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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
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
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
#ifndef SOL_ALG
#define SOL_ALG 279
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
* 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.
Stephan Mueller <>