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<ul id="index">
  <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>

<h1 id="NAME">NAME</h1>

<p>EVP_PKEY_verify_recover_init, EVP_PKEY_verify_recover - recover signature using a public key algorithm</p>

<h1 id="SYNOPSIS">SYNOPSIS</h1>

<pre><code> #include &lt;openssl/evp.h&gt;

 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);</code></pre>

<h1 id="DESCRIPTION">DESCRIPTION</h1>

<p>The EVP_PKEY_verify_recover_init() function initializes a public key algorithm context using key <b>pkey</b> for a verify recover operation.</p>

<p>The EVP_PKEY_verify_recover() function recovers signed data using <b>ctx</b>. The signature is specified using the <b>sig</b> and <b>siglen</b> parameters. If <b>rout</b> is <b>NULL</b> then the maximum size of the output buffer is written to the <b>routlen</b> parameter. If <b>rout</b> is not <b>NULL</b> then before the call the <b>routlen</b> parameter should contain the length of the <b>rout</b> buffer, if the call is successful recovered data is written to <b>rout</b> and the amount of data written to <b>routlen</b>.</p>

<h1 id="NOTES">NOTES</h1>

<p>Normally an application is only interested in whether a signature verification operation is successful in those cases the EVP_verify() 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 EVP_PKEY_verify_recover_init() algorithm specific control operations can be performed to set any appropriate parameters for the operation.</p>

<p>The function EVP_PKEY_verify_recover() can be called more than once on the same context if several operations are performed using the same parameters.</p>

<h1 id="RETURN-VALUES">RETURN VALUES</h1>

<p>EVP_PKEY_verify_recover_init() and EVP_PKEY_verify_recover() 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>

<h1 id="EXAMPLES">EXAMPLES</h1>

<p>Recover digest originally signed using PKCS#1 and SHA256 digest:</p>

<pre><code> #include &lt;openssl/evp.h&gt;
 #include &lt;openssl/rsa.h&gt;

 EVP_PKEY_CTX *ctx;
 unsigned char *rout, *sig;
 size_t routlen, siglen;
 EVP_PKEY *verify_key;

 /*
  * 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 */

 /* Determine buffer length */
 if (EVP_PKEY_verify_recover(ctx, NULL, &amp;routlen, sig, siglen) &lt;= 0)
     /* Error */

 rout = OPENSSL_malloc(routlen);

 if (!rout)
     /* malloc failure */

 if (EVP_PKEY_verify_recover(ctx, rout, &amp;routlen, sig, siglen) &lt;= 0)
     /* Error */

 /* Recovered data is routlen bytes written to buffer rout */</code></pre>

<h1 id="SEE-ALSO">SEE ALSO</h1>

<p><a href="../man3/EVP_PKEY_CTX_new.html">EVP_PKEY_CTX_new(3)</a>, <a href="../man3/EVP_PKEY_encrypt.html">EVP_PKEY_encrypt(3)</a>, <a href="../man3/EVP_PKEY_decrypt.html">EVP_PKEY_decrypt(3)</a>, <a href="../man3/EVP_PKEY_sign.html">EVP_PKEY_sign(3)</a>, <a href="../man3/EVP_PKEY_verify.html">EVP_PKEY_verify(3)</a>, <a href="../man3/EVP_PKEY_derive.html">EVP_PKEY_derive(3)</a></p>

<h1 id="HISTORY">HISTORY</h1>

<p>These functions were added in OpenSSL 1.0.0.</p>

<h1 id="COPYRIGHT">COPYRIGHT</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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