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hmac(3)                                            OpenSSL                                           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, ENGINE *impl);
        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_ripemd16_() 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.



0.9.8                                            2006-01-30                                          hmac(3)