197 lines
5.5 KiB
C
197 lines
5.5 KiB
C
/* $NetBSD: ip_id.c,v 1.2 2003/09/16 00:31:55 itojun Exp $ */
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/* $OpenBSD: ip_id.c,v 1.6 2002/03/15 18:19:52 millert Exp $ */
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/*
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* Copyright 1998 Niels Provos <provos@citi.umich.edu>
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* All rights reserved.
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*
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* Theo de Raadt <deraadt@openbsd.org> came up with the idea of using
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* such a mathematical system to generate more random (yet non-repeating)
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* ids to solve the resolver/named problem. But Niels designed the
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* actual system based on the constraints.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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* 3. All advertising materials mentioning features or use of this software
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* must display the following acknowledgement:
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* This product includes software developed by Niels Provos.
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* 4. The name of the author may not be used to endorse or promote products
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* derived from this software without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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/*
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* seed = random 15bit
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* n = prime, g0 = generator to n,
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* j = random so that gcd(j,n-1) == 1
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* g = g0^j mod n will be a generator again.
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*
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* X[0] = random seed.
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* X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
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* with a = 7^(even random) mod m,
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* b = random with gcd(b,m) == 1
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* m = 31104 and a maximal period of m-1.
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*
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* The transaction id is determined by:
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* id[n] = seed xor (g^X[n] mod n)
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*
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* Effectivly the id is restricted to the lower 15 bits, thus
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* yielding two different cycles by toggling the msb on and off.
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* This avoids reuse issues caused by reseeding.
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*/
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#include <sys/cdefs.h>
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__KERNEL_RCSID(0, "$NetBSD: ip_id.c,v 1.2 2003/09/16 00:31:55 itojun Exp $");
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#include <sys/types.h>
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#include <sys/param.h>
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#include <sys/kernel.h>
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#include <lib/libkern/libkern.h>
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#include <net/if.h>
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#include <netinet/in.h>
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#include <netinet/ip_var.h>
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#define RU_OUT 180 /* Time after wich will be reseeded */
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#define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */
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#define RU_GEN 2 /* Starting generator */
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#define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */
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#define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */
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#define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */
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#define PFAC_N 3
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const static u_int16_t pfacts[PFAC_N] = {
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2,
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3,
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2729
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};
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static u_int16_t ru_x;
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static u_int16_t ru_seed, ru_seed2;
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static u_int16_t ru_a, ru_b;
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static u_int16_t ru_g;
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static u_int16_t ru_counter = 0;
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static u_int16_t ru_msb = 0;
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static long ru_reseed;
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static u_int32_t tmp; /* Storage for unused random */
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static u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t);
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static void ip_initid(void);
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/*
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* Do a fast modular exponation, returned value will be in the range
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* of 0 - (mod-1)
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*/
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static u_int16_t
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pmod(u_int16_t gen, u_int16_t expo, u_int16_t mod)
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{
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u_int16_t s, t, u;
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s = 1;
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t = gen;
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u = expo;
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while (u) {
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if (u & 1)
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s = (s * t) % mod;
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u >>= 1;
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t = (t * t) % mod;
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}
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return (s);
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}
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/*
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* Initalizes the seed and chooses a suitable generator. Also toggles
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* the msb flag. The msb flag is used to generate two distinct
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* cycles of random numbers and thus avoiding reuse of ids.
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*
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* This function is called from id_randomid() when needed, an
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* application does not have to worry about it.
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*/
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static void
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ip_initid(void)
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{
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u_int16_t j, i;
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int noprime = 1;
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ru_x = ((tmp = arc4random()) & 0xFFFF) % RU_M;
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/* 15 bits of random seed */
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ru_seed = (tmp >> 16) & 0x7FFF;
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ru_seed2 = arc4random() & 0x7FFF;
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/* Determine the LCG we use */
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ru_b = ((tmp = arc4random()) & 0xfffe) | 1;
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ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
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while (ru_b % 3 == 0)
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ru_b += 2;
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j = (tmp = arc4random()) % RU_N;
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tmp = tmp >> 16;
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/*
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* Do a fast gcd(j,RU_N-1), so we can find a j with
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* gcd(j, RU_N-1) == 1, giving a new generator for
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* RU_GEN^j mod RU_N
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*/
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while (noprime) {
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for (i = 0; i < PFAC_N; i++)
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if (j % pfacts[i] == 0)
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break;
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if (i >= PFAC_N)
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noprime = 0;
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else
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j = (j + 1) % RU_N;
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}
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ru_g = pmod(RU_GEN, j, RU_N);
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ru_counter = 0;
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ru_reseed = time.tv_sec + RU_OUT;
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ru_msb = ru_msb == 0x8000 ? 0 : 0x8000;
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}
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u_int16_t
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ip_randomid(void)
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{
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int i, n;
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if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed)
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ip_initid();
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if (!tmp)
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tmp = arc4random();
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/* Skip a random number of ids */
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n = tmp & 0x3; tmp = tmp >> 2;
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if (ru_counter + n >= RU_MAX)
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ip_initid();
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for (i = 0; i <= n; i++)
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/* Linear Congruential Generator */
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ru_x = (ru_a * ru_x + ru_b) % RU_M;
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ru_counter += i;
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return (ru_seed ^ pmod(ru_g, ru_seed2 ^ ru_x,RU_N)) | ru_msb;
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}
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