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