp); - return; - endif - - /* On error, discard the response, and zero the session key */ - /* from the response immediately */ - - key = get_decryption_key(resp.enc-part.kvno, -resp.enc-part.etype, - resp.padata); - unencrypted part of resp := decode of decrypt of resp.enc-part - using resp.enc-part.etype and key; - zero(key); - - if (common_as_rep_tgs_rep_checks fail) then - destroy resp.key; - return error; - endif - - if near(resp.princ_exp) then - print(warning message); - endif - save_for_later(ticket,session,client,server,times,flags); - - A.4. KRB_AS_REP and KRB_TGS_REP common checks - - if (decryption_error() or - (req.cname != resp.cname) or - (req.realm != resp.crealm) or - (req.sname != resp.sname) or - (req.realm != resp.realm) or - (req.nonce != resp.nonce) or - (req.addresses != resp.caddr)) then - destroy resp.key; - return KRB_AP_ERR_MODIFIED; - endif - - /* make sure no flags are set that shouldn't be, and that all -that */ - /* should be are set -*/ - if (!check_flags_for_compatability(req.kdc-options,resp.flags)) -then - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - destroy resp.key; - return KRB_AP_ERR_MODIFIED; - endif - - if ((req.from = 0) and - (resp.starttime is not within allowable skew)) then - destroy resp.key; - return KRB_AP_ERR_SKEW; - endif - if ((req.from != 0) and (req.from != resp.starttime)) then - destroy resp.key; - return KRB_AP_ERR_MODIFIED; - endif - if ((req.till != 0) and (resp.endtime > req.till)) then - destroy resp.key; - return KRB_AP_ERR_MODIFIED; - endif - - if ((req.kdc-options.RENEWABLE is set) and - (req.rtime != 0) and (resp.renew-till > req.rtime)) then - destroy resp.key; - return KRB_AP_ERR_MODIFIED; - endif - if ((req.kdc-options.RENEWABLE-OK is set) and - (resp.flags.RENEWABLE) and - (req.till != 0) and - (resp.renew-till > req.till)) then - destroy resp.key; - return KRB_AP_ERR_MODIFIED; - endif - - A.5. KRB_TGS_REQ generation - - /* Note that make_application_request might have to recursivly -*/ - /* call this routine to get the appropriate ticket-granting -ticket */ - - request.pvno := protocol version; /* pvno = 5 */ - request.msg-type := message type; /* type = KRB_TGS_REQ */ - - body.kdc-options := users's preferences; - /* If the TGT is not for the realm of the end-server */ - /* then the sname will be for a TGT for the end-realm */ - /* and the realm of the requested ticket (body.realm) */ - /* will be that of the TGS to which the TGT we are */ - /* sending applies */ - body.sname := service's name; - body.realm := service's realm; - - if (body.kdc-options.POSTDATED is set) then - body.from := requested starting time; - else - omit body.from; - endif - body.till := requested end time; - if (body.kdc-options.RENEWABLE is set) then - body.rtime := requested final renewal time; - endif - body.nonce := random_nonce(); - body.etype := requested etypes; - if (user supplied addresses) then - body.addresses := user's addresses; - else - omit body.addresses; - endif - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - body.enc-authorization-data := user-supplied data; - if (body.kdc-options.ENC-TKT-IN-SKEY) then - body.additional-tickets_ticket := second TGT; - endif - - request.req-body := body; - check := generate_checksum (req.body,checksumtype); - - request.padata[0].padata-type := PA-TGS-REQ; - request.padata[0].padata-value := create a KRB_AP_REQ using - the TGT and checksum - - /* add in any other padata as required/supplied */ - - kerberos := lookup(name of local kerberose server (or -servers)); - send(packet,kerberos); - - wait(for response); - if (timed_out) then - retry or use alternate server; - endif - - A.6. KRB_TGS_REQ verification and KRB_TGS_REP generation - - /* note that reading the application request requires first - determining the server for which a ticket was issued, and -choosing the - correct key for decryption. The name of the server appears in -the - plaintext part of the ticket. */ - - if (no KRB_AP_REQ in req.padata) then - error_out(KDC_ERR_PADATA_TYPE_NOSUPP); - endif - verify KRB_AP_REQ in req.padata; - - /* Note that the realm in which the Kerberos server is -operating is - determined by the instance from the ticket-granting ticket. -The realm - in the ticket-granting ticket is the realm under which the -ticket - granting ticket was issued. It is possible for a single -Kerberos - server to support more than one realm. */ - - auth_hdr := KRB_AP_REQ; - tgt := auth_hdr.ticket; - - if (tgt.sname is not a TGT for local realm and is not -req.sname) then - error_out(KRB_AP_ERR_NOT_US); - - realm := realm_tgt_is_for(tgt); - - decode remainder of request; - - if (auth_hdr.authenticator.cksum is missing) then - error_out(KRB_AP_ERR_INAPP_CKSUM); - endif - - if (auth_hdr.authenticator.cksum type is not supported) then - error_out(KDC_ERR_SUMTYPE_NOSUPP); - endif - if (auth_hdr.authenticator.cksum is not both collision-proof -and keyed) then - error_out(KRB_AP_ERR_INAPP_CKSUM); - endif - - set computed_checksum := checksum(req); - if (computed_checksum != auth_hdr.authenticatory.cksum) then - error_out(KRB_AP_ERR_MODIFIED); - endif - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - - server := lookup(req.sname,realm); - - if (!server) then - if (is_foreign_tgt_name(req.sname)) then - server := best_intermediate_tgs(req.sname); - else - /* no server in Database */ - error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN); - endif - endif - - session := generate_random_session_key(); - - use_etype := first supported etype in req.etypes; - - if (no support for req.etypes) then - error_out(KDC_ERR_ETYPE_NOSUPP); - endif - - new_tkt.vno := ticket version; /* = 5 */ - new_tkt.sname := req.sname; - new_tkt.srealm := realm; - reset all flags in new_tkt.flags; - - /* It should be noted that local policy may affect the */ - /* processing of any of these flags. For example, some */ - /* realms may refuse to issue renewable tickets */ - - new_tkt.caddr := tgt.caddr; - resp.caddr := NULL; /* We only include this if they change */ - if (req.kdc-options.FORWARDABLE is set) then - if (tgt.flags.FORWARDABLE is reset) then - error_out(KDC_ERR_BADOPTION); - endif - set new_tkt.flags.FORWARDABLE; - endif - if (req.kdc-options.FORWARDED is set) then - if (tgt.flags.FORWARDABLE is reset) then - error_out(KDC_ERR_BADOPTION); - endif - set new_tkt.flags.FORWARDED; - new_tkt.caddr := req.addresses; - resp.caddr := req.addresses; - endif - if (tgt.flags.FORWARDED is set) then - set new_tkt.flags.FORWARDED; - endif - - if (req.kdc-options.PROXIABLE is set) then - if (tgt.flags.PROXIABLE is reset) - error_out(KDC_ERR_BADOPTION); - endif - set new_tkt.flags.PROXIABLE; - endif - if (req.kdc-options.PROXY is set) then - if (tgt.flags.PROXIABLE is reset) then - error_out(KDC_ERR_BADOPTION); - endif - set new_tkt.flags.PROXY; - new_tkt.caddr := req.addresses; - resp.caddr := req.addresses; - endif - - if (req.kdc-options.ALLOW-POSTDATE is set) then - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - if (tgt.flags.MAY-POSTDATE is reset) - error_out(KDC_ERR_BADOPTION); - endif - set new_tkt.flags.MAY-POSTDATE; - endif - if (req.kdc-options.POSTDATED is set) then - if (tgt.flags.MAY-POSTDATE is reset) then - error_out(KDC_ERR_BADOPTION); - endif - set new_tkt.flags.POSTDATED; - set new_tkt.flags.INVALID; - if (against_postdate_policy(req.from)) then - error_out(KDC_ERR_POLICY); - endif - new_tkt.starttime := req.from; - endif - - if (req.kdc-options.VALIDATE is set) then - if (tgt.flags.INVALID is reset) then - error_out(KDC_ERR_POLICY); - endif - if (tgt.starttime > kdc_time) then - error_out(KRB_AP_ERR_NYV); - endif - if (check_hot_list(tgt)) then - error_out(KRB_AP_ERR_REPEAT); - endif - tkt := tgt; - reset new_tkt.flags.INVALID; - endif - - if (req.kdc-options.(any flag except ENC-TKT-IN-SKEY, RENEW, - and those already processed) is set) then - error_out(KDC_ERR_BADOPTION); - endif - - new_tkt.authtime := tgt.authtime; - - if (req.kdc-options.RENEW is set) then - /* Note that if the endtime has already passed, the ticket -would */ - /* have been rejected in the initial authentication stage, so -*/ - /* there is no need to check again here -*/ - if (tgt.flags.RENEWABLE is reset) then - error_out(KDC_ERR_BADOPTION); - endif - if (tgt.renew-till < kdc_time) then - error_out(KRB_AP_ERR_TKT_EXPIRED); - endif - tkt := tgt; - new_tkt.starttime := kdc_time; - old_life := tgt.endttime - tgt.starttime; - new_tkt.endtime := min(tgt.renew-till, - new_tkt.starttime + old_life); - else - new_tkt.starttime := kdc_time; - if (req.till = 0) then - till := infinity; - else - till := req.till; - endif - new_tkt.endtime := min(till, - -new_tkt.starttime+client.max_life, - -new_tkt.starttime+server.max_life, - -new_tkt.starttime+max_life_for_realm, - tgt.endtime); - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - - if ((req.kdc-options.RENEWABLE-OK is set) and - (new_tkt.endtime < req.till) and - (tgt.flags.RENEWABLE is set) then - /* we set the RENEWABLE option for later -processing */ - set req.kdc-options.RENEWABLE; - req.rtime := min(req.till, tgt.renew-till); - endif - endif - - if (req.rtime = 0) then - rtime := infinity; - else - rtime := req.rtime; - endif - - if ((req.kdc-options.RENEWABLE is set) and - (tgt.flags.RENEWABLE is set)) then - set new_tkt.flags.RENEWABLE; - new_tkt.renew-till := min(rtime, - -new_tkt.starttime+client.max_rlife, - -new_tkt.starttime+server.max_rlife, - -new_tkt.starttime+max_rlife_for_realm, - tgt.renew-till); - else - new_tkt.renew-till := OMIT; /* leave the renew-till -field out */ - endif - if (req.enc-authorization-data is present) then - decrypt req.enc-authorization-data into -decrypted_authorization_data - using auth_hdr.authenticator.subkey; - if (decrypt_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - endif - endif - new_tkt.authorization_data := -req.auth_hdr.ticket.authorization_data + - decrypted_authorization_data; - - new_tkt.key := session; - new_tkt.crealm := tgt.crealm; - new_tkt.cname := req.auth_hdr.ticket.cname; - - if (realm_tgt_is_for(tgt) := tgt.realm) then - /* tgt issued by local realm */ - new_tkt.transited := tgt.transited; - else - /* was issued for this realm by some other realm */ - if (tgt.transited.tr-type not supported) then - error_out(KDC_ERR_TRTYPE_NOSUPP); - endif - new_tkt.transited := compress_transited(tgt.transited + -tgt.realm) - /* Don't check tranited field if TGT for foreign realm, - * or requested not to check */ - if (is_not_foreign_tgt_name(new_tkt.server) - && req.kdc-options.DISABLE-TRANSITED-CHECK not set) -then - /* Check it, so end-server does not have to - * but don't fail, end-server may still accept -it */ - if (check_transited_field(new_tkt.transited) == -OK) - set -new_tkt.flags.TRANSITED-POLICY-CHECKED; - endif - endif - endif - - encode encrypted part of new_tkt into OCTET STRING; - if (req.kdc-options.ENC-TKT-IN-SKEY is set) then - if (server not specified) then - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - server = req.second_ticket.client; - endif - if ((req.second_ticket is not a TGT) or - (req.second_ticket.client != server)) then - error_out(KDC_ERR_POLICY); - endif - - new_tkt.enc-part := encrypt OCTET STRING using - using etype_for_key(second-ticket.key), -second-ticket.key; - else - new_tkt.enc-part := encrypt OCTET STRING - using etype_for_key(server.key), server.key, -server.p_kvno; - endif - - resp.pvno := 5; - resp.msg-type := KRB_TGS_REP; - resp.crealm := tgt.crealm; - resp.cname := tgt.cname; - resp.ticket := new_tkt; - - resp.key := session; - resp.nonce := req.nonce; - resp.last-req := fetch_last_request_info(client); - resp.flags := new_tkt.flags; - - resp.authtime := new_tkt.authtime; - resp.starttime := new_tkt.starttime; - resp.endtime := new_tkt.endtime; - - omit resp.key-expiration; - - resp.sname := new_tkt.sname; - resp.realm := new_tkt.realm; - - if (new_tkt.flags.RENEWABLE) then - resp.renew-till := new_tkt.renew-till; - endif - - encode body of reply into OCTET STRING; - - if (req.padata.authenticator.subkey) - resp.enc-part := encrypt OCTET STRING using use_etype, - req.padata.authenticator.subkey; - else resp.enc-part := encrypt OCTET STRING using use_etype, -tgt.key; - - send(resp); - - A.7. KRB_TGS_REP verification - - decode response into resp; - - if (resp.msg-type = KRB_ERROR) then - process_error(resp); - return; - endif - - /* On error, discard the response, and zero the session key -from - the response immediately */ - - if (req.padata.authenticator.subkey) - unencrypted part of resp := decode of decrypt of -resp.enc-part - using resp.enc-part.etype and subkey; - else unencrypted part of resp := decode of decrypt of -resp.enc-part - using resp.enc-part.etype and tgt's -session key; - if (common_as_rep_tgs_rep_checks fail) then - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - destroy resp.key; - return error; - endif - - check authorization_data as necessary; - save_for_later(ticket,session,client,server,times,flags); - - A.8. Authenticator generation - - body.authenticator-vno := authenticator vno; /* = 5 */ - body.cname, body.crealm := client name; - if (supplying checksum) then - body.cksum := checksum; - endif - get system_time; - body.ctime, body.cusec := system_time; - if (selecting sub-session key) then - select sub-session key; - body.subkey := sub-session key; - endif - if (using sequence numbers) then - select initial sequence number; - body.seq-number := initial sequence; - endif - - A.9. KRB_AP_REQ generation - - obtain ticket and session_key from cache; - - packet.pvno := protocol version; /* 5 */ - packet.msg-type := message type; /* KRB_AP_REQ */ - - if (desired(MUTUAL_AUTHENTICATION)) then - set packet.ap-options.MUTUAL-REQUIRED; - else - reset packet.ap-options.MUTUAL-REQUIRED; - endif - if (using session key for ticket) then - set packet.ap-options.USE-SESSION-KEY; - else - reset packet.ap-options.USE-SESSION-KEY; - endif - packet.ticket := ticket; /* ticket */ - generate authenticator; - encode authenticator into OCTET STRING; - encrypt OCTET STRING into packet.authenticator using -session_key; - - A.10. KRB_AP_REQ verification - - receive packet; - if (packet.pvno != 5) then - either process using other protocol spec - or error_out(KRB_AP_ERR_BADVERSION); - endif - if (packet.msg-type != KRB_AP_REQ) then - error_out(KRB_AP_ERR_MSG_TYPE); - endif - if (packet.ticket.tkt_vno != 5) then - either process using other protocol spec - or error_out(KRB_AP_ERR_BADVERSION); - endif - if (packet.ap_options.USE-SESSION-KEY is set) then - retrieve session key from ticket-granting ticket for - packet.ticket.{sname,srealm,enc-part.etype}; - else - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - retrieve service key for - -packet.ticket.{sname,srealm,enc-part.etype,enc-part.skvno}; - endif - if (no_key_available) then - if (cannot_find_specified_skvno) then - error_out(KRB_AP_ERR_BADKEYVER); - else - error_out(KRB_AP_ERR_NOKEY); - endif - endif - decrypt packet.ticket.enc-part into decr_ticket using retrieved -key; - if (decryption_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - endif - decrypt packet.authenticator into decr_authenticator - using decr_ticket.key; - if (decryption_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - endif - if (decr_authenticator.{cname,crealm} != - decr_ticket.{cname,crealm}) then - error_out(KRB_AP_ERR_BADMATCH); - endif - if (decr_ticket.caddr is present) then - if (sender_address(packet) is not in decr_ticket.caddr) -then - error_out(KRB_AP_ERR_BADADDR); - endif - elseif (application requires addresses) then - error_out(KRB_AP_ERR_BADADDR); - endif - if (not in_clock_skew(decr_authenticator.ctime, - decr_authenticator.cusec)) then - error_out(KRB_AP_ERR_SKEW); - endif - if (repeated(decr_authenticator.{ctime,cusec,cname,crealm})) -then - error_out(KRB_AP_ERR_REPEAT); - endif - save_identifier(decr_authenticator.{ctime,cusec,cname,crealm}); - get system_time; - if ((decr_ticket.starttime-system_time > CLOCK_SKEW) or - (decr_ticket.flags.INVALID is set)) then - /* it hasn't yet become valid */ - error_out(KRB_AP_ERR_TKT_NYV); - endif - if (system_time-decr_ticket.endtime > CLOCK_SKEW) then - error_out(KRB_AP_ERR_TKT_EXPIRED); - endif - if (decr_ticket.transited) then - /* caller may ignore the TRANSITED-POLICY-CHECKED and do - * check anyway */ - if (decr_ticket.flags.TRANSITED-POLICY-CHECKED not set) -then - if (check_transited_field(decr_ticket.transited) then - error_out(KDC_AP_PATH_NOT_ACCPETED); - endif - endif - endif - /* caller must check decr_ticket.flags for any pertinent -details */ - return(OK, decr_ticket, packet.ap_options.MUTUAL-REQUIRED); - - A.11. KRB_AP_REP generation - - packet.pvno := protocol version; /* 5 */ - packet.msg-type := message type; /* KRB_AP_REP */ - - body.ctime := packet.ctime; - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - body.cusec := packet.cusec; - if (selecting sub-session key) then - select sub-session key; - body.subkey := sub-session key; - endif - if (using sequence numbers) then - select initial sequence number; - body.seq-number := initial sequence; - endif - - encode body into OCTET STRING; - - select encryption type; - encrypt OCTET STRING into packet.enc-part; - - A.12. KRB_AP_REP verification - - receive packet; - if (packet.pvno != 5) then - either process using other protocol spec - or error_out(KRB_AP_ERR_BADVERSION); - endif - if (packet.msg-type != KRB_AP_REP) then - error_out(KRB_AP_ERR_MSG_TYPE); - endif - cleartext := decrypt(packet.enc-part) using ticket's session -key; - if (decryption_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - endif - if (cleartext.ctime != authenticator.ctime) then - error_out(KRB_AP_ERR_MUT_FAIL); - endif - if (cleartext.cusec != authenticator.cusec) then - error_out(KRB_AP_ERR_MUT_FAIL); - endif - if (cleartext.subkey is present) then - save cleartext.subkey for future use; - endif - if (cleartext.seq-number is present) then - save cleartext.seq-number for future verifications; - endif - return(AUTHENTICATION_SUCCEEDED); - - A.13. KRB_SAFE generation - - collect user data in buffer; - - /* assemble packet: */ - packet.pvno := protocol version; /* 5 */ - packet.msg-type := message type; /* KRB_SAFE */ - - body.user-data := buffer; /* DATA */ - if (using timestamp) then - get system_time; - body.timestamp, body.usec := system_time; - endif - if (using sequence numbers) then - body.seq-number := sequence number; - endif - body.s-address := sender host addresses; - if (only one recipient) then - body.r-address := recipient host address; - endif - checksum.cksumtype := checksum type; - compute checksum over body; - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - checksum.checksum := checksum value; /* checksum.checksum */ - packet.cksum := checksum; - packet.safe-body := body; - - A.14. KRB_SAFE verification - - receive packet; - if (packet.pvno != 5) then - either process using other protocol spec - or error_out(KRB_AP_ERR_BADVERSION); - endif - if (packet.msg-type != KRB_SAFE) then - error_out(KRB_AP_ERR_MSG_TYPE); - endif - if (packet.checksum.cksumtype is not both collision-proof and -keyed) then - error_out(KRB_AP_ERR_INAPP_CKSUM); - endif - if (safe_priv_common_checks_ok(packet)) then - set computed_checksum := checksum(packet.body); - if (computed_checksum != packet.checksum) then - error_out(KRB_AP_ERR_MODIFIED); - endif - return (packet, PACKET_IS_GENUINE); - else - return common_checks_error; - endif - - A.15. KRB_SAFE and KRB_PRIV common checks - - if (packet.s-address != O/S_sender(packet)) then - /* O/S report of sender not who claims to have sent it -*/ - error_out(KRB_AP_ERR_BADADDR); - endif - if ((packet.r-address is present) and - (packet.r-address != local_host_address)) then - /* was not sent to proper place */ - error_out(KRB_AP_ERR_BADADDR); - endif - if (((packet.timestamp is present) and - (not in_clock_skew(packet.timestamp,packet.usec))) or - (packet.timestamp is not present and timestamp expected)) -then - error_out(KRB_AP_ERR_SKEW); - endif - if (repeated(packet.timestamp,packet.usec,packet.s-address)) -then - error_out(KRB_AP_ERR_REPEAT); - endif - - if (((packet.seq-number is present) and - ((not in_sequence(packet.seq-number)))) or - (packet.seq-number is not present and sequence expected)) -then - error_out(KRB_AP_ERR_BADORDER); - endif - if (packet.timestamp not present and packet.seq-number not -present) then - error_out(KRB_AP_ERR_MODIFIED); - endif - - save_identifier(packet.{timestamp,usec,s-address}, - sender_principal(packet)); - - return PACKET_IS_OK; - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - A.16. KRB_PRIV generation - - collect user data in buffer; - - /* assemble packet: */ - packet.pvno := protocol version; /* 5 */ - packet.msg-type := message type; /* KRB_PRIV */ - - packet.enc-part.etype := encryption type; - - body.user-data := buffer; - if (using timestamp) then - get system_time; - body.timestamp, body.usec := system_time; - endif - if (using sequence numbers) then - body.seq-number := sequence number; - endif - body.s-address := sender host addresses; - if (only one recipient) then - body.r-address := recipient host address; - endif - - encode body into OCTET STRING; - - select encryption type; - encrypt OCTET STRING into packet.enc-part.cipher; - - A.17. KRB_PRIV verification - - receive packet; - if (packet.pvno != 5) then - either process using other protocol spec - or error_out(KRB_AP_ERR_BADVERSION); - endif - if (packet.msg-type != KRB_PRIV) then - error_out(KRB_AP_ERR_MSG_TYPE); - endif - - cleartext := decrypt(packet.enc-part) using negotiated key; - if (decryption_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - endif - - if (safe_priv_common_checks_ok(cleartext)) then - return(cleartext.DATA, -PACKET_IS_GENUINE_AND_UNMODIFIED); - else - return common_checks_error; - endif - - A.18. KRB_CRED generation - - invoke KRB_TGS; /* obtain tickets to be provided to peer */ - - /* assemble packet: */ - packet.pvno := protocol version; /* 5 */ - packet.msg-type := message type; /* KRB_CRED */ - - for (tickets[n] in tickets to be forwarded) do - packet.tickets[n] = tickets[n].ticket; - done - - packet.enc-part.etype := encryption type; - - for (ticket[n] in tickets to be forwarded) do - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - body.ticket-info[n].key = tickets[n].session; - body.ticket-info[n].prealm = tickets[n].crealm; - body.ticket-info[n].pname = tickets[n].cname; - body.ticket-info[n].flags = tickets[n].flags; - body.ticket-info[n].authtime = tickets[n].authtime; - body.ticket-info[n].starttime = tickets[n].starttime; - body.ticket-info[n].endtime = tickets[n].endtime; - body.ticket-info[n].renew-till = tickets[n].renew-till; - body.ticket-info[n].srealm = tickets[n].srealm; - body.ticket-info[n].sname = tickets[n].sname; - body.ticket-info[n].caddr = tickets[n].caddr; - done - - get system_time; - body.timestamp, body.usec := system_time; - - if (using nonce) then - body.nonce := nonce; - endif - - if (using s-address) then - body.s-address := sender host addresses; - endif - if (limited recipients) then - body.r-address := recipient host address; - endif - - encode body into OCTET STRING; - - select encryption type; - encrypt OCTET STRING into packet.enc-part.cipher - using negotiated encryption key; - - A.19. KRB_CRED verification - - receive packet; - if (packet.pvno != 5) then - either process using other protocol spec - or error_out(KRB_AP_ERR_BADVERSION); - endif - if (packet.msg-type != KRB_CRED) then - error_out(KRB_AP_ERR_MSG_TYPE); - endif - - cleartext := decrypt(packet.enc-part) using negotiated key; - if (decryption_error()) then - error_out(KRB_AP_ERR_BAD_INTEGRITY); - endif - if ((packet.r-address is present or required) and - (packet.s-address != O/S_sender(packet)) then - /* O/S report of sender not who claims to have sent it -*/ - error_out(KRB_AP_ERR_BADADDR); - endif - if ((packet.r-address is present) and - (packet.r-address != local_host_address)) then - /* was not sent to proper place */ - error_out(KRB_AP_ERR_BADADDR); - endif - if (not in_clock_skew(packet.timestamp,packet.usec)) then - error_out(KRB_AP_ERR_SKEW); - endif - if (repeated(packet.timestamp,packet.usec,packet.s-address)) -then - error_out(KRB_AP_ERR_REPEAT); - endif - if (packet.nonce is required or present) and - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - (packet.nonce != expected-nonce) then - error_out(KRB_AP_ERR_MODIFIED); - endif - - for (ticket[n] in tickets that were forwarded) do - save_for_later(ticket[n],key[n],principal[n], - server[n],times[n],flags[n]); - return - - A.20. KRB_ERROR generation - - /* assemble packet: */ - packet.pvno := protocol version; /* 5 */ - packet.msg-type := message type; /* KRB_ERROR */ - - get system_time; - packet.stime, packet.susec := system_time; - packet.realm, packet.sname := server name; - - if (client time available) then - packet.ctime, packet.cusec := client_time; - endif - packet.error-code := error code; - if (client name available) then - packet.cname, packet.crealm := client name; - endif - if (error text available) then - packet.e-text := error text; - endif - if (error data available) then - packet.e-data := error data; - endif - - B. Definition of common authorization data elements - - This appendix contains the definitions of common authorization data - elements. These common authorization data elements are recursivly - defined, meaning the ad-data for these types will itself contain a - sequence of authorization data whose interpretation is affected by the - encapsulating element. Depending on the meaning of the encapsulating - element, the encapsulated elements may be ignored, might be - interpreted as issued directly by the KDC, or they might be stored in - a separate plaintext part of the ticket. The types of the - encapsulating elements are specified as part of the Kerberos - specification because the behavior based on these values should be - understood across implementations whereas other elements need only be - understood by the applications which they affect. - - In the definitions that follow, the value of the ad-type for the - element will be specified in the subsection number, and the value of - the ad-data will be as shown in the ASN.1 structure that follows the - subsection heading. - - B.1. If relevant - - AD-IF-RELEVANT AuthorizationData - - AD elements encapsulated within the if-relevant element are intended - for interpretation only by application servers that understand the - particular ad-type of the embedded element. Application servers that - do not understand the type of an element embedded within the - if-relevant element may ignore the uninterpretable element. This - element promotes interoperability across implementations which may - have local extensions for authorization. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - B.2. Intended for server - - AD-INTENDED-FOR-SERVER SEQUENCE { - intended-server[0] SEQUENCE OF PrincipalName - elements[1] AuthorizationData - } - - AD elements encapsulated within the intended-for-server element may be - ignored if the application server is not in the list of principal - names of intended servers. Further, a KDC issuing a ticket for an - application server can remove this element if the application server - is not in the list of intended servers. - - Application servers should check for their principal name in the - intended-server field of this element. If their principal name is not - found, this element should be ignored. If found, then the encapsulated - elements should be evaluated in the same manner as if they were - present in the top level authorization data field. Applications and - application servers that do not implement this element should reject - tickets that contain authorization data elements of this type. - - B.3. Intended for application class - - AD-INTENDED-FOR-APPLICATION-CLASS SEQUENCE { - intended-application-class[0] SEQUENCE OF GeneralString elements[1] - AuthorizationData } AD elements encapsulated within the - intended-for-application-class element may be ignored if the - application server is not in one of the named classes of application - servers. Examples of application server classes include "FILESYSTEM", - and other kinds of servers. - - This element and the elements it encapulates may be safely ignored by - applications, application servers, and KDCs that do not implement this - element. - - B.4. KDC Issued - - AD-KDCIssued SEQUENCE { - ad-checksum[0] Checksum, - i-realm[1] Realm OPTIONAL, - i-sname[2] PrincipalName OPTIONAL, - elements[3] AuthorizationData. - } - - ad-checksum - A checksum over the elements field using a cryptographic checksum - method that is identical to the checksum used to protect the - ticket itself (i.e. using the same hash function and the same - encryption algorithm used to encrypt the ticket) and using a key - derived from the same key used to protect the ticket. - i-realm, i-sname - The name of the issuing principal if different from the KDC - itself. This field would be used when the KDC can verify the - authenticity of elements signed by the issuing principal and it - allows this KDC to notify the application server of the validity - of those elements. - elements - A sequence of authorization data elements issued by the KDC. - The KDC-issued ad-data field is intended to provide a means for - Kerberos principal credentials to embed within themselves privilege - attributes and other mechanisms for positive authorization, amplifying - the priveleges of the principal beyond what can be done using a - credentials without such an a-data element. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - This can not be provided without this element because the definition - of the authorization-data field allows elements to be added at will by - the bearer of a TGT at the time that they request service tickets and - elements may also be added to a delegated ticket by inclusion in the - authenticator. - - For KDC-issued elements this is prevented because the elements are - signed by the KDC by including a checksum encrypted using the server's - key (the same key used to encrypt the ticket - or a key derived from - that key). Elements encapsulated with in the KDC-issued element will - be ignored by the application server if this "signature" is not - present. Further, elements encapsulated within this element from a - ticket granting ticket may be interpreted by the KDC, and used as a - basis according to policy for including new signed elements within - derivative tickets, but they will not be copied to a derivative ticket - directly. If they are copied directly to a derivative ticket by a KDC - that is not aware of this element, the signature will not be correct - for the application ticket elements, and the field will be ignored by - the application server. - - This element and the elements it encapulates may be safely ignored by - applications, application servers, and KDCs that do not implement this - element. - - B.5. And-Or - - AD-AND-OR SEQUENCE { - condition-count[0] INTEGER, - elements[1] AuthorizationData - } - - When restrictive AD elements encapsulated within the and-or element - are encountered, only the number specified in condition-count of the - encapsulated conditions must be met in order to satisfy this element. - This element may be used to implement an "or" operation by setting the - condition-count field to 1, and it may specify an "and" operation by - setting the condition count to the number of embedded elements. - Application servers that do not implement this element must reject - tickets that contain authorization data elements of this type. - - B.6. Mandatory ticket extensions - - AD-Mandatory-Ticket-Extensions SEQUENCE { - te-type[0] INTEGER, - te-checksum[0] Checksum - } - - An authorization data element of type mandatory-ticket-extensions - specifies the type and a collision-proof checksum using the same hash - algorithm used to protect the integrity of the ticket itself. This - checksum will be calculated over an individual extension field of the - type indicated. If there are more than one extension, multiple - Mandatory-Ticket-Extensions authorization data elements may be - present, each with a checksum for a different extension field. This - restriction indicates that the ticket should not be accepted if a - ticket extension is not present in the ticket for which the type and - checksum do not match that checksum specified in the authorization - data element. Note that although the type is redundant for the - purposes of the comparison, it makes the comparison easier when - multiple extensions are present. Application servers that do not - implement this element must reject tickets that contain authorization - data elements of this type. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - B.7. Authorization Data in ticket extensions - - AD-IN-Ticket-Extensions Checksum - - An authorization data element of type in-ticket-extensions specifies a - collision-proof checksum using the same hash algorithm used to protect - the integrity of the ticket itself. This checksum is calculated over a - separate external AuthorizationData field carried in the ticket - extensions. Application servers that do not implement this element - must reject tickets that contain authorization data elements of this - type. Application servers that do implement this element will search - the ticket extensions for authorization data fields, calculate the - specified checksum over each authorization data field and look for one - matching the checksum in this in-ticket-extensions element. If not - found, then the ticket must be rejected. If found, the corresponding - authorization data elements will be interpreted in the same manner as - if they were contained in the top level authorization data field. - - Note that if multiple external authorization data fields are present - in a ticket, each will have a corresponding element of type - in-ticket-extensions in the top level authorization data field, and - the external entries will be linked to the corresponding element by - their checksums. - - C. Definition of common ticket extensions - - This appendix contains the definitions of common ticket extensions. - Support for these extensions is optional. However, certain extensions - have associated authorization data elements that may require rejection - of a ticket containing an extension by application servers that do not - implement the particular extension. Other extensions have been defined - beyond those described in this specification. Such extensions are - described elswhere and for some of those extensions the reserved - number may be found in the list of constants. - - It is known that older versions of Kerberos did not support this - field, and that some clients will strip this field from a ticket when - they parse and then reassemble a ticket as it is passed to the - application servers. The presence of the extension will not break such - clients, but any functionaly dependent on the extensions will not work - when such tickets are handled by old clients. In such situations, some - implementation may use alternate methods to transmit the information - in the extensions field. - - C.1. Null ticket extension - - TE-NullExtension OctetString -- The empty Octet String - - The te-data field in the null ticket extension is an octet string of - lenght zero. This extension may be included in a ticket granting - ticket so that the KDC can determine on presentation of the ticket - granting ticket whether the client software will strip the extensions - field. - - C.2. External Authorization Data - - TE-ExternalAuthorizationData AuthorizationData - - The te-data field in the external authorization data ticket extension - is field of type AuthorizationData containing one or more - authorization data elements. If present, a corresponding authorization - data element will be present in the primary authorization data for the - ticket and that element will contain a checksum of the external - authorization data ticket extension. - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - ---------------------------------------------------------------------- - [TM] Project Athena, Athena, and Kerberos are trademarks of the - Massachusetts Institute of Technology (MIT). No commercial use of - these trademarks may be made without prior written permission of MIT. - - [1] Note, however, that many applications use Kerberos' functions only - upon the initiation of a stream-based network connection. Unless an - application subsequently provides integrity protection for the data - stream, the identity verification applies only to the initiation of - the connection, and does not guarantee that subsequent messages on the - connection originate from the same principal. - - [2] Secret and private are often used interchangeably in the - literature. In our usage, it takes two (or more) to share a secret, - thus a shared DES key is a secret key. Something is only private when - no one but its owner knows it. Thus, in public key cryptosystems, one - has a public and a private key. - - [3] Of course, with appropriate permission the client could arrange - registration of a separately-named prin- cipal in a remote realm, and - engage in normal exchanges with that realm's services. However, for - even small numbers of clients this becomes cumbersome, and more - automatic methods as described here are necessary. - - [4] Though it is permissible to request or issue tick- ets with no - network addresses specified. - - [5] The password-changing request must not be honored unless the - requester can provide the old password (the user's current secret - key). Otherwise, it would be possible for someone to walk up to an - unattended ses- sion and change another user's password. - - [6] To authenticate a user logging on to a local system, the - credentials obtained in the AS exchange may first be used in a TGS - exchange to obtain credentials for a local server. Those credentials - must then be verified by a local server through successful completion - of the Client/Server exchange. - - [7] "Random" means that, among other things, it should be impossible - to guess the next session key based on knowledge of past session keys. - This can only be achieved in a pseudo-random number generator if it is - based on cryptographic principles. It is more desirable to use a truly - random number generator, such as one based on measurements of random - physical phenomena. - - [8] Tickets contain both an encrypted and unencrypted portion, so - cleartext here refers to the entire unit, which can be copied from one - message and replayed in another without any cryptographic skill. - - [9] Note that this can make applications based on unreliable - transports difficult to code correctly. If the transport might deliver - duplicated messages, either a new authenticator must be generated for - each retry, or the application server must match requests and replies - and replay the first reply in response to a detected duplicate. - - [10] This is used for user-to-user authentication as described in [8]. - - [11] Note that the rejection here is restricted to authenticators from - the same principal to the same server. Other client principals - communicating with the same server principal should not be have their - authenticators rejected if the time and microsecond fields happen to - match some other client's authenticator. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - [12] In the Kerberos version 4 protocol, the timestamp in the reply - was the client's timestamp plus one. This is not necessary in version - 5 because version 5 messages are formatted in such a way that it is - not possible to create the reply by judicious message surgery (even in - encrypted form) without knowledge of the appropriate encryption keys. - - [13] Note that for encrypting the KRB_AP_REP message, the sub-session - key is not used, even if present in the Authenticator. - - [14] Implementations of the protocol may wish to provide routines to - choose subkeys based on session keys and random numbers and to - generate a negotiated key to be returned in the KRB_AP_REP message. - - [15]This can be accomplished in several ways. It might be known - beforehand (since the realm is part of the principal identifier), it - might be stored in a nameserver, or it might be obtained from a - configura- tion file. If the realm to be used is obtained from a - nameserver, there is a danger of being spoofed if the nameservice - providing the realm name is not authenti- cated. This might result in - the use of a realm which has been compromised, and would result in an - attacker's ability to compromise the authentication of the application - server to the client. - - [16] If the client selects a sub-session key, care must be taken to - ensure the randomness of the selected sub- session key. One approach - would be to generate a random number and XOR it with the session key - from the ticket-granting ticket. - - [17] This allows easy implementation of user-to-user authentication - [8], which uses ticket-granting ticket session keys in lieu of secret - server keys in situa- tions where such secret keys could be easily - comprom- ised. - - [18] For the purpose of appending, the realm preceding the first - listed realm is considered to be the null realm (""). - - [19] For the purpose of interpreting null subfields, the client's - realm is considered to precede those in the transited field, and the - server's realm is considered to follow them. - - [20] This means that a client and server running on the same host and - communicating with one another using the KRB_SAFE messages should not - share a common replay cache to detect KRB_SAFE replays. - - [21] The implementation of the Kerberos server need not combine the - database and the server on the same machine; it is feasible to store - the principal database in, say, a network name service, as long as the - entries stored therein are protected from disclosure to and - modification by unauthorized parties. However, we recommend against - such strategies, as they can make system management and threat - analysis quite complex. - - [22] See the discussion of the padata field in section 5.4.2 for - details on why this can be useful. - - [23] Warning for implementations that unpack and repack data - structures during the generation and verification of embedded - checksums: Because any checksums applied to data structures must be - checked against the original data the length of bit strings must be - preserved within a data structure between the time that a checksum is - generated through transmission to the time that the checksum is - verified. - - -Neuman, Ts'o, Kohl Expires: 14 January -2001 - -^L - -INTERNET-DRAFT draft-ietf-cat-kerberos-revisions-06 July 14, -2000 - - [24] It is NOT recommended that this time value be used to adjust the - workstation's clock since the workstation cannot reliably determine - that such a KRB_AS_REP actually came from the proper KDC in a timely - manner. - - [25] Note, however, that if the time is used as the nonce, one must - make sure that the workstation time is monotonically increasing. If - the time is ever reset backwards, there is a small, but finite, - probability that a nonce will be reused. - - [27] An application code in the encrypted part of a message provides - an additional check that the message was decrypted properly. - - [29] An application code in the encrypted part of a message provides - an additional check that the message was decrypted properly. - - [31] An application code in the encrypted part of a message provides - an additional check that the message was decrypted properly. - - [32] If supported by the encryption method in use, an initialization - vector may be passed to the encryption procedure, in order to achieve - proper cipher chaining. The initialization vector might come from the - last block of the ciphertext from the previous KRB_PRIV message, but - it is the application's choice whether or not to use such an - initialization vector. If left out, the default initialization vector - for the encryption algorithm will be used. - - [33] This prevents an attacker who generates an incorrect AS request - from obtaining verifiable plaintext for use in an off-line password - guessing attack. - - [35] In the above specification, UNTAGGED OCTET STRING(length) is the - notation for an octet string with its tag and length removed. It is - not a valid ASN.1 type. The tag bits and length must be removed from - the confounder since the purpose of the confounder is so that the - message starts with random data, but the tag and its length are fixed. - For other fields, the length and tag would be redundant if they were - included because they are specified by the encryption type. [36] The - ordering of the fields in the CipherText is important. Additionally, - messages encoded in this format must include a length as part of the - msg-seq field. This allows the recipient to verify that the message - has not been truncated. Without a length, an attacker could use a - chosen plaintext attack to generate a message which could be - truncated, while leaving the checksum intact. Note that if the msg-seq - is an encoding of an ASN.1 SEQUENCE or OCTET STRING, then the length - is part of that encoding. - - [37] In some cases, it may be necessary to use a different "mix-in" - string for compatibility reasons; see the discussion of padata in - section 5.4.2. - - [38] In some cases, it may be necessary to use a different "mix-in" - string for compatibility reasons; see the discussion of padata in - section 5.4.2. - - [39] A variant of the key is used to limit the use of a key to a - particular function, separating the functions of generating a checksum - from other encryption performed using the session key. The constant - F0F0F0F0F0F0F0F0 was chosen because it maintains key parity. The - properties of DES precluded the use of the complement. The same - constant is used for similar purpose in the Message Integrity Check in - the Privacy Enhanced Mail standard. - - [40] This error carries additional information in the e- data field. - The contents of the e-data field for this message is described in - section 5.9.1. - - |
