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, 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 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.


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.

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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