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HMAC(3)                             OpenSSL                            HMAC(3)
NAME
       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
SYNOPSIS
        #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);
       Deprecated:
        #if OPENSSL_API_COMPAT < 0x10100000L
        int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
                      const EVP_MD *md);
        #endif
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. 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_new().
       HMAC_CTX_free() erases the key and other data from the HMAC_CTX,
       releases any associated resources and finally frees the HMAC_CTX
       itself.
       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
       EVP_MD_CTX_set_flags(3).
       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
       output.
RETURN VALUES
       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.
CONFORMING TO
       RFC 2104
SEE ALSO
       SHA1(3), evp(7)
HISTORY
       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
       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
       <https://www.openssl.org/source/license.html>;.
1.1.1k                            2021-03-25                           HMAC(3)