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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_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.

26/Feb/2001		      0.9.6				1

hmac(3)			     OpenSSL			  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.

       authentication code"

26/Feb/2001		      0.9.6				2

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