hmac(3)

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_CTX_init(HMAC_CTX *ctx);

 void HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
               const EVP_MD *md);
 void HMAC_Init_ex(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_CTX_cleanup(HMAC_CTX *ctx);
 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_CTX_init() initialises a HMAC_CTX before first use. It must be called.

HMAC_CTX_cleanup() erases the key and other data from the HMAC_CTX and releases any associated resources. It must be called when an HMAC_CTX 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 HMAC_CTX 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 HMAC_CTX structure to use the function evp_md and key key. Either can be NULL, in which case the existing one will be reused. HMAC_CTX_init() must have been called before the first use of an HMAC_CTX 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
HMAC() returns a pointer to the message authentication code.

HMAC_CTX_init(), HMAC_Init_ex(), HMAC_Update(), HMAC_Final() and HMAC_CTX_cleanup() do not return values.

CONFORMING TO
RFC 2104

SEE ALSO
sha(3), evp(3)

HISTORY
HMAC(), 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.