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hmac(3)				    OpenSSL			       hmac(3)

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

	#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);

	int HMAC_Init(HMAC_CTX *ctx, const void	*key, int key_len,
		      const EVP_MD *md);
	int HMAC_Init_ex(HMAC_CTX *ctx,	const void *key, int key_len,
			  const	EVP_MD *md, ENGINE *impl);
	int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
	int HMAC_Final(HMAC_CTX	*ctx, unsigned char *md, unsigned int *len);

	void HMAC_CTX_cleanup(HMAC_CTX *ctx);
	void HMAC_cleanup(HMAC_CTX *ctx);

       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

       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.

       HMAC_CTX_init() initialises a HMAC_CTX before first use.	It must	be

       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.

       HMAC() returns a	pointer	to the message authentication code or NULL if
       an error	occurred.

       HMAC_Init_ex(), HMAC_Update() and HMAC_Final() return 1 for success or
       0 if an error occurred.

       HMAC_CTX_init() and HMAC_CTX_cleanup() do not return values.

       RFC 2104

       sha(3), evp(3)

       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.

       HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in
       versions	of OpenSSL before 1.0.0.

1.0.1e				  2013-02-11			       hmac(3)


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