hmac(3)

NAME
HMAC, HMAC_CTX_init, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_cleanup, HMAC_cleanup - HMAC message authentication code

LIBRARY
libcrypto, -lcrypto

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
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.

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 hash function evp_md and key key. If both are NULL (or evp_md is the same as the previous digest used by ctx and key is NULL) the existing key is reused. ctx must have been created with HMAC_CTX_new() 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.

NB: if HMAC_Init_ex() is called with key NULL and evp_md is not the same as the previous digest used by ctx then an error is returned because reuse of an existing key with a different digest is not supported.

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 or NULL 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
RFC 2104

SEE ALSO
openssl_sha(3), openssl_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.

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