- import of strongswan-2.7.0
- applied patch for charon
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Martin Willi
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<HTML><HEAD><TITLE>Manpage of IPSEC_PRNG</TITLE>
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</HEAD><BODY>
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<H1>IPSEC_PRNG</H1>
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Section: C Library Functions (3)<BR>Updated: 1 April 2002<BR><A HREF="#index">Index</A>
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<A HREF="http://localhost/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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ipsec prng_init - initialize IPsec pseudorandom-number generator
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<BR>
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ipsec prng_bytes - get bytes from IPsec pseudorandom-number generator
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<BR>
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ipsec prng_final - close down IPsec pseudorandom-number generator
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>#include <<A HREF="file:/usr/include/freeswan.h">freeswan.h</A>></B>
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<P>
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<B>void prng_init(struct prng *prng,</B>
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<BR>
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<B>const unsigned char *key, size_t keylen);</B>
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<BR>
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<B>void prng_bytes(struct prng *prng, char *dst,</B>
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<BR>
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<B>size_t dstlen);</B>
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<BR>
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<B>unsigned long prng_count(struct prng *prng);</B>
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<BR>
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<B>void prng_final(struct prng *prng);</B>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<I>Prng_init</I>
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initializes a crypto-quality pseudo-random-number generator from a key;
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<I>prng_bytes</I>
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obtains pseudo-random bytes from it;
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<I>prng_count</I>
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reports the number of bytes extracted from it to date;
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<I>prng_final</I>
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closes it down.
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It is the user's responsibility to initialize a PRNG before using it,
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and not to use it again after it is closed down.
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<P>
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<I>Prng_init</I>
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initializes,
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or re-initializes,
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the specified
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<I>prng</I>
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from the
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<I>key</I>,
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whose length is given by
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<I>keylen</I>.
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The user must allocate the
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<B>struct prng</B>
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pointed to by
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<I>prng</I>.
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There is no particular constraint on the length of the key,
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although a key longer than 256 bytes is unnecessary because
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only the first 256 would be used.
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Initialization requires on the order of 3000 integer operations,
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independent of key length.
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<P>
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<I>Prng_bytes</I>
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obtains
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<I>dstlen</I>
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pseudo-random bytes from the PRNG and puts them in
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<I>buf</I>.
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This is quite fast,
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on the order of 10 integer operations per byte.
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<P>
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<I>Prng_count</I>
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reports the number of bytes obtained from the PRNG
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since it was (last) initialized.
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<P>
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<I>Prng_final</I>
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closes down a PRNG by
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zeroing its internal memory,
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obliterating all trace of the state used to generate its previous output.
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This requires on the order of 250 integer operations.
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<P>
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The
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<B><<A HREF="file:/usr/include/freeswan.h">freeswan.h</A>></B>
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header file supplies the definition of the
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<B>prng</B>
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structure.
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Examination of its innards is discouraged, as they may change.
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<P>
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The PRNG algorithm
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used by these functions is currently identical to that of RC4(TM).
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This algorithm is cryptographically strong,
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sufficiently unpredictable that even a hostile observer will
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have difficulty determining the next byte of output from past history,
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provided it is initialized from a reasonably large key composed of
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highly random bytes (see
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<I><A HREF="random.4.html">random</A></I>(4)).
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The usual run of software pseudo-random-number generators
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(e.g.
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<I><A HREF="random.3.html">random</A></I>(3))
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are
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<I>not</I>
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cryptographically strong.
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<P>
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The well-known attacks against RC4(TM),
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e.g. as found in 802.11b's WEP encryption system,
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apply only if multiple PRNGs are initialized with closely-related keys
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(e.g., using a counter appended to a base key).
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If such keys are used, the first few hundred pseudo-random bytes
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from each PRNG should be discarded,
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to give the PRNGs a chance to randomize their innards properly.
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No useful attacks are known if the key is well randomized to begin with.
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<A NAME="lbAE"> </A>
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<H2>SEE ALSO</H2>
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<A HREF="random.3.html">random</A>(3), <A HREF="random.4.html">random</A>(4)
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<BR>
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Bruce Schneier,
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<I>Applied Cryptography</I>, 2nd ed., 1996, ISBN 0-471-11709-9,
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pp. 397-8.
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<A NAME="lbAF"> </A>
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<H2>HISTORY</H2>
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Written for the FreeS/WAN project by Henry Spencer.
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<A NAME="lbAG"> </A>
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<H2>BUGS</H2>
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If an attempt is made to obtain more than 4e9 bytes
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between initializations,
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the PRNG will continue to work but
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<I>prng_count</I>'s
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output will stick at
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<B>4000000000</B>.
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Fixing this would require a longer integer type and does
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not seem worth the trouble,
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since you should probably re-initialize before then anyway...
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<P>
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``RC4'' is a trademark of RSA Data Security, Inc.
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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><A HREF="#lbAB">NAME</A><DD>
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<DT><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT><A HREF="#lbAE">SEE ALSO</A><DD>
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<DT><A HREF="#lbAF">HISTORY</A><DD>
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<DT><A HREF="#lbAG">BUGS</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="http://localhost/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 21:40:18 GMT, November 11, 2003
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