388 lines
14 KiB
ReStructuredText
388 lines
14 KiB
ReStructuredText
User Space Interface
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====================
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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
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crypto API high level discussion for the in-kernel use cases applies
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here as well.
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The major difference, however, is that user space can only act as a
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consumer and never as a provider of a transformation or cipher
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algorithm.
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The following covers the user space interface exported by the kernel
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crypto API. A working example of this description is libkcapi that can
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be obtained from [1]. That library can be used by user space
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applications that require cryptographic services from the kernel.
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Some details of the in-kernel kernel crypto API aspects do not apply to
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user space, however. This includes the difference between synchronous
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and asynchronous invocations. The user space API call is fully
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synchronous.
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[1] http://www.chronox.de/libkcapi.html
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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
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following 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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- AEAD ciphers
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- Random Number Generators
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The interface is provided via socket type using the type AF_ALG. In
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addition, the setsockopt option type is SOL_ALG. In case the user space
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header files do not export these flags yet, use the following macros:
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::
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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
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calls. This includes the generic vs. unique naming schema for ciphers as
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well as the enforcement of priorities for generic names.
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To interact with the kernel crypto API, a socket must be created by the
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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
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obtained with the read()/recv() system call family.
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The following API calls assume that the socket descriptor is already
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opened by the user space application and discusses only the kernel
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crypto API specific invocations.
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To initialize the socket interface, the following sequence has to be
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performed by the consumer:
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1. Create a socket of type AF_ALG with the struct sockaddr_alg
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parameter 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 the
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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
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space interface allows the cipher operation in-place. That means that
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the input buffer used for the send/write system call and the output
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buffer used by the read/recv system call may be one and the same. This
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is of particular interest for symmetric cipher operations where a
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copying of the output data to its final 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
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to provide different memory pointers for the encryption and decryption
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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
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when invoking the bind syscall. bind requires the caller to provide a
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filled struct sockaddr data structure. This data structure must be
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filled as follows:
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::
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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
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digests. Though, a keyed message digest is referenced by the appropriate
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salg_name. Please see below for the setsockopt interface that explains
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how the key can be set for a keyed message digest.
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Using the send() system call, the application provides the data that
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should be processed with the message digest. The send system call allows
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the following 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 yet
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calculated. If the flag is not set, the send system call calculates
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the final message digest immediately.
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With the recv() system call, the application can read the message digest
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from the kernel crypto API. If the buffer is too small for the message
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digest, the 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
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the setsockopt() option of ALG_SET_KEY. If the key is not set the HMAC
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operation is performed without the initial HMAC state change caused by
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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
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follows:
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::
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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
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family, the consumer must set the key. The key setting is described with
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the setsockopt invocation below.
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Using the sendmsg() system call, the application provides the data that
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should be processed for encryption or decryption. In addition, the IV is
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specified 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
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information on how the cmsghdr data structure is used together with the
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send/recv system call family. That cmsghdr data structure holds the
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following information 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 with a
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subsequent invocation of the send system call.
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Note: The kernel reports -EINVAL for any unexpected data. The caller
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must make sure that all data matches the constraints given in
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/proc/crypto for the 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
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at least as large as to hold all blocks of the encrypted or decrypted
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data. If the output data size is smaller, only as many blocks are
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returned that fit into that output buffer size.
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AEAD Cipher API
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---------------
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The operation is very similar to the symmetric cipher discussion. During
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initialization, the struct sockaddr data structure must be filled as
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follows:
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::
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struct sockaddr_alg sa = {
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.salg_family = AF_ALG,
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.salg_type = "aead", /* this selects the symmetric cipher */
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.salg_name = "gcm(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
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family, the consumer must set the key. The key setting is described with
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the setsockopt invocation below.
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In addition, before data can be sent to the kernel using the write/send
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system call family, the consumer must set the authentication tag size.
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To set the authentication tag size, the caller must use the setsockopt
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invocation described below.
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Using the sendmsg() system call, the application provides the data that
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should be processed for encryption or decryption. In addition, the IV is
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specified 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
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information on how the cmsghdr data structure is used together with the
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send/recv system call family. That cmsghdr data structure holds the
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following information 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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- specification of the associated authentication data (AAD) with the
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flag ALG_SET_AEAD_ASSOCLEN. The AAD is sent to the kernel together
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with the plaintext / ciphertext. See below for the memory structure.
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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 with a
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subsequent invocation of the send system call.
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Note: The kernel reports -EINVAL for any unexpected data. The caller
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must make sure that all data matches the constraints given in
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/proc/crypto for the 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
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at least as large as defined with the memory structure below. If the
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output data size is smaller, the cipher operation is not performed.
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The authenticated decryption operation may indicate an integrity error.
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Such breach in integrity is marked with the -EBADMSG error code.
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AEAD Memory Structure
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~~~~~~~~~~~~~~~~~~~~~
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The AEAD cipher operates with the following information that is
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communicated between user and kernel space as one data stream:
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- plaintext or ciphertext
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- associated authentication data (AAD)
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- authentication tag
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The sizes of the AAD and the authentication tag are provided with the
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sendmsg and setsockopt calls (see there). As the kernel knows the size
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of the entire data stream, the kernel is now able to calculate the right
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offsets of the data components in the data stream.
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The user space caller must arrange the aforementioned information in the
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following order:
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- AEAD encryption input: AAD \|\| plaintext
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- AEAD decryption input: AAD \|\| ciphertext \|\| authentication tag
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The output buffer the user space caller provides must be at least as
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large to hold the following data:
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- AEAD encryption output: ciphertext \|\| authentication tag
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- AEAD decryption output: plaintext
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Random Number Generator API
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---------------------------
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Again, the operation is very similar to the other APIs. During
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initialization, the struct sockaddr data structure must be filled as
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follows:
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::
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struct sockaddr_alg sa = {
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.salg_family = AF_ALG,
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.salg_type = "rng", /* this selects the symmetric cipher */
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.salg_name = "drbg_nopr_sha256" /* this is the cipher name */
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};
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Depending on the RNG type, the RNG must be seeded. The seed is provided
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using the setsockopt interface to set the key. For example, the
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ansi_cprng requires a seed. The DRBGs do not require a seed, but may be
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seeded.
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Using the read()/recvmsg() system calls, random numbers can be obtained.
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The kernel generates at most 128 bytes in one call. If user space
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requires more data, multiple calls to read()/recvmsg() must be made.
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WARNING: The user space caller may invoke the initially mentioned accept
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system call multiple times. In this case, the returned file descriptors
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have the same state.
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Zero-Copy Interface
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-------------------
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In addition to the send/write/read/recv system call family, the AF_ALG
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interface can be accessed with the zero-copy interface of
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splice/vmsplice. As the name indicates, the kernel tries to avoid a copy
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operation into kernel space.
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The zero-copy operation requires data to be aligned at the page
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boundary. Non-aligned data can be used as well, but may require more
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operations of the kernel which would defeat the speed gains obtained
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from the zero-copy interface.
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The system-inherent limit for the size of one zero-copy operation is 16
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pages. If more data is to be sent to AF_ALG, user space must slice the
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input into segments with a maximum size of 16 pages.
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Zero-copy can be used with the following code example (a complete
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working example is provided with libkcapi):
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::
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int pipes[2];
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pipe(pipes);
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/* input data in iov */
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vmsplice(pipes[1], iov, iovlen, SPLICE_F_GIFT);
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/* opfd is the file descriptor returned from accept() system call */
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splice(pipes[0], NULL, opfd, NULL, ret, 0);
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read(opfd, out, outlen);
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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
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and retrieve data subject to the cipher operation, a consumer also needs
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to set the additional information for the cipher operation. This
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additional information is set using the setsockopt system call that must
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be invoked with the file descriptor of the open cipher (i.e. the file
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descriptor returned by the 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
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mentioned 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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- the AEAD cipher type
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- the RNG cipher type to provide the seed
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- ALG_SET_AEAD_AUTHSIZE -- Setting the authentication tag size for
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AEAD ciphers. For a encryption operation, the authentication tag of
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the given size will be generated. For a decryption operation, the
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provided ciphertext is assumed to contain an authentication tag of
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the given size (see section about AEAD memory layout below).
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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
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the aforementioned Netlink kernel interface. [1] also contains a test
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application that invokes all libkcapi API calls.
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[1] http://www.chronox.de/libkcapi.html
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