HMAC(3)

NAME
HMAC, HMAC_CTX_init, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_cleanup, HMAC_cleanup, HMAC_CTX_copy, HMAC_CTX_set_flags, HMAC_size - 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);
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);
int
HMAC_CTX_copy(HMAC_CTX *dctx, HMAC_CTX *sctx);
void
HMAC_CTX_set_flags(HMAC_CTX *ctx, unsigned long flags);
size_t
HMAC_size(const HMAC_CTX *e);

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(3), EVP_ripemd160(3), etc.

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 backward compatibility with 0.9.6b. It is deprecated and implemented as a macro.

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.

HMAC_CTX_copy() copies all of the internal state from sctx into dctx.

HMAC_CTX_set_flags() applies the specified flags to the internal EVP_MD_CTX objects. Possible flag values EVP_MD_CTX_FLAG_* are defined in <openssl/evp.h>.

HMAC_size() returns the length in bytes of the underlying hash function output. It is implemented as a macro.

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

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

HMAC_size() returns the length in bytes of the underlying hash function output or 0 on error.

SEE ALSO
evp(3)

STANDARDS
RFC 2104

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.

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