120 lines
5.3 KiB
HTML
120 lines
5.3 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of RAND</TITLE>
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</HEAD><BODY>
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<H1>RAND</H1>
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Section: OpenSSL (7SSL)<BR>Updated: 2021-03-22<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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RAND - the OpenSSL random generator
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<A NAME="lbAC"> </A>
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<H2>DESCRIPTION</H2>
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Random numbers are a vital part of cryptography, they are needed to provide
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unpredictability for tasks like key generation, creating salts, and many more.
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Software-based generators must be seeded with external randomness before they
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can be used as a cryptographically-secure pseudo-random number generator
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(<FONT SIZE="-1">CSPRNG</FONT>).
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The availability of common hardware with special instructions and
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modern operating systems, which may use items such as interrupt jitter
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and network packet timings, can be reasonable sources of seeding material.
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<P>
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OpenSSL comes with a default implementation of the <FONT SIZE="-1">RAND API</FONT> which is based on
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the deterministic random bit generator (<FONT SIZE="-1">DRBG</FONT>) model as described in
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[<FONT SIZE="-1">NIST SP 800-90A</FONT> Rev. 1]. The default random generator will initialize
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automatically on first use and will be fully functional without having
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to be initialized ('seeded') explicitly.
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It seeds and reseeds itself automatically using trusted random sources
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provided by the operating system.
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<P>
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As a normal application developer, you do not have to worry about any details,
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just use <B><A HREF="/cgi-bin/man/man2html?3+RAND_bytes">RAND_bytes</A></B>(3) to obtain random data.
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Having said that, there is one important rule to obey: Always check the error
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return value of <B><A HREF="/cgi-bin/man/man2html?3+RAND_bytes">RAND_bytes</A></B>(3) and do not take randomness for granted.
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Although (re-)seeding is automatic, it can fail because no trusted random source
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is available or the trusted source(s) temporarily fail to provide sufficient
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random seed material.
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In this case the <FONT SIZE="-1">CSPRNG</FONT> enters an error state and ceases to provide output,
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until it is able to recover from the error by reseeding itself.
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For more details on reseeding and error recovery, see <FONT SIZE="-1"><B><A HREF="/cgi-bin/man/man2html?7+RAND_DRBG">RAND_DRBG</A></B></FONT><B></B>(7).
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<P>
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For values that should remain secret, you can use <B><A HREF="/cgi-bin/man/man2html?3+RAND_priv_bytes">RAND_priv_bytes</A></B>(3)
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instead.
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This method does not provide 'better' randomness, it uses the same type of <FONT SIZE="-1">CSPRNG.</FONT>
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The intention behind using a dedicated <FONT SIZE="-1">CSPRNG</FONT> exclusively for private
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values is that none of its output should be visible to an attacker (e.g.,
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used as salt value), in order to reveal as little information as
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possible about its internal state, and that a compromise of the ``public''
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<FONT SIZE="-1">CSPRNG</FONT> instance will not affect the secrecy of these private values.
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<P>
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In the rare case where the default implementation does not satisfy your special
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requirements, there are two options:
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<DL COMPACT>
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<DT id="1">•<DD>
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Replace the default <FONT SIZE="-1">RAND</FONT> method by your own <FONT SIZE="-1">RAND</FONT> method using
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<B><A HREF="/cgi-bin/man/man2html?3+RAND_set_rand_method">RAND_set_rand_method</A></B>(3).
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<DT id="2">•<DD>
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Modify the default settings of the OpenSSL <FONT SIZE="-1">RAND</FONT> method by modifying the security
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parameters of the underlying <FONT SIZE="-1">DRBG,</FONT> which is described in detail in <FONT SIZE="-1"><B><A HREF="/cgi-bin/man/man2html?7+RAND_DRBG">RAND_DRBG</A></B></FONT><B></B>(7).
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</DL>
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<P>
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Changing the default random generator or its default parameters should be necessary
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only in exceptional cases and is not recommended, unless you have a profound knowledge
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of cryptographic principles and understand the implications of your changes.
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<A NAME="lbAD"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?3+RAND_add">RAND_add</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+RAND_bytes">RAND_bytes</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+RAND_priv_bytes">RAND_priv_bytes</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+RAND_get_rand_method">RAND_get_rand_method</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+RAND_set_rand_method">RAND_set_rand_method</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+RAND_OpenSSL">RAND_OpenSSL</A></B>(3),
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<FONT SIZE="-1"><B><A HREF="/cgi-bin/man/man2html?7+RAND_DRBG">RAND_DRBG</A></B></FONT><B></B>(7)
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<A NAME="lbAE"> </A>
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<H2>COPYRIGHT</H2>
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Copyright 2018-2019 The OpenSSL Project Authors. All Rights Reserved.
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<P>
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Licensed under the OpenSSL license (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 <FONT SIZE="-1">LICENSE</FONT> 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>>.
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="3"><A HREF="#lbAB">NAME</A><DD>
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<DT id="4"><A HREF="#lbAC">DESCRIPTION</A><DD>
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<DT id="5"><A HREF="#lbAD">SEE ALSO</A><DD>
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<DT id="6"><A HREF="#lbAE">COPYRIGHT</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:06:09 GMT, March 31, 2021
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</BODY>
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</HTML>
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