98 lines
3.2 KiB
HTML
Executable File
98 lines
3.2 KiB
HTML
Executable File
<?xml version="1.0" ?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<title>BN_num_bytes</title>
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<meta http-equiv="content-type" content="text/html; charset=utf-8" />
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<link rev="made" href="mailto:root@localhost" />
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</head>
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<body style="background-color: white">
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<!-- INDEX BEGIN -->
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<div name="index">
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<p><a name="__index__"></a></p>
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<ul>
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<li><a href="#name">NAME</a></li>
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<li><a href="#synopsis">SYNOPSIS</a></li>
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<li><a href="#description">DESCRIPTION</a></li>
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<li><a href="#return_values">RETURN VALUES</a></li>
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<li><a href="#notes">NOTES</a></li>
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<li><a href="#see_also">SEE ALSO</a></li>
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<li><a href="#copyright">COPYRIGHT</a></li>
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</ul>
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<hr name="index" />
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</div>
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<!-- INDEX END -->
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<p>
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</p>
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<hr />
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<h1><a name="name">NAME</a></h1>
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<p>BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size</p>
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<p>
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</p>
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<hr />
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<h1><a name="synopsis">SYNOPSIS</a></h1>
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<pre>
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#include <openssl/bn.h></pre>
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<pre>
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int BN_num_bytes(const BIGNUM *a);</pre>
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<pre>
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int BN_num_bits(const BIGNUM *a);</pre>
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<pre>
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int BN_num_bits_word(BN_ULONG w);</pre>
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<p>
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</p>
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<hr />
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<h1><a name="description">DESCRIPTION</a></h1>
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<p><code>BN_num_bytes()</code> returns the size of a <strong>BIGNUM</strong> in bytes.</p>
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<p><code>BN_num_bits_word()</code> returns the number of significant bits in a word.
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If we take 0x00000432 as an example, it returns 11, not 16, not 32.
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Basically, except for a zero, it returns <em>floor(log2(w))+1</em>.</p>
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<p><code>BN_num_bits()</code> returns the number of significant bits in a <strong>BIGNUM</strong>,
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following the same principle as <code>BN_num_bits_word()</code>.</p>
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<p><code>BN_num_bytes()</code> is a macro.</p>
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<p>
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</p>
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<hr />
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<h1><a name="return_values">RETURN VALUES</a></h1>
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<p>The size.</p>
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<p>
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</p>
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<hr />
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<h1><a name="notes">NOTES</a></h1>
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<p>Some have tried using <code>BN_num_bits()</code> on individual numbers in RSA keys,
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DH keys and DSA keys, and found that they don't always come up with
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the number of bits they expected (something like 512, 1024, 2048,
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...). This is because generating a number with some specific number
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of bits doesn't always set the highest bits, thereby making the number
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of <em>significant</em> bits a little lower. If you want to know the "key
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size" of such a key, either use functions like <code>RSA_size()</code>, <code>DH_size()</code>
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and <code>DSA_size()</code>, or use <code>BN_num_bytes()</code> and multiply with 8 (although
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there's no real guarantee that will match the "key size", just a lot
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more probability).</p>
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<p>
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</p>
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<hr />
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<h1><a name="see_also">SEE ALSO</a></h1>
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<p><em>DH_size(3)</em>, <em>DSA_size(3)</em>,
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<em>RSA_size(3)</em></p>
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<p>
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</p>
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<hr />
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<h1><a name="copyright">COPYRIGHT</a></h1>
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<p>Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.</p>
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<p>Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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<a href="https://www.openssl.org/source/license.html">https://www.openssl.org/source/license.html</a>.</p>
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</body>
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</html>
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