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_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    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       RFC 2104

SEE ALSO    [Toc]    [Back]

       SHA1(3), EVP_DigestInit(3)

HISTORY    [Toc]    [Back]

       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.


OpenBSD 3.6                 2002-09-10                          2