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hmac(3)				    OpenSSL			       hmac(3)

NAME
       HMAC, HMAC_CTX_init, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final,
       HMAC_CTX_cleanup, 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);

	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);

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.

       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
       hash function evp_md and	key key. If both are NULL (or evp_md is	the
       same as the previous digest used	by ctx and key is NULL)	the existing
       key is reused. ctx must have been created with HMAC_CTX_new() 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.

       NB: if HMAC_Init_ex() is	called with key	NULL and evp_md	is not the
       same as the previous digest used	by ctx then an error is	returned
       because reuse of	an existing key	with a different digest	is not
       supported.

       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 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.

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.

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

1.0.2l				  2017-05-25			       hmac(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | CONFORMING TO | SEE ALSO | HISTORY

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