HMAC ( const EVP_MD *evp_md , const void *key , int key_len , const unsigned char *d , size_t n , unsigned char *md , unsigned int *md_len );

HMAC_CTX * HMAC_CTX_new ( void );

HMAC_CTX_reset ( HMAC_CTX *ctx );

HMAC_CTX_free ( HMAC_CTX *ctx );

HMAC_Init_ex ( HMAC_CTX *ctx , const void *key , int key_len , const EVP_MD *md , ENGINE *impl );

HMAC_Init ( HMAC_CTX *ctx , const void *key , int key_len , const EVP_MD *md );

HMAC_Update ( HMAC_CTX *ctx , const unsigned char *data , size_t len );

HMAC_Final ( HMAC_CTX *ctx , unsigned char *md , unsigned int *len );

HMAC_CTX_copy ( HMAC_CTX *dctx , HMAC_CTX *sctx );

HMAC_CTX_set_flags ( HMAC_CTX *ctx , unsigned long flags );

const EVP_MD * HMAC_CTX_get_md ( const HMAC_CTX *ctx );

size_t HMAC_size ( const HMAC_CTX *e );

DESCRIPTION ¶

HMAC is a MAC (message authentication code), i.e. a keyed hash function used for message authentication, which is based on a hash function.

HMAC () computes the message authentication code of the n bytes at d using the hash function evp_md and the key key which is key_len bytes long.

It places the result in md , which must have space for the output of the hash function, which is no more than EVP_MAX_MD_SIZE bytes. If md is NULL , the digest is placed in a static array, which is not thread safe. The size of the output is placed in md_len , unless it is NULL .

evp_md can be EVP_sha1(3) , EVP_ripemd160(3) , etc.

HMAC_CTX_new () allocates and initializes a new HMAC_CTX object.

HMAC_CTX_reset () zeroes and re-initializes ctx and associated resources, making it suitable for new computations as if it was deleted with HMAC_CTX_free () and newly created with HMAC_CTX_new ().

HMAC_CTX_free () erases the key and other data from ctx , releases any associated resources, and finally frees ctx itself.

The following functions may be used if the message is not completely stored in memory:

HMAC_Init_ex () sets up or reuses ctx to use the hash function evp_md and the key key . Either can be NULL , in which case the existing one is reused. The ctx must have been created with HMAC_CTX_new () before the first use in this function. If HMAC_Init_ex () is called with a NULL key but evp_md is neither NULL nor the same as the previous digest used by ctx , then an error is returned because reuse of an existing key with a different digest is not supported.

HMAC_Init () is a deprecated wrapper around HMAC_Init_ex () which performs no longer useful extra initialization in some circumstances.

HMAC_Update () can be called repeatedly with chunks of the message to be authenticated ( len bytes at data ).

HMAC_Final () places the message authentication code in md , which must have space for the hash function output.

HMAC_CTX_copy () copies all of the internal state from sctx into dctx .

HMAC_CTX_set_flags () applies the specified flags to the internal EVP_MD_CTX objects. Possible flag values EVP_MD_CTX_FLAG_* are defined in < openssl/evp.h > .

HMAC_size () returns the length in bytes of the underlying hash function output. It is implemented as a macro.

RETURN VALUES ¶

HMAC () returns a pointer to the message authentication code or NULL if an error occurred.

HMAC_CTX_new () returns a pointer to the new HMAC_CTX object or NULL if an error occurred.

HMAC_CTX_reset (), HMAC_Init_ex (), HMAC_Update (), HMAC_Final (), and HMAC_CTX_copy () return 1 for success or 0 if an error occurred.

HMAC_CTX_get_md () returns the message digest that was previously set for ctx with HMAC_Init_ex (), or NULL if none was set.

HMAC_size () returns the length in bytes of the underlying hash function output or 0 on error.

SEE ALSO ¶

CMAC_Init(3) , EVP_DigestInit(3)

STANDARDS ¶

RFC 2104

HISTORY ¶

HMAC (), HMAC_Init (), HMAC_Update (), HMAC_Final (), and HMAC_size () first appeared in SSLeay 0.9.0 and have been available since OpenBSD 2.4 .

HMAC_Init_ex () first appeared in OpenSSL 0.9.7 and have been available since OpenBSD 3.2 .

HMAC_CTX_set_flags () first appeared in OpenSSL 0.9.7f and have been available since OpenBSD 3.8 .

HMAC_CTX_copy () first appeared in OpenSSL 1.0.0 and has been available since OpenBSD 4.9 .

HMAC_CTX_new (), HMAC_CTX_reset (), HMAC_CTX_free (), and HMAC_CTX_get_md () first appeared in OpenSSL 1.1.0 and have been available since OpenBSD 6.3 .