hmac(3)

     hmac(3)                0.9.6h (2000-09-14)                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_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);

     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.  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_cleanup() erases the key and other data from the
          HMAC_CTX.

          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.

          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.

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     hmac(3)                0.9.6h (2000-09-14)                hmac(3)

     RETURN VALUES
          HMAC() returns a pointer to the message authentication code.

          HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_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.

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