=pod =head1 NAME HMAC, HMAC_Init, HMAC_Update, HMAC_Final, HMAC_cleanup - HMAC message authentication code =head1 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); =head1 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 B<n> bytes at B<d> using the hash function B<evp_md> and the key B<key> which is B<key_len> bytes long. It places the result in B<md> (which must have space for the output of the hash function, which is no more than B<EVP_MAX_MD_SIZE> bytes). If B<md> is NULL, the digest is placed in a static array. The size of the output is placed in B<md_len>, unless it is B<NULL>. B<evp_md> can be EVP_sha1(), EVP_ripemd160() etc. B<key> and B<evp_md> may be B<NULL> if a key and hash function have been set in a previous call to HMAC_Init() for that B<HMAC_CTX>. HMAC_cleanup() erases the key and other data from the B<HMAC_CTX>. The following functions may be used if the message is not completely stored in memory: HMAC_Init() initializes a B<HMAC_CTX> structure to use the hash function B<evp_md> and the key B<key> which is B<key_len> bytes long. HMAC_Update() can be called repeatedly with chunks of the message to be authenticated (B<len> bytes at B<data>). HMAC_Final() places the message authentication code in B<md>, which must have space for the hash function output. =head1 RETURN VALUES HMAC() returns a pointer to the message authentication code. HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup() do not return values. =head1 CONFORMING TO RFC 2104 =head1 SEE ALSO L<sha(3)|sha(3)>, L<evp(3)|evp(3)> =head1 HISTORY HMAC(), HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup() are available since SSLeay 0.9.0. =cut