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HMAC(3)				    OpenSSL			       HMAC(3)

       HMAC, HMAC_CTX_new, HMAC_CTX_reset, HMAC_CTX_free, HMAC_Init,
       HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_copy,
       HMAC_CTX_set_flags, HMAC_CTX_get_md, HMAC_size -	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, size_t	n,
			    unsigned char *md, unsigned	int *md_len);

	HMAC_CTX *HMAC_CTX_new(void);
	int HMAC_CTX_reset(HMAC_CTX *ctx);

	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, size_t len);
	int HMAC_Final(HMAC_CTX	*ctx, unsigned char *md, unsigned int *len);

	void HMAC_CTX_free(HMAC_CTX *ctx);

	int HMAC_CTX_copy(HMAC_CTX *dctx, HMAC_CTX *sctx);
	void HMAC_CTX_set_flags(HMAC_CTX *ctx, unsigned	long flags);
	const EVP_MD *HMAC_CTX_get_md(const HMAC_CTX *ctx);

	size_t HMAC_size(const HMAC_CTX	*e);


	#if OPENSSL_API_COMPAT < 0x10100000L
	int HMAC_Init(HMAC_CTX *ctx, const void	*key, int key_len,
		      const EVP_MD *md);

       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. Note: passing a NULL value for
       md  to use the static array is not thread safe.

       evp_md is a message digest such as EVP_sha1(), EVP_ripemd160() etc.
       HMAC does not support variable output length digests such as
       EVP_shake128() and EVP_shake256().

       HMAC_CTX_new() creates a	new HMAC_CTX in	heap memory.

       HMAC_CTX_reset()	zeros an existing HMAC_CTX and associated resources,
       making it suitable for new computations as if it	was newly created with

       HMAC_CTX_free() erases the key and other	data from the HMAC_CTX,
       releases	any associated resources and finally frees the HMAC_CTX

       The following functions may be used if the message is not completely
       stored in memory:

       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 if key is	NULL
       and evp_md is the same as the previous call, then 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.

       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_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.

       HMAC_CTX_copy() copies all of the internal state	from sctx into dctx.

       HMAC_CTX_set_flags() applies the	specified flags	to the internal
       EVP_MD_CTXs.  These flags have the same meaning as for

       HMAC_CTX_get_md() returns the EVP_MD that has previously	been set for
       the supplied HMAC_CTX.

       HMAC_size() returns the length in bytes of the underlying hash function

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

       HMAC_CTX_new() returns a	pointer	to a new HMAC_CTX on success or	NULL
       if an error occurred.

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

       HMAC_CTX_get_md() return	the EVP_MD previously set for the supplied
       HMAC_CTX	or NULL	if no EVP_MD has been set.

       HMAC_size() returns the length in bytes of the underlying hash function
       output or zero on error.

       RFC 2104

       SHA1(3),	evp(7)

       HMAC_CTX_init() was replaced with HMAC_CTX_reset() in OpenSSL 1.1.0.

       HMAC_CTX_cleanup() existed in OpenSSL before version 1.1.0.

       HMAC_CTX_new(), HMAC_CTX_free() and HMAC_CTX_get_md() are new in
       OpenSSL 1.1.0.

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

       Copyright 2000-2020 The OpenSSL Project Authors.	All Rights Reserved.

       Licensed	under the OpenSSL license (the "License").  You	may not	use
       this file except	in compliance with the License.	 You can obtain	a copy
       in the file LICENSE in the source distribution or at

1.1.1k				  2021-03-25			       HMAC(3)


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