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     hmac(3)               18/Jul/2002 (0.9.7e)                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);
           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_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_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.


     Page 2                                         (printed 10/20/05)