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/* $OpenBSD: res_random.c,v 1.16 2005/03/25 13:24:12 otto Exp $ */
/*
* Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
* All rights reserved.
*
* Theo de Raadt <deraadt@openbsd.org> came up with the idea of using
* such a mathematical system to generate more random (yet non-repeating)
* ids to solve the resolver/named problem. But Niels designed the
* actual system based on the constraints.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
/*
* seed = random 15bit
* n = prime, g0 = generator to n,
* j = random so that gcd(j,n-1) == 1
* g = g0^j mod n will be a generator again.
*
* X[0] = random seed.
* X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
* with a = 7^(even random) mod m,
* b = random with gcd(b,m) == 1
* m = 31104 and a maximal period of m-1.
*
* The transaction id is determined by:
* id[n] = seed xor (g^X[n] mod n)
*
* Effectivly the id is restricted to the lower 15 bits, thus
* yielding two different cycles by toggling the msb on and off.
* This avoids reuse issues caused by reseeding.
*
* The 16 bit space is very small and brute force attempts are
* entirly feasible, we skip a random number of transaction ids
* so that an attacker will not get sequential ids.
*/
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/time.h>
#include <resolv.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#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
const static u_int16_t pfacts[PFAC_N] = {
2,
3,
2729
};
static u_int16_t ru_x;
static u_int16_t ru_seed, ru_seed2;
static u_int16_t ru_a, ru_b;
static u_int16_t ru_g;
static u_int16_t ru_counter = 0;
static u_int16_t ru_msb = 0;
static long ru_reseed;
static u_int32_t tmp; /* Storage for unused random */
static struct timeval tv;
static u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t);
static void res_initid(void);
/*
* Do a fast modular exponation, returned value will be in the range
* of 0 - (mod-1)
*/
static u_int16_t
pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod)
{
u_int16_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 res_randomid() when needed, an
* application does not have to worry about it.
*/
static void
res_initid(void)
{
u_int16_t j, i;
int noprime = 1;
tmp = arc4random();
ru_x = (tmp & 0xFFFF) % RU_M;
/* 15 bits of random seed */
ru_seed = (tmp >> 16) & 0x7FFF;
tmp = arc4random();
ru_seed2 = tmp & 0x7FFF;
tmp = arc4random();
/* Determine the LCG we use */
ru_b = (tmp & 0xfffe) | 1;
ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
while (ru_b % 3 == 0)
ru_b += 2;
tmp = arc4random();
j = tmp % RU_N;
tmp = tmp >> 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) {
for (i = 0; i < PFAC_N; i++)
if (j % pfacts[i] == 0)
break;
if (i >= PFAC_N)
noprime = 0;
else
j = (j + 1) % RU_N;
}
ru_g = pmod(RU_GEN, j, RU_N);
ru_counter = 0;
gettimeofday(&tv, NULL);
ru_reseed = tv.tv_sec + RU_OUT;
ru_msb = ru_msb == 0x8000 ? 0 : 0x8000;
}
u_int
res_randomid(void)
{
int i, n;
gettimeofday(&tv, NULL);
if (ru_counter >= RU_MAX || tv.tv_sec > ru_reseed)
res_initid();
#if 0
if (!tmp)
tmp = arc4random();
/* Skip a random number of ids */
n = tmp & 0x7; tmp = tmp >> 3;
if (ru_counter + n >= RU_MAX)
res_initid();
#else
n = 0;
#endif
for (i = 0; i <= n; i++)
/* Linear Congruential Generator */
ru_x = (ru_a * ru_x + ru_b) % RU_M;
ru_counter += i;
return (ru_seed ^ pmod(ru_g, ru_seed2 + ru_x, RU_N)) | ru_msb;
}
#if 0
void
main(int argc, char **argv)
{
int i, n;
u_int16_t wert;
res_initid();
printf("Generator: %u\n", ru_g);
printf("Seed: %u\n", ru_seed);
printf("Reseed at %ld\n", ru_reseed);
printf("Ru_X: %u\n", ru_x);
printf("Ru_A: %u\n", ru_a);
printf("Ru_B: %u\n", ru_b);
n = atoi(argv[1]);
for (i=0;i<n;i++) {
wert = res_randomid();
printf("%06d\n", wert);
}
}
#endif
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