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/* $OpenBSD: ip_id.c,v 1.6 2002/03/15 18:19:52 millert Exp $ */
/*
* Copyright 1998 Niels Provos <provos@citi.umich.edu>
* 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.
* 3. All advertising materials mentioning features or use of this software
* must display the following acknowledgement:
* This product includes software developed by Niels Provos.
* 4. The name of the author may not be used to endorse or promote products
* derived from this software without specific prior written permission.
*
* 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.
*/
#include <sys/param.h>
#include <sys/kernel.h>
#include <dev/rndvar.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 u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t);
static void ip_initid(void);
u_int16_t ip_randomid(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);
}
/*
* Initalizes 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 id_randomid() when needed, an
* application does not have to worry about it.
*/
static void
ip_initid(void)
{
u_int16_t j, i;
int noprime = 1;
ru_x = ((tmp = arc4random()) & 0xFFFF) % RU_M;
/* 15 bits of random seed */
ru_seed = (tmp >> 16) & 0x7FFF;
ru_seed2 = arc4random() & 0x7FFF;
/* Determine the LCG we use */
ru_b = ((tmp = arc4random()) & 0xfffe) | 1;
ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
while (ru_b % 3 == 0)
ru_b += 2;
j = (tmp = arc4random()) % 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;
ru_reseed = time.tv_sec + RU_OUT;
ru_msb = ru_msb == 0x8000 ? 0 : 0x8000;
}
u_int16_t
ip_randomid(void)
{
int i, n;
if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed)
ip_initid();
if (!tmp)
tmp = arc4random();
/* Skip a random number of ids */
n = tmp & 0x3; tmp = tmp >> 2;
if (ru_counter + n >= RU_MAX)
ip_initid();
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;
}
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