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