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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


 #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 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 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_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() 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. HMAC_Init() had this undocumented behaviour in previous versions of OpenSSL - failure to switch to HMAC_Init_ex() 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 supported.
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() 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


sha(3), evp(3)


HMAC_CTX_init() was replaced with HMAC_CTX_reset() in OpenSSL versions 1.1.0.
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 <>.
2018-03-27 1.1.0h