- NAME
- SYNOPSIS
- DESCRIPTION
- RETURN VALUES
- CONFORMING TO
- SEE ALSO
- HISTORY
- COPYRIGHT

# 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.