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<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="#notes">NOTES</a></li>
	<li><a href="#return_values">RETURN VALUES</a></li>
	<li><a href="#examples">EXAMPLES</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>
</ul>

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<p>
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<hr />
<h1><a name="name">NAME</a></h1>
<p>EVP_PKEY_verify_recover_init, EVP_PKEY_verify_recover - recover signature using a public key algorithm</p>
<p>
</p>
<hr />
<h1><a name="synopsis">SYNOPSIS</a></h1>
<pre>
 #include &lt;openssl/evp.h&gt;</pre>
<pre>
 int EVP_PKEY_verify_recover_init(EVP_PKEY_CTX *ctx);
 int EVP_PKEY_verify_recover(EVP_PKEY_CTX *ctx,
                             unsigned char *rout, size_t *routlen,
                             const unsigned char *sig, size_t siglen);</pre>
<p>
</p>
<hr />
<h1><a name="description">DESCRIPTION</a></h1>
<p>The <code>EVP_PKEY_verify_recover_init()</code> function initializes a public key algorithm
context using key <strong>pkey</strong> for a verify recover operation.</p>
<p>The <code>EVP_PKEY_verify_recover()</code> function recovers signed data
using <strong>ctx</strong>. The signature is specified using the <strong>sig</strong> and
<strong>siglen</strong> parameters. If <strong>rout</strong> is <strong>NULL</strong> then the maximum size of the output
buffer is written to the <strong>routlen</strong> parameter. If <strong>rout</strong> is not <strong>NULL</strong> then
before the call the <strong>routlen</strong> parameter should contain the length of the
<strong>rout</strong> buffer, if the call is successful recovered data is written to
<strong>rout</strong> and the amount of data written to <strong>routlen</strong>.</p>
<p>
</p>
<hr />
<h1><a name="notes">NOTES</a></h1>
<p>Normally an application is only interested in whether a signature verification
operation is successful in those cases the <code>EVP_verify()</code> function should be
used.</p>
<p>Sometimes however it is useful to obtain the data originally signed using a
signing operation. Only certain public key algorithms can recover a signature
in this way (for example RSA in PKCS padding mode).</p>
<p>After the call to <code>EVP_PKEY_verify_recover_init()</code> algorithm specific control
operations can be performed to set any appropriate parameters for the
operation.</p>
<p>The function <code>EVP_PKEY_verify_recover()</code> can be called more than once on the same
context if several operations are performed using the same parameters.</p>
<p>
</p>
<hr />
<h1><a name="return_values">RETURN VALUES</a></h1>
<p><code>EVP_PKEY_verify_recover_init()</code> and <code>EVP_PKEY_verify_recover()</code> return 1 for success
and 0 or a negative value for failure. In particular a return value of -2
indicates the operation is not supported by the public key algorithm.</p>
<p>
</p>
<hr />
<h1><a name="examples">EXAMPLES</a></h1>
<p>Recover digest originally signed using PKCS#1 and SHA256 digest:</p>
<pre>
 #include &lt;openssl/evp.h&gt;
 #include &lt;openssl/rsa.h&gt;</pre>
<pre>
 EVP_PKEY_CTX *ctx;
 unsigned char *rout, *sig;
 size_t routlen, siglen;
 EVP_PKEY *verify_key;</pre>
<pre>
 /*
  * NB: assumes verify_key, sig and siglen are already set up
  * and that verify_key is an RSA public key
  */
 ctx = EVP_PKEY_CTX_new(verify_key, NULL /* no engine */);
 if (!ctx)
     /* Error occurred */
 if (EVP_PKEY_verify_recover_init(ctx) &lt;= 0)
     /* Error */
 if (EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_PADDING) &lt;= 0)
     /* Error */
 if (EVP_PKEY_CTX_set_signature_md(ctx, EVP_sha256()) &lt;= 0)
     /* Error */</pre>
<pre>
 /* Determine buffer length */
 if (EVP_PKEY_verify_recover(ctx, NULL, &amp;routlen, sig, siglen) &lt;= 0)
     /* Error */</pre>
<pre>
 rout = OPENSSL_malloc(routlen);</pre>
<pre>
 if (!rout)
     /* malloc failure */</pre>
<pre>
 if (EVP_PKEY_verify_recover(ctx, rout, &amp;routlen, sig, siglen) &lt;= 0)
     /* Error */</pre>
<pre>
 /* Recovered data is routlen bytes written to buffer rout */</pre>
<p>
</p>
<hr />
<h1><a name="see_also">SEE ALSO</a></h1>
<p><em>EVP_PKEY_CTX_new(3)</em>,
<em>EVP_PKEY_encrypt(3)</em>,
<em>EVP_PKEY_decrypt(3)</em>,
<em>EVP_PKEY_sign(3)</em>,
<em>EVP_PKEY_verify(3)</em>,
<em>EVP_PKEY_derive(3)</em></p>
<p>
</p>
<hr />
<h1><a name="history">HISTORY</a></h1>
<p>These functions were added in OpenSSL 1.0.0.</p>
<p>
</p>
<hr />
<h1><a name="copyright">COPYRIGHT</a></h1>
<p>Copyright 2013-2019 The OpenSSL Project Authors. All Rights Reserved.</p>
<p>Licensed under the OpenSSL license (the &quot;License&quot;).  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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