Updated: 2021/Apr/14

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 message authentication code

       libcrypto, -lcrypto

        #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);

        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, int 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);


        #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 can be EVP_sha1(), EVP_ripemd160() etc.

       HMAC_CTX_new() creates a new HMAC_CTX in heap memory.

       HMAC_CTX_reset() zeroes 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() 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. HHMMAACC__IInniitt(()) had
       this undocumented behaviour in previous versions of OpenSSL - failure
       to switch to HHMMAACC__IInniitt__eexx(()) in programs that expect it will cause them
       to stop working.

       NOTE: 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

       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() 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.

       RFC 2104

       openssl_sha(3), openssl_evp(3)

       HMAC_CTX_init() was replaced with HMAC_CTX_reset() in OpenSSL versions

       HMAC_CTX_cleanup() existed in OpenSSL versions before 1.1.0.

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

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

       Copyright 2000-2016 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

1.1.0g                            2018-01-15                           HMAC(3)