HMAC(3) — Subroutines
NAME
HMAC, HMAC_Init, HMAC_Update, HMAC_Final, HMAC_cleanup − HMAC message authentication code
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 message authentication code (MAC), i.e. a keyed hash function used for message authentication, which is based on a hash function.
The HMAC() function 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.
The evp_md can be EVP_sha1(), EVP_ripemd160(), etc. The key and evp_md can be NULL if a key and hash function have been set in a previous call to HMAC_Init() for that HMAC_CTX.
The HMAC_cleanup() function 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:
The HMAC_Init() function initializes a HMAC_CTX structure to use the hash function evp_md and the key key which is key_len bytes long.
The HMAC_Update() function can be called repeatedly with chunks of the message to be authenticated (len bytes at data).
The HMAC_Final() function places the message authentication code in md, which must have space for the hash function output.
HMAC conforms to RFC 2104.
RETURN VALUES
The HMAC() function returns a pointer to the message authentication code.
The HMAC_Init(), HMAC_Update(), HMAC_Final(), and HMAC_cleanup() do not return values.
HISTORY
The HMAC(), HMAC_Init(), HMAC_Update(), HMAC_Final(), and HMAC_cleanup() functions are available since SSLeay 0.9.0.