crypto: doc - AEAD / RNG AF_ALG interface

The patch moves the information provided in
Documentation/crypto/crypto-API-userspace.txt into a separate chapter in
the kernel crypto API DocBook. Some corrections are applied (such as
removing a reference to Netlink when the AF_ALG socket is referred to).

In addition, the AEAD and RNG interface description is now added.

Also, a brief description of the zero-copy interface with an example
code snippet is provided.
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/crypto/crypto-API-userspace.txt

-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>