=pod =head1 NAME HMAC, HMAC_CTX_init, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_cleanup - HMAC message authentication code =head1 SYNOPSIS #include 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); int 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); =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 bytes at B using the hash function B and the key B which is B bytes long. It places the result in B (which must have space for the output of the hash function, which is no more than B bytes). If B is NULL, the digest is placed in a static array. The size of the output is placed in B, unless it is B. B can be EVP_sha1(), EVP_ripemd160() etc. HMAC_CTX_init() initialises a B before first use. It must be called. HMAC_CTX_cleanup() erases the key and other data from the B and releases any associated resources. It must be called when an B is no longer required. The following functions may be used if the message is not completely stored in memory: HMAC_Init() initializes a B structure to use the hash function B and the key B which is B bytes long. It is deprecated and only included for backward compatibility with OpenSSL 0.9.6b. HMAC_Init_ex() initializes or reuses a B structure to use the function B and key B. 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 B in this function. B. HMAC_Update() can be called repeatedly with chunks of the message to be authenticated (B bytes at B). HMAC_Final() places the message authentication code in B, which must have space for the hash function output. =head1 RETURN VALUES HMAC() returns a pointer to the message authentication code or NULL if an error occurred. HMAC_CTX_init(), HMAC_Init_ex(), HMAC_Update() and HMAC_Final() return 1 for success or 0 if an error occurred. HMAC_CTX_cleanup() do not return values. =head1 CONFORMING TO RFC 2104 =head1 SEE ALSO L, L =head1 HISTORY HMAC_CTX_init() did not return a value in OpenSSL versions before 1.1. HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in versions of OpenSSL before 1.0.0. =cut