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author | markm <markm@FreeBSD.org> | 2000-01-09 20:58:00 +0000 |
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committer | markm <markm@FreeBSD.org> | 2000-01-09 20:58:00 +0000 |
commit | 5f68254a360fdb4f3ebc30f6ad6507556425dd0a (patch) | |
tree | a96b9cd24173c10bba3fd7e2acf5d68e5bb0773e /crypto/heimdal/doc/whatis.texi | |
parent | d88c9767e1793993cc2242ccfb9a8ba002a061c6 (diff) | |
parent | 4ecbd6db44d79348bc815f31096e53104f50838b (diff) | |
download | FreeBSD-src-5f68254a360fdb4f3ebc30f6ad6507556425dd0a.zip FreeBSD-src-5f68254a360fdb4f3ebc30f6ad6507556425dd0a.tar.gz |
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diff --git a/crypto/heimdal/doc/whatis.texi b/crypto/heimdal/doc/whatis.texi new file mode 100644 index 0000000..97d4da2 --- /dev/null +++ b/crypto/heimdal/doc/whatis.texi @@ -0,0 +1,149 @@ +@node What is Kerberos?, Building and Installing, Introduction, Top +@chapter What is Kerberos? + +@quotation +@flushleft + Now this Cerberus had three heads of dogs, + the tail of a dragon, and on his back the + heads of all sorts of snakes. + --- Pseudo-Apollodorus Library 2.5.12 +@end flushleft +@end quotation + +Kerberos is a system for authenticating users and services on a network. +It is built upon the assumption that the network is ``unsafe''. For +example, data sent over the network can be eavesdropped and altered, and +addresses can also be faked. Therefore they cannot be used for +authentication purposes. +@cindex authentication + +Kerberos is a trusted third-party service. That means that there is a +third party (the kerberos server) that is trusted by all the entities on +the network (users and services, usually called @dfn{principals}). All +principals share a secret password (or key) with the kerberos server and +this enables principals to verify that the messages from the kerberos +server are authentic. Thus trusting the kerberos server, users and +services can authenticate each other. + +@section Basic mechanism + +@ifinfo +@macro sub{arg} +<\arg\> +@end macro +@end ifinfo + +@tex +@def@xsub#1{$_{#1}$} +@global@let@sub=@xsub +@end tex + +@ifhtml +@macro sub{arg} +<\arg\> +@end macro +@end ifhtml + +@quotation +@strong{Note:} This discussion is about Kerberos version 4, but version +5 works similarly. +@end quotation + +In Kerberos, principals use @dfn{tickets} to prove that they are who +they claim to be. In the following example, @var{A} is the initiator of +the authentication exchange, usually a user, and @var{B} is the service +that @var{A} wishes to use. + +To obtain a ticket for a specific service, @var{A} sends a ticket +request to the kerberos server. The request contains @var{A}'s and +@var{B}'s names (along with some other fields). The kerberos server +checks that both @var{A} and @var{B} are valid principals. + +Having verified the validity of the principals, it creates a packet +containing @var{A}'s and @var{B}'s names, @var{A}'s network address +(@var{A@sub{addr}}), the current time (@var{t@sub{issue}}), the lifetime +of the ticket (@var{life}), and a secret @dfn{session key} +@cindex session key +(@var{K@sub{AB}}). This packet is encrypted with @var{B}'s secret key +(@var{K@sub{B}}). The actual ticket (@var{T@sub{AB}}) looks like this: +(@{@var{A}, @var{B}, @var{A@sub{addr}}, @var{t@sub{issue}}, @var{life}, +@var{K@sub{AB}}@}@var{K@sub{B}}). + +The reply to @var{A} consists of the ticket (@var{T@sub{AB}}), @var{B}'s +name, the current time, the lifetime of the ticket, and the session key, all +encrypted in @var{A}'s secret key (@{@var{B}, @var{t@sub{issue}}, +@var{life}, @var{K@sub{AB}}, @var{T@sub{AB}}@}@var{K@sub{A}}). @var{A} +decrypts the reply and retains it for later use. + +@sp 1 + +Before sending a message to @var{B}, @var{A} creates an authenticator +consisting of @var{A}'s name, @var{A}'s address, the current time, and a +``checksum'' chosen by @var{A}, all encrypted with the secret session +key (@{@var{A}, @var{A@sub{addr}}, @var{t@sub{current}}, +@var{checksum}@}@var{K@sub{AB}}). This is sent together with the ticket +received from the kerberos server to @var{B}. Upon reception, @var{B} +decrypts the ticket using @var{B}'s secret key. Since the ticket +contains the session key that the authenticator was encrypted with, +@var{B} can now also decrypt the authenticator. To verify that @var{A} +really is @var{A}, @var{B} now has to compare the contents of the ticket +with that of the authenticator. If everything matches, @var{B} now +considers @var{A} as properly authenticated. + +@c (here we should have some more explanations) + +@section Different attacks + +@subheading Impersonating A + +An impostor, @var{C} could steal the authenticator and the ticket as it +is transmitted across the network, and use them to impersonate +@var{A}. The address in the ticket and the authenticator was added to +make it more difficult to perform this attack. To succeed @var{C} will +have to either use the same machine as @var{A} or fake the source +addresses of the packets. By including the time stamp in the +authenticator, @var{C} does not have much time in which to mount the +attack. + +@subheading Impersonating B + +@var{C} can hijack @var{B}'s network address, and when @var{A} sends +her credentials, @var{C} just pretend to verify them. @var{C} can't +be sure that she is talking to @var{A}. + +@section Defense strategies + +It would be possible to add a @dfn{replay cache} +@cindex replay cache +to the server side. The idea is to save the authenticators sent during +the last few minutes, so that @var{B} can detect when someone is trying +to retransmit an already used message. This is somewhat impractical +(mostly regarding efficiency), and is not part of Kerberos 4; MIT +Kerberos 5 contains it. + +To authenticate @var{B}, @var{A} might request that @var{B} sends +something back that proves that @var{B} has access to the session +key. An example of this is the checksum that @var{A} sent as part of the +authenticator. One typical procedure is to add one to the checksum, +encrypt it with the session key and send it back to @var{A}. This is +called @dfn{mutual authentication}. + +The session key can also be used to add cryptographic checksums to the +messages sent between @var{A} and @var{B} (known as @dfn{message +integrity}). Encryption can also be added (@dfn{message +confidentiality}). This is probably the best approach in all cases. +@cindex integrity +@cindex confidentiality + +@section Further reading + +The original paper on Kerberos from 1988 is @cite{Kerberos: An +Authentication Service for Open Network Systems}, by Jennifer Steiner, +Clifford Neuman and Jeffrey I. Schiller. + +A less technical description can be found in @cite{Designing an +Authentication System: a Dialogue in Four Scenes} by Bill Bryant, also +from 1988. + +These documents can be found on our web-page at +@url{http://www.pdc.kth.se/kth-krb/}. |