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CRYPTO(4) Device Drivers Manual CRYPTO(4)


crypto, cryptodevuser-mode access to hardware-accelerated cryptography


device crypto
device cryptodev

#include <sys/ioctl.h>
#include <sys/time.h>
#include <crypto/cryptodev.h>


The crypto driver gives user-mode applications access to hardware-accelerated cryptographic transforms as implemented by the crypto(9) in-kernel interface.

The /dev/crypto special device provides an ioctl(2) based interface. User-mode applications open the special device and then issue ioctl(2) calls on the descriptor. User-mode access to /dev/crypto is controlled by two sysctl(8) variables: kern.userasymcrypto and kern.cryptodevallowsoft.

The crypto device provides two distinct modes of operation: one mode for symmetric-keyed cryptographic requests and digests, and a second mode for both asymmetric-key (public-key/private-key) requests and modular arithmetic (for Diffie-Hellman key exchange and other cryptographic protocols). The two modes are described separately below.


Regardless of whether symmetric-key or asymmetric-key operations are to be performed, use of the device requires a basic series of steps:

  1. Open the /dev/crypto device.
  2. Create a new cryptography file descriptor via CRIOGET to use for all subsequent ioctl(2) commands.
  3. Close the /dev/crypto device.
  4. If any symmetric-keyed cryptographic or digest operations will be performed, create a session with CIOCGSESSION. Most applications will require at least one symmetric session. Since cipher and MAC keys are tied to sessions, many applications will require more. Asymmetric operations do not use sessions.
  5. Submit requests, synchronously with CIOCCRYPT (symmetric), CIOCCRYPTAEAD (symmetric), or CIOCKEY (asymmetric).
  6. Optionally destroy a session with CIOCFSESSION.
  7. Close the cryptography file descriptor with close(2). This will automatically close any remaining sessions associated with the file desriptor.


The symmetric-key operation mode provides a context-based API to traditional symmetric-key encryption (or privacy) algorithms, or to keyed and unkeyed one-way hash (HMAC and MAC) algorithms. The symmetric-key mode also permits fused operation, where the hardware performs both a privacy algorithm and an integrity-check algorithm in a single pass over the data: either a fused encrypt/HMAC-generate operation, or a fused HMAC-verify/decrypt operation.

To use symmetric mode, you must first create a session specifying the algorithm(s) and key(s) to use; then issue encrypt or decrypt requests against the session.


For a list of supported algorithms, see crypto(7) and crypto(9).

IOCTL Request Descriptions

int *fd
Clone the fd argument to ioctl(2), yielding a new file descriptor for the creation of sessions.
struct crypt_find_op *fop
struct crypt_find_op {
    int     crid;       /* driver id + flags */
    char    name[32];   /* device/driver name */

If crid is -1, then find the driver named name and return the id in crid. If crid is not -1, return the name of the driver with crid in name. In either case, if the driver is not found, ENOENT is returned.
struct session_op *sessp
struct session_op {
    u_int32_t cipher;	/* e.g. CRYPTO_DES_CBC */
    u_int32_t mac;	/* e.g. CRYPTO_MD5_HMAC */

    u_int32_t keylen;	/* cipher key */
    const void *key;
    int mackeylen;	/* mac key */
    const void *mackey;

    u_int32_t ses;	/* returns: ses # */

Create a new cryptographic session on a file descriptor for the device; that is, a persistent object specific to the chosen privacy algorithm, integrity algorithm, and keys specified in sessp. The special value 0 for either privacy or integrity is reserved to indicate that the indicated operation (privacy or integrity) is not desired for this session.

Multiple sessions may be bound to a single file descriptor. The session ID returned in sessp->ses is supplied as a required field in the symmetric-operation structure crypt_op for future encryption or hashing requests.

For non-zero symmetric-key privacy algorithms, the privacy algorithm must be specified in sessp->cipher, the key length in sessp->keylen, and the key value in the octets addressed by sessp->key.

For keyed one-way hash algorithms, the one-way hash must be specified in sessp->mac, the key length in sessp->mackey, and the key value in the octets addressed by sessp->mackeylen.

Support for a specific combination of fused privacy and integrity-check algorithms depends on whether the underlying hardware supports that combination. Not all combinations are supported by all hardware, even if the hardware supports each operation as a stand-alone non-fused operation.

struct session2_op *sessp
struct session2_op {
    u_int32_t cipher;	/* e.g. CRYPTO_DES_CBC */
    u_int32_t mac;	/* e.g. CRYPTO_MD5_HMAC */

    u_int32_t keylen;	/* cipher key */
    const void *key;
    int mackeylen;	/* mac key */
    const void *mackey;

    u_int32_t ses;	/* returns: ses # */
    int	crid;		/* driver id + flags (rw) */
    int	pad[4];		/* for future expansion */

This request is similar to CIOGSESSION except that sessp->crid requests either a specific crypto device or a class of devices (software vs hardware). The sessp->pad field must be initialized to zero.
struct crypt_op *cr_op
struct crypt_op {
    u_int32_t ses;
    u_int16_t op;	/* e.g. COP_ENCRYPT */
    u_int16_t flags;
    u_int len;
    caddr_t src, dst;
    caddr_t mac;		/* must be large enough for result */
    caddr_t iv;

Request a symmetric-key (or hash) operation. To encrypt, set cr_op->op to COP_ENCRYPT. To decrypt, set cr_op->op to COP_DECRYPT. The field cr_op->len supplies the length of the input buffer; the fields cr_op->src, cr_op->dst, cr_op->mac, cr_op->iv supply the addresses of the input buffer, output buffer, one-way hash, and initialization vector, respectively. If a session is using both a privacy algorithm and a hash algorithm, the request will generate a hash of the input buffer before generating the output buffer by default. If the COP_F_CIPHER_FIRST flag is included in the cr_op->flags field, then the request will generate a hash of the output buffer after executing the privacy algorithm.
struct crypt_aead *cr_aead
struct crypt_aead {
    u_int32_t ses;
    u_int16_t op;	/* e.g. COP_ENCRYPT */
    u_int16_t flags;
    u_int len;
    u_int aadlen;
    u_int ivlen;
    caddr_t src, dst;
    caddr_t aad;
    caddr_t tag;		/* must be large enough for result */
    caddr_t iv;

The CIOCCRYPTAEAD is similar to the CIOCCRYPT but provides additional data in cr_aead->aad to include in the authentication mode.
u_int32_t ses_id
Destroys the session identified by ses_id.


Asymmetric-key algorithms

Contingent upon hardware support, the following asymmetric (public-key/private-key; or key-exchange subroutine) operations may also be available:

Input parameter Output parameter
Count Count
3 1
6 1
5 2
7 0
3 1

See below for discussion of the input and output parameter counts.

Asymmetric-key commands

int *feature_mask
Returns a bitmask of supported asymmetric-key operations. Each of the above-listed asymmetric operations is present if and only if the bit position numbered by the code for that operation is set. For example, CRK_MOD_EXP is available if and only if the bit (1 << CRK_MOD_EXP) is set.
struct crypt_kop *kop
struct crypt_kop {
    u_int crk_op;		/* e.g. CRK_MOD_EXP */
    u_int crk_status;		/* return status */
    u_short crk_iparams;	/* # of input params */
    u_short crk_oparams;	/* # of output params */
    u_int crk_pad1;
    struct crparam crk_param[CRK_MAXPARAM];

/* Bignum parameter, in packed bytes. */
struct crparam {
    void * crp_p;
    u_int crp_nbits;

Performs an asymmetric-key operation from the list above. The specific operation is supplied in kop->crk_op; final status for the operation is returned in kop->crk_status. The number of input arguments and the number of output arguments is specified in kop->crk_iparams and kop->crk_iparams, respectively. The field crk_param[] must be filled in with exactly kop->crk_iparams + kop->crk_oparams arguments, each encoded as a struct crparam (address, bitlength) pair.

The semantics of these arguments are currently undocumented.


aesni(4), hifn(4), ipsec(4), padlock(4), safe(4), ubsec(4), crypto(7), geli(8), crypto(9)


The crypto driver first appeared in OpenBSD 3.0. The crypto driver was imported to FreeBSD 5.0.


Error checking and reporting is weak.

The values specified for symmetric-key key sizes to CIOCGSESSION must exactly match the values expected by opencrypto(9). The output buffer and MAC buffers supplied to CIOCCRYPT must follow whether privacy or integrity algorithms were specified for session: if you request a non-NULL algorithm, you must supply a suitably-sized buffer.

The scheme for passing arguments for asymmetric requests is baroque.

CRIOGET should not exist. It should be possible to use the CIOC* commands directly on a /dev/crypto file descriptor.

December 17, 2019 Debian