| 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 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] https://www.chronox.de/libkcapi.html |
