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.
1 April 2002 IPSEC_PRNG_INIT(3)