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.TH IPSEC_PRNG 3 "1 April 2002"
.\" RCSID $Id: prng.3,v 1.1 2004/03/15 20:35:26 as Exp $
.SH NAME
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
.SH SYNOPSIS
.B "#include <freeswan.h>
.sp
.B "void prng_init(struct prng *prng,"
.ti +1c
.B "const unsigned char *key, size_t keylen);"
.br
.B "void prng_bytes(struct prng *prng, char *dst,"
.ti +1c
.B "size_t dstlen);"
.br
.B "unsigned long prng_count(struct prng *prng);"
.br
.B "void prng_final(struct prng *prng);"
.SH DESCRIPTION
.I Prng_init
initializes a crypto-quality pseudo-random-number generator from a key;
.I prng_bytes
obtains pseudo-random bytes from it;
.I prng_count
reports the number of bytes extracted from it to date;
.I prng_final
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.
.PP
.I Prng_init
initializes,
or re-initializes,
the specified
.I prng
from the
.IR key ,
whose length is given by
.IR keylen .
The user must allocate the
.B "struct prng"
pointed to by
.IR prng .
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.
.PP
.I Prng_bytes
obtains
.I dstlen
pseudo-random bytes from the PRNG and puts them in
.IR buf .
This is quite fast,
on the order of 10 integer operations per byte.
.PP
.I Prng_count
reports the number of bytes obtained from the PRNG
since it was (last) initialized.
.PP
.I Prng_final
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.
.PP
The
.B <freeswan.h>
header file supplies the definition of the
.B prng
structure.
Examination of its innards is discouraged, as they may change.
.PP
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
.IR random (4)).
The usual run of software pseudo-random-number generators
(e.g.
.IR random (3))
are
.I not
cryptographically strong.
.PP
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.
.SH SEE ALSO
random(3), random(4)
.br
Bruce Schneier,
\fIApplied Cryptography\fR, 2nd ed., 1996, ISBN 0-471-11709-9,
pp. 397-8.
.SH HISTORY
Written for the FreeS/WAN project by Henry Spencer.
.SH BUGS
If an attempt is made to obtain more than 4e9 bytes
between initializations,
the PRNG will continue to work but
.IR prng_count 's
output will stick at
.BR 4000000000 .
Fixing this would require a longer integer type and does
not seem worth the trouble,
since you should probably re-initialize before then anyway...
.PP
``RC4'' is a trademark of RSA Data Security, Inc.
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