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/*
* Portions Copyright (C) 2002 Internet Software Consortium.
* Portions Copyright (C) 1997 Niels Provos.
*
* Permission to use, copy, modify, and distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND INTERNET SOFTWARE CONSORTIUM
* DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL
* INTERNET SOFTWARE CONSORTIUM BE LIABLE FOR ANY SPECIAL, DIRECT,
* INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING
* FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
* NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION
* WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
/* $OpenBSD: lcg.c,v 1.2 2004/09/28 17:14:07 jakob Exp $ */
#include <config.h>
#include <stdlib.h>
#include <isc/lcg.h>
#include <isc/random.h>
#include <isc/time.h>
#include <isc/util.h>
#define VALID_LCG(x) (x != NULL)
#define RU_OUT 180 /* Time after wich will be reseeded */
#define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */
#define RU_GEN 2 /* Starting generator */
#define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */
#define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */
#define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */
#define PFAC_N 3
static const isc_uint16_t pfacts[PFAC_N] = {
2,
3,
2729
};
/*
* Do a fast modular exponation, returned value will be in the range
* of 0 - (mod-1)
*/
static isc_uint16_t
pmod(isc_uint16_t gen, isc_uint16_t exp, isc_uint16_t mod)
{
isc_uint16_t s, t, u;
s = 1;
t = gen;
u = exp;
while (u) {
if (u & 1)
s = (s*t) % mod;
u >>= 1;
t = (t*t) % mod;
}
return (s);
}
/*
* Initializes the seed and chooses a suitable generator. Also toggles
* the msb flag. The msb flag is used to generate two distinct
* cycles of random numbers and thus avoiding reuse of ids.
*
* This function is called from isc_lcg_generate() when needed, an
* application does not have to worry about it.
*/
static void
reseed(isc_lcg_t *lcg)
{
isc_time_t isctime;
isc_boolean_t noprime = ISC_TRUE;
isc_uint16_t j, i;
isc_random_get(&lcg->random);
lcg->ru_x = (lcg->random & 0xFFFF) % RU_M;
/* 15 bits of random seed */
lcg->ru_seed = (lcg->random >> 16) & 0x7FFF;
isc_random_get(&lcg->random);
lcg->ru_seed2 = lcg->random & 0x7FFF;
isc_random_get(&lcg->random);
/* Determine the LCG we use */
lcg->ru_b = (lcg->random & 0xfffe) | 1;
lcg->ru_a = pmod(RU_AGEN, (lcg->random >> 16) & 0xfffe, RU_M);
while (lcg->ru_b % 3 == 0)
lcg->ru_b += 2;
isc_random_get(&lcg->random);
j = lcg->random % RU_N;
lcg->random = lcg->random >> 16;
/*
* Do a fast gcd(j,RU_N-1), so we can find a j with
* gcd(j, RU_N-1) == 1, giving a new generator for
* RU_GEN^j mod RU_N
*/
while (noprime == ISC_TRUE) {
for (i=0; i<PFAC_N; i++)
if (j % pfacts[i] == 0)
break;
if (i >= PFAC_N)
noprime = ISC_FALSE;
else
j = (j+1) % RU_N;
}
lcg->ru_g = pmod(RU_GEN, j, RU_N);
lcg->ru_counter = 0;
isc_time_now(&isctime);
lcg->ru_reseed = isc_time_seconds(&isctime) + RU_OUT;
lcg->ru_msb = lcg->ru_msb == 0x8000 ? 0 : 0x8000;
}
void
isc_lcg_init(isc_lcg_t *lcg)
{
REQUIRE(VALID_LCG(lcg));
lcg->ru_x = 0;
lcg->ru_seed = 0;
lcg->ru_seed2 = 0;
lcg->ru_a = 0;
lcg->ru_b = 0;
lcg->ru_g = 0;
lcg->ru_counter = 0;
lcg->ru_msb = 0;
lcg->ru_reseed = 0;
lcg->random = 0;
}
isc_uint16_t
isc_lcg_generate16(isc_lcg_t *lcg)
{
isc_time_t isctime;
int i, n;
REQUIRE(VALID_LCG(lcg));
isc_time_now(&isctime);
if (lcg->ru_counter >= RU_MAX ||
isc_time_seconds(&isctime) > lcg->ru_reseed)
reseed(lcg);
if (! lcg->random)
isc_random_get(&lcg->random);
/* Skip a random number of ids */
n = lcg->random & 0x7; lcg->random = lcg->random >> 3;
if (lcg->ru_counter + n >= RU_MAX)
reseed(lcg);
for (i=0; i<=n; i++)
/* Linear Congruential Generator */
lcg->ru_x = (lcg->ru_a*lcg->ru_x + lcg->ru_b) % RU_M;
lcg->ru_counter += i;
return (lcg->ru_seed ^
pmod(lcg->ru_g, lcg->ru_seed2 ^ lcg->ru_x, RU_N)) | lcg->ru_msb;
}
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