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<title>HMAC</title>
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<!-- INDEX BEGIN -->
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<p><a name="__index__"></a></p>
<ul>
<li><a href="#name">NAME</a></li>
<li><a href="#synopsis">SYNOPSIS</a></li>
<li><a href="#description">DESCRIPTION</a></li>
<li><a href="#return_values">RETURN VALUES</a></li>
<li><a href="#conforming_to">CONFORMING TO</a></li>
<li><a href="#see_also">SEE ALSO</a></li>
<li><a href="#history">HISTORY</a></li>
<li><a href="#copyright">COPYRIGHT</a></li>
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<p>
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<hr />
<h1><a name="name">NAME</a></h1>
<p>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</p>
<p>
</p>
<hr />
<h1><a name="synopsis">SYNOPSIS</a></h1>
<pre>
#include <openssl/hmac.h></pre>
<pre>
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);</pre>
<pre>
HMAC_CTX *HMAC_CTX_new(void);
int HMAC_CTX_reset(HMAC_CTX *ctx);</pre>
<pre>
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);</pre>
<pre>
void HMAC_CTX_free(HMAC_CTX *ctx);</pre>
<pre>
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);</pre>
<pre>
size_t HMAC_size(const HMAC_CTX *e);</pre>
<p>Deprecated:</p>
<pre>
#if OPENSSL_API_COMPAT < 0x10100000L
int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
const EVP_MD *md);
#endif</pre>
<p>
</p>
<hr />
<h1><a name="description">DESCRIPTION</a></h1>
<p>HMAC is a MAC (message authentication code), i.e. a keyed hash
function used for message authentication, which is based on a hash
function.</p>
<p><code>HMAC()</code> computes the message authentication code of the <strong>n</strong> bytes at
<strong>d</strong> using the hash function <strong>evp_md</strong> and the key <strong>key</strong> which is
<strong>key_len</strong> bytes long.</p>
<p>It places the result in <strong>md</strong> (which must have space for the output of
the hash function, which is no more than <strong>EVP_MAX_MD_SIZE</strong> bytes).
If <strong>md</strong> is NULL, the digest is placed in a static array. The size of
the output is placed in <strong>md_len</strong>, unless it is <strong>NULL</strong>. Note: passing a NULL
value for <strong>md</strong> to use the static array is not thread safe.</p>
<p><strong>evp_md</strong> 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().</p>
<p><code>HMAC_CTX_new()</code> creates a new HMAC_CTX in heap memory.</p>
<p><code>HMAC_CTX_reset()</code> zeros an existing <strong>HMAC_CTX</strong> and associated
resources, making it suitable for new computations as if it was newly
created with <code>HMAC_CTX_new()</code>.</p>
<p><code>HMAC_CTX_free()</code> erases the key and other data from the <strong>HMAC_CTX</strong>,
releases any associated resources and finally frees the <strong>HMAC_CTX</strong>
itself.</p>
<p>The following functions may be used if the message is not completely
stored in memory:</p>
<p><code>HMAC_Init_ex()</code> initializes or reuses a <strong>HMAC_CTX</strong> structure to use the hash
function <strong>evp_md</strong> and key <strong>key</strong>. If both are NULL, or if <strong>key</strong> is NULL
and <strong>evp_md</strong> is the same as the previous call, then the
existing key is
reused. <strong>ctx</strong> must have been created with <code>HMAC_CTX_new()</code> before the first use
of an <strong>HMAC_CTX</strong> in this function.</p>
<p>If <code>HMAC_Init_ex()</code> is called with <strong>key</strong> NULL and <strong>evp_md</strong> is not the
same as the previous digest used by <strong>ctx</strong> then an error is returned
because reuse of an existing key with a different digest is not supported.</p>
<p><code>HMAC_Init()</code> initializes a <strong>HMAC_CTX</strong> structure to use the hash
function <strong>evp_md</strong> and the key <strong>key</strong> which is <strong>key_len</strong> bytes
long.</p>
<p><code>HMAC_Update()</code> can be called repeatedly with chunks of the message to
be authenticated (<strong>len</strong> bytes at <strong>data</strong>).</p>
<p><code>HMAC_Final()</code> places the message authentication code in <strong>md</strong>, which
must have space for the hash function output.</p>
<p><code>HMAC_CTX_copy()</code> copies all of the internal state from <strong>sctx</strong> into <strong>dctx</strong>.</p>
<p><code>HMAC_CTX_set_flags()</code> applies the specified flags to the internal EVP_MD_CTXs.
These flags have the same meaning as for <em>EVP_MD_CTX_set_flags(3)</em>.</p>
<p><code>HMAC_CTX_get_md()</code> returns the EVP_MD that has previously been set for the
supplied HMAC_CTX.</p>
<p><code>HMAC_size()</code> returns the length in bytes of the underlying hash function output.</p>
<p>
</p>
<hr />
<h1><a name="return_values">RETURN VALUES</a></h1>
<p><code>HMAC()</code> returns a pointer to the message authentication code or NULL if
an error occurred.</p>
<p><code>HMAC_CTX_new()</code> returns a pointer to a new <strong>HMAC_CTX</strong> on success or
<strong>NULL</strong> if an error occurred.</p>
<p><code>HMAC_CTX_reset()</code>, <code>HMAC_Init_ex()</code>, <code>HMAC_Update()</code>, <code>HMAC_Final()</code> and
<code>HMAC_CTX_copy()</code> return 1 for success or 0 if an error occurred.</p>
<p><code>HMAC_CTX_get_md()</code> return the EVP_MD previously set for the supplied HMAC_CTX or
NULL if no EVP_MD has been set.</p>
<p><code>HMAC_size()</code> returns the length in bytes of the underlying hash function output
or zero on error.</p>
<p>
</p>
<hr />
<h1><a name="conforming_to">CONFORMING TO</a></h1>
<p>RFC 2104</p>
<p>
</p>
<hr />
<h1><a name="see_also">SEE ALSO</a></h1>
<p><em>SHA1(3)</em>, <em>evp(7)</em></p>
<p>
</p>
<hr />
<h1><a name="history">HISTORY</a></h1>
<p><code>HMAC_CTX_init()</code> was replaced with <code>HMAC_CTX_reset()</code> in OpenSSL 1.1.0.</p>
<p><code>HMAC_CTX_cleanup()</code> existed in OpenSSL before version 1.1.0.</p>
<p><code>HMAC_CTX_new()</code>, <code>HMAC_CTX_free()</code> and <code>HMAC_CTX_get_md()</code> are new in OpenSSL 1.1.0.</p>
<p><code>HMAC_Init_ex()</code>, <code>HMAC_Update()</code> and <code>HMAC_Final()</code> did not return values in
OpenSSL before version 1.0.0.</p>
<p>
</p>
<hr />
<h1><a name="copyright">COPYRIGHT</a></h1>
<p>Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.</p>
<p>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
<a href="https://www.openssl.org/source/license.html">https://www.openssl.org/source/license.html</a>.</p>
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