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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="#RETURN-VALUES">RETURN VALUES</a></li>
  <li><a href="#NOTES">NOTES</a></li>
  <li><a href="#SEE-ALSO">SEE ALSO</a></li>
  <li><a href="#COPYRIGHT">COPYRIGHT</a></li>
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<h1 id="NAME">NAME</h1>

<p>BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size</p>

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

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

 int BN_num_bytes(const BIGNUM *a);

 int BN_num_bits(const BIGNUM *a);

 int BN_num_bits_word(BN_ULONG w);</code></pre>

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

<p>BN_num_bytes() returns the size of a <b>BIGNUM</b> in bytes.</p>

<p>BN_num_bits_word() returns the number of significant bits in a word. If we take 0x00000432 as an example, it returns 11, not 16, not 32. Basically, except for a zero, it returns <i>floor(log2(w))+1</i>.</p>

<p>BN_num_bits() returns the number of significant bits in a <b>BIGNUM</b>, following the same principle as BN_num_bits_word().</p>

<p>BN_num_bytes() is a macro.</p>

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

<p>The size.</p>

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

<p>Some have tried using BN_num_bits() on individual numbers in RSA keys, DH keys and DSA keys, and found that they don&#39;t always come up with the number of bits they expected (something like 512, 1024, 2048, ...). This is because generating a number with some specific number of bits doesn&#39;t always set the highest bits, thereby making the number of <i>significant</i> bits a little lower. If you want to know the &quot;key size&quot; of such a key, either use functions like RSA_size(), DH_size() and DSA_size(), or use BN_num_bytes() and multiply with 8 (although there&#39;s no real guarantee that will match the &quot;key size&quot;, just a lot more probability).</p>

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

<p><a href="../man3/DH_size.html">DH_size(3)</a>, <a href="../man3/DSA_size.html">DSA_size(3)</a>, <a href="../man3/RSA_size.html">RSA_size(3)</a></p>

<h1 id="COPYRIGHT">COPYRIGHT</h1>

<p>Copyright 2000-2017 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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