Cryptography and SSL/TLS Toolkit



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, 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);

 size_t HMAC_size(const HMAC_CTX *e);

Deprecated since OpenSSL 1.1.0, can be hidden entirely by defining OPENSSL_API_COMPAT with a suitable version value, see openssl_user_macros(7):

 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 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() zeroes 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.


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 2000-2016 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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