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_CTX_init(HMAC_CTX *ctx);
int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
const EVP_MD *md);
int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len,
const EVP_MD *md, ENGINE *impl);
int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
int HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);
void HMAC_CTX_cleanup(HMAC_CTX *ctx);
void HMAC_cleanup(HMAC_CTX *ctx);

DESCRIPTION

is a (message authentication code), i.e. a keyed hash function used for message authentication, which is based on a hash 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 bytes). If md is the digest is placed in a static array. The size of the output is placed in md_len, unless it is .

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

HMAC_CTX_init() initialises a before first use. It must be called.

HMAC_CTX_cleanup() erases the key and other data from the and releases any associated resources. It must be called when an is no longer required.

HMAC_cleanup() is an alias for HMAC_CTX_cleanup() included for back compatibility with 0.9.6b, it is deprecated.

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

HMAC_Init() initializes a structure to use the hash function evp_md and the key key which is key_len bytes long. It is deprecated and only included for backward compatibility with OpenSSL 0.9.6b.

HMAC_Init_ex() initializes or reuses a structure to use the function evp_md and key key. Either can be in which case the existing one will be reused. HMAC_CTX_init() must have been called before the first use of an in this function. N.B. HMAC_Init() had this undocumented behaviour in previous versions of OpenSSL - failure to switch to HMAC_Init_ex() in programs that expect it will cause them to stop working.

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

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

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

HMAC_CTX_init() and HMAC_CTX_cleanup() do not return values.

CONFORMING TO

HISTORY

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

HMAC_CTX_init(), HMAC_Init_ex() and HMAC_CTX_cleanup() are available since OpenSSL 0.9.7.

HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in versions of OpenSSL before 1.0.0.