NAME    [Toc]    [Back]

       HMAC,  HMAC_Init,  HMAC_Update, HMAC_Final, HMAC_cleanup -
       HMAC message authentication code

SYNOPSIS    [Toc]    [Back]

       #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    [Toc]    [Back]

       HMAC  is a message authentication code (MAC), i.e. a keyed
       hash function used for message  authentication,  which  is
       based on a hash function.

       The  HMAC()  function  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.

       The  evp_md  can  be EVP_sha1(), EVP_ripemd160(), etc. The
       key and evp_md can be NULL if a key and hash function have
       been  set  in  a  previous  call  to  HMAC_Init() for that
       HMAC_CTX.

       The HMAC_cleanup() function 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:

       The HMAC_Init() function initializes a HMAC_CTX  structure
       to  use  the hash function evp_md and the key key which is
       key_len bytes long.

       The HMAC_Update() function can be called  repeatedly  with
       chunks  of  the  message to be authenticated (len bytes at
       data).

       The HMAC_Final() function places the  message  authentication
  code in md, which must have space for the hash function
 output.

       HMAC conforms to RFC 2104.

RETURN VALUES    [Toc]    [Back]

       The HMAC() function  returns  a  pointer  to  the  message
       authentication code.

       The    HMAC_Init(),   HMAC_Update(),   HMAC_Final(),   and
       HMAC_cleanup() do not return values.

HISTORY    [Toc]    [Back]

       The HMAC(), HMAC_Init(), HMAC_Update(), HMAC_Final(),  and
       HMAC_cleanup() functions are available since SSLeay 0.9.0.

SEE ALSO    [Toc]    [Back]

       Functions: sha(3), evp(3)



                                                          HMAC(3)