MAC#

A Message Authentication Code (MAC) provides data integrity and authenticity of a message.

HMAC#

An HMAC is a specific construction of a MAC that involves a cryptographic hash function (see [RFC 2104]). Thus, an HMAC comes in multiple instantiations. HACL Packages supports the following ones:

  • HMAC-BLAKE2b,

  • HMAC-BLAKE2s,

  • HMAC-SHA-2-256,

  • HMAC-SHA-2-384,

  • HMAC-SHA-2-512, and

  • HMAC-SHA-1.

Keys must be chosen using a cryptographically strong pseudo-random generator and periodically refreshed.

API Reference#

#include "Hacl_Spec.h"
typedef uint8_t Spec_Hash_Definitions_hash_alg#

The available hash functions.

The allowed values with recommended HMAC key length (KEY_LEN) and HMAC digest output length (DIGEST_LEN) are …

  • Spec_Hash_Definitions_SHA2_224 (KEY_LEN=64, DIGEST_LEN=28),

  • Spec_Hash_Definitions_SHA2_256 (KEY_LEN=64, DIGEST_LEN=32),

  • Spec_Hash_Definitions_SHA2_384 (KEY_LEN=128, DIGEST_LEN=48),

  • Spec_Hash_Definitions_SHA2_512 (KEY_LEN=128, DIGEST_LEN=64),

  • Spec_Hash_Definitions_SHA1 (KEY_LEN=64, DIGEST_LEN=20),

  • Spec_Hash_Definitions_MD5 (KEY_LEN=64, DIGEST_LEN=16),

  • Spec_Hash_Definitions_Blake2S (KEY_LEN=64, DIGEST_LEN=32), and

  • Spec_Hash_Definitions_Blake2B (KEY_LEN=128, DIGEST_LEN=64).

Note that the HMAC key can be of any length and will be preprocessed accordingly before use.

bool EverCrypt_HMAC_is_supported_alg(Spec_Hash_Definitions_hash_alg uu___)#

Check that a hash function is supported to be used in the HMAC construction.

void EverCrypt_HMAC_compute(Spec_Hash_Definitions_hash_alg a, uint8_t *mac, uint8_t *key, uint32_t keylen, uint8_t *data, uint32_t datalen)#

Write the MAC of a message (data) by using the hash algorithm a with a key (key) into dst.

The recommended size for the key depends on the used hash algorithm (see KEY_LEN above). However, the key can be any length and will be hashed if it is longer and padded if it is shorter. The length of dst depends on the output length of the used hash algorithm (see DIGEST_LEN above).