OSCL-LXR linux-6.0.1/​Documentation/​crypto/​userspace-if.rst	Source navigation Diff markup Identifier search General search
	Version: linux-6.0.1 spark-3.0.0 hadoop-3.2.0-src hadoop-3.3.0-src spark-2.4.7 linux-4.15.13 hadoop-2.7.4-src Revert   Change

 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] https://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 random number generator */
         .salg_name = "drbg_nopr_sha256" /* this is the RNG 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. The seed is also known as a *Personalization String* in NIST SP 800-90A
 standard.
 
 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.
 
 Following CAVP testing interfaces are enabled when kernel is built with
 CRYPTO_USER_API_RNG_CAVP option:
 
 -  the concatenation of *Entropy* and *Nonce* can be provided to the RNG via
    ALG_SET_DRBG_ENTROPY setsockopt interface. Setting the entropy requires
    CAP_SYS_ADMIN permission.
 
 -  *Additional Data* can be provided using the send()/sendmsg() system calls,
    but only after the entropy has been set.
 
 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).
 
 -  ALG_SET_DRBG_ENTROPY -- Setting the entropy of the random number generator.
    This option is applicable to RNG cipher type only.
 
 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] https://www.chronox.de/libkcapi.html

This page was automatically generated by the 2.1.0 LXR engine. The LXR team