From dbe5fe7e1b3b3632bef2c09964a5f5505de4d744 Mon Sep 17 00:00:00 2001 From: Stephan Mueller Date: Fri, 6 Mar 2015 21:34:22 +0100 Subject: [PATCH] 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 Signed-off-by: Herbert Xu --- Documentation/DocBook/crypto-API.tmpl | 596 ++++++++++++++++++ Documentation/crypto/crypto-API-userspace.txt | 205 ------ 2 files changed, 596 insertions(+), 205 deletions(-) delete mode 100644 Documentation/crypto/crypto-API-userspace.txt diff --git a/Documentation/DocBook/crypto-API.tmpl b/Documentation/DocBook/crypto-API.tmpl index 33f63cfc00ca..efc8d90a9a3f 100644 --- a/Documentation/DocBook/crypto-API.tmpl +++ b/Documentation/DocBook/crypto-API.tmpl @@ -1072,6 +1072,602 @@ kernel crypto API | Caller + 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: + + + + + + Create a socket of type AF_ALG with the struct sockaddr_alg + parameter specified below for the different cipher types. + + + + + + Invoke bind with the socket descriptor + + + + + + 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 familty, 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-interent 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 + + + + + + Programming Interface Block Cipher Context Data Structures !Pinclude/linux/crypto.h Block Cipher Context Data Structures diff --git a/Documentation/crypto/crypto-API-userspace.txt b/Documentation/crypto/crypto-API-userspace.txt deleted file mode 100644 index ac619cd90300..000000000000 --- a/Documentation/crypto/crypto-API-userspace.txt +++ /dev/null @@ -1,205 +0,0 @@ -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 -- 2.30.2