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| -rw-r--r-- | Documentation/crypto/crypto-API-userspace.txt | 205 |
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diff --git a/Documentation/crypto/crypto-API-userspace.txt b/Documentation/crypto/crypto-API-userspace.txt deleted file mode 100644 index ac619cd..0000000 --- a/Documentation/crypto/crypto-API-userspace.txt +++ /dev/null @@ -1,205 +0,0 @@ -Introduction -============ - -The concepts of the kernel crypto API visible to kernel space is fully -applicable to the user space interface as well. Therefore, the kernel crypto API -high level discussion for the in-kernel use cases applies here as well. - -The major difference, however, is that user space can only act as a consumer -and never as a provider of a transformation or cipher algorithm. - -The following covers the user space interface exported by the kernel crypto -API. A working example of this description is libkcapi that can be obtained from -[1]. That library can be used by user space applications that require -cryptographic services from the kernel. - -Some details of the in-kernel kernel crypto API aspects do not -apply to user space, however. This includes the difference between synchronous -and asynchronous invocations. The user space API call is fully synchronous. -In addition, only a subset of all cipher types are available as documented -below. - - -User space API general remarks -============================== - -The kernel crypto API is accessible from user space. Currently, the following -ciphers are accessible: - - * Message digest including keyed message digest (HMAC, CMAC) - - * Symmetric ciphers - -Note, AEAD ciphers are currently not supported via the symmetric cipher -interface. - -The interface is provided via Netlink using the type AF_ALG. In addition, the -setsockopt option type is SOL_ALG. In case the user space header files do not -export these flags yet, use the following macros: - -#ifndef AF_ALG -#define AF_ALG 38 -#endif -#ifndef SOL_ALG -#define SOL_ALG 279 -#endif - -A cipher is accessed with the same name as done for the in-kernel API calls. -This includes the generic vs. unique naming schema for ciphers as well as the -enforcement of priorities for generic names. - -To interact with the kernel crypto API, a Netlink socket must be created by -the user space application. User space invokes the cipher operation with the -send/write system call family. The result of the cipher operation is obtained -with the read/recv system call family. - -The following API calls assume that the Netlink socket descriptor is already -opened by the user space application and discusses only the kernel crypto API -specific invocations. - -To initialize a Netlink interface, the following sequence has to be performed -by the consumer: - - 1. Create a socket of type AF_ALG with the struct sockaddr_alg parameter - specified below for the different cipher types. - - 2. Invoke bind with the socket descriptor - - 3. Invoke accept with the socket descriptor. The accept system call - returns a new file descriptor that is to be used to interact with - the particular cipher instance. When invoking send/write or recv/read - system calls to send data to the kernel or obtain data from the - kernel, the file descriptor returned by accept must be used. - -In-place cipher operation -========================= - -Just like the in-kernel operation of the kernel crypto API, the user space -interface allows the cipher operation in-place. That means that the input buffer -used for the send/write system call and the output buffer used by the read/recv -system call may be one and the same. This is of particular interest for -symmetric cipher operations where a copying of the output data to its final -destination can be avoided. - -If a consumer on the other hand wants to maintain the plaintext and the -ciphertext in different memory locations, all a consumer needs to do is to -provide different memory pointers for the encryption and decryption operation. - -Message digest API -================== - -The message digest type to be used for the cipher operation is selected when -invoking the bind syscall. bind requires the caller to provide a filled -struct sockaddr data structure. This data structure must be filled as follows: - -struct sockaddr_alg sa = { - .salg_family = AF_ALG, - .salg_type = "hash", /* this selects the hash logic in the kernel */ - .salg_name = "sha1" /* this is the cipher name */ -}; - -The salg_type value "hash" applies to message digests and keyed message digests. -Though, a keyed message digest is referenced by the appropriate salg_name. -Please see below for the setsockopt interface that explains how the key can be -set for a keyed message digest. - -Using the send() system call, the application provides the data that should be -processed with the message digest. The send system call allows the following -flags to be specified: - - * MSG_MORE: If this flag is set, the send system call acts like a - message digest update function where the final hash is not - yet calculated. If the flag is not set, the send system call - calculates the final message digest immediately. - -With the recv() system call, the application can read the message digest from -the kernel crypto API. If the buffer is too small for the message digest, the -flag MSG_TRUNC is set by the kernel. - -In order to set a message digest key, the calling application must use the -setsockopt() option of ALG_SET_KEY. If the key is not set the HMAC operation is -performed without the initial HMAC state change caused by the key. - - -Symmetric cipher API -==================== - -The operation is very similar to the message digest discussion. During -initialization, the struct sockaddr data structure must be filled as follows: - -struct sockaddr_alg sa = { - .salg_family = AF_ALG, - .salg_type = "skcipher", /* this selects the symmetric cipher */ - .salg_name = "cbc(aes)" /* this is the cipher name */ -}; - -Before data can be sent to the kernel using the write/send system call family, -the consumer must set the key. The key setting is described with the setsockopt -invocation below. - -Using the sendmsg() system call, the application provides the data that should -be processed for encryption or decryption. In addition, the IV is specified -with the data structure provided by the sendmsg() system call. - -The sendmsg system call parameter of struct msghdr is embedded into the -struct cmsghdr data structure. See recv(2) and cmsg(3) for more information -on how the cmsghdr data structure is used together with the send/recv system -call family. That cmsghdr data structure holds the following information -specified with a separate header instances: - - * specification of the cipher operation type with one of these flags: - ALG_OP_ENCRYPT - encryption of data - ALG_OP_DECRYPT - decryption of data - - * specification of the IV information marked with the flag ALG_SET_IV - -The send system call family allows the following flag to be specified: - - * MSG_MORE: If this flag is set, the send system call acts like a - cipher update function where more input data is expected - with a subsequent invocation of the send system call. - -Note: The kernel reports -EINVAL for any unexpected data. The caller must -make sure that all data matches the constraints given in /proc/crypto for the -selected cipher. - -With the recv() system call, the application can read the result of the -cipher operation from the kernel crypto API. The output buffer must be at least -as large as to hold all blocks of the encrypted or decrypted data. If the output -data size is smaller, only as many blocks are returned that fit into that -output buffer size. - -Setsockopt interface -==================== - -In addition to the read/recv and send/write system call handling to send and -retrieve data subject to the cipher operation, a consumer also needs to set -the additional information for the cipher operation. This additional information -is set using the setsockopt system call that must be invoked with the file -descriptor of the open cipher (i.e. the file descriptor returned by the -accept system call). - -Each setsockopt invocation must use the level SOL_ALG. - -The setsockopt interface allows setting the following data using the mentioned -optname: - - * ALG_SET_KEY -- Setting the key. Key setting is applicable to: - - - the skcipher cipher type (symmetric ciphers) - - - the hash cipher type (keyed message digests) - -User space API example -====================== - -Please see [1] for libkcapi which provides an easy-to-use wrapper around the -aforementioned Netlink kernel interface. [1] also contains a test application -that invokes all libkcapi API calls. - -[1] http://www.chronox.de/libkcapi.html - -Author -====== - -Stephan Mueller <smueller@chronox.de> |
