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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 new file mode 100644 index 0000000..ac619cd --- /dev/null +++ b/Documentation/crypto/crypto-API-userspace.txt @@ -0,0 +1,205 @@ +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> |
