IPSEC_PRNG(3)

NAME
ipsec prng_init - initialize IPsec pseudorandom-number generator
ipsec prng_bytes - get bytes from IPsec pseudorandom-number generator
ipsec prng_final - close down IPsec pseudorandom-number generator

SYNOPSIS
#include <freeswan.h>

void prng_init(struct prng *prng,
const unsigned char *key, size_t keylen);
void prng_bytes(struct prng *prng, char *dst,
size_t dstlen);
unsigned long prng_count(struct prng *prng);
void prng_final(struct prng *prng);

DESCRIPTION
Prng_init initializes a crypto-quality pseudo-random-number generator from a key; prng_bytes obtains pseudo-random bytes from it; prng_count reports the number of bytes extracted from it to date; 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.

Prng_init initializes, or re-initializes, the specified prng from the key, whose length is given by keylen. The user must allocate the struct prng pointed to by 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.

Prng_bytes obtains dstlen pseudo-random bytes from the PRNG and puts them in buf. This is quite fast, on the order of 10 integer operations per byte.

Prng_count reports the number of bytes obtained from the PRNG since it was (last) initialized.

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.

The <freeswan.h> header file supplies the definition of the prng structure. Examination of its innards is discouraged, as they may change.

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 random(4)). The usual run of software pseudo-random-number generators (e.g. random(3)) are not cryptographically strong.

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.

SEE ALSO
random(3), random(4)
Bruce Schneier, Applied Cryptography, 2nd ed., 1996, ISBN 0-471-11709-9, pp. 397-8.

HISTORY
Written for the FreeS/WAN project by Henry Spencer.

BUGS
If an attempt is made to obtain more than 4e9 bytes between initializations, the PRNG will continue to work but prng_count's output will stick at 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...

``RC4'' is a trademark of RSA Data Security, Inc.