hmac(3)

NAME
HMAC, HMAC_Init, HMAC_Update, HMAC_Final - HMAC message authentication code

LIBRARY
libcrypto, -lcrypto

SYNOPSIS

 #include <openssl/hmac.h>
 unsigned char *HMAC(const EVP_MD *evp_md, const void *key,
               int key_len, const unsigned char *d, int n,
               unsigned char *md, unsigned int *md_len);
 void HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
               const EVP_MD *md);
 void HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
 void HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);
 void HMAC_cleanup(HMAC_CTX *ctx);

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. The size of the output is placed in md_len, unless it is NULL.

evp_md can be EVP_sha1(), EVP_ripemd160() etc. key and evp_md may be NULL if a key and hash function have been set in a previous call to HMAC_Init() for that HMAC_CTX.

HMAC_cleanup() erases the key and other data from the HMAC_CTX.

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

HMAC_Init() initializes a HMAC_CTX structure to use the hash function evp_md and the key key which is key_len bytes long.

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.

RETURN VALUES
HMAC() returns a pointer to the message authentication code.

HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup() do not return values.

CONFORMING TO
RFC 2104

SEE ALSO
the sha(3)|sha(3) manpage, the evp(3)|evp(3) manpage

HISTORY
HMAC(), HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup() are available since SSLeay 0.9.0.