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+User Space Interface
+====================
+
+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.
+
+[1] http://www.chronox.de/libkcapi.html
+
+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
+
+-  AEAD ciphers
+
+-  Random Number Generators
+
+The interface is provided via socket type 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 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 socket descriptor is already
+opened by the user space application and discusses only the kernel
+crypto API specific invocations.
+
+To initialize the socket 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.
+
+AEAD Cipher API
+---------------
+
+The operation is very similar to the symmetric cipher discussion. During
+initialization, the struct sockaddr data structure must be filled as
+follows:
+
+::
+
+    struct sockaddr_alg sa = {
+        .salg_family = AF_ALG,
+        .salg_type = "aead", /* this selects the symmetric cipher */
+        .salg_name = "gcm(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.
+
+In addition, before data can be sent to the kernel using the write/send
+system call family, the consumer must set the authentication tag size.
+To set the authentication tag size, the caller must use the setsockopt
+invocation described 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
+
+-  specification of the associated authentication data (AAD) with the
+   flag ALG_SET_AEAD_ASSOCLEN. The AAD is sent to the kernel together
+   with the plaintext / ciphertext. See below for the memory structure.
+
+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 defined with the memory structure below. If the
+output data size is smaller, the cipher operation is not performed.
+
+The authenticated decryption operation may indicate an integrity error.
+Such breach in integrity is marked with the -EBADMSG error code.
+
+AEAD Memory Structure
+~~~~~~~~~~~~~~~~~~~~~
+
+The AEAD cipher operates with the following information that is
+communicated between user and kernel space as one data stream:
+
+-  plaintext or ciphertext
+
+-  associated authentication data (AAD)
+
+-  authentication tag
+
+The sizes of the AAD and the authentication tag are provided with the
+sendmsg and setsockopt calls (see there). As the kernel knows the size
+of the entire data stream, the kernel is now able to calculate the right
+offsets of the data components in the data stream.
+
+The user space caller must arrange the aforementioned information in the
+following order:
+
+-  AEAD encryption input: AAD \|\| plaintext
+
+-  AEAD decryption input: AAD \|\| ciphertext \|\| authentication tag
+
+The output buffer the user space caller provides must be at least as
+large to hold the following data:
+
+-  AEAD encryption output: ciphertext \|\| authentication tag
+
+-  AEAD decryption output: plaintext
+
+Random Number Generator API
+---------------------------
+
+Again, the operation is very similar to the other APIs. During
+initialization, the struct sockaddr data structure must be filled as
+follows:
+
+::
+
+    struct sockaddr_alg sa = {
+        .salg_family = AF_ALG,
+        .salg_type = "rng", /* this selects the symmetric cipher */
+        .salg_name = "drbg_nopr_sha256" /* this is the cipher name */
+    };
+
+
+Depending on the RNG type, the RNG must be seeded. The seed is provided
+using the setsockopt interface to set the key. For example, the
+ansi_cprng requires a seed. The DRBGs do not require a seed, but may be
+seeded.
+
+Using the read()/recvmsg() system calls, random numbers can be obtained.
+The kernel generates at most 128 bytes in one call. If user space
+requires more data, multiple calls to read()/recvmsg() must be made.
+
+WARNING: The user space caller may invoke the initially mentioned accept
+system call multiple times. In this case, the returned file descriptors
+have the same state.
+
+Zero-Copy Interface
+-------------------
+
+In addition to the send/write/read/recv system call family, the AF_ALG
+interface can be accessed with the zero-copy interface of
+splice/vmsplice. As the name indicates, the kernel tries to avoid a copy
+operation into kernel space.
+
+The zero-copy operation requires data to be aligned at the page
+boundary. Non-aligned data can be used as well, but may require more
+operations of the kernel which would defeat the speed gains obtained
+from the zero-copy interface.
+
+The system-inherent limit for the size of one zero-copy operation is 16
+pages. If more data is to be sent to AF_ALG, user space must slice the
+input into segments with a maximum size of 16 pages.
+
+Zero-copy can be used with the following code example (a complete
+working example is provided with libkcapi):
+
+::
+
+    int pipes[2];
+
+    pipe(pipes);
+    /* input data in iov */
+    vmsplice(pipes[1], iov, iovlen, SPLICE_F_GIFT);
+    /* opfd is the file descriptor returned from accept() system call */
+    splice(pipes[0], NULL, opfd, NULL, ret, 0);
+    read(opfd, out, outlen);
+
+
+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)
+
+   -  the AEAD cipher type
+
+   -  the RNG cipher type to provide the seed
+
+-  ALG_SET_AEAD_AUTHSIZE -- Setting the authentication tag size for
+   AEAD ciphers. For a encryption operation, the authentication tag of
+   the given size will be generated. For a decryption operation, the
+   provided ciphertext is assumed to contain an authentication tag of
+   the given size (see section about AEAD memory layout below).
+
+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