/* $OpenBSD: ip_id.c,v 1.1 1998/12/26 12:35:12 provos Exp $ */ /* * Copyright 1998 Niels Provos * All rights reserved. * * Theo de Raadt 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 #include #include #include #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; 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 __P((u_int16_t, u_int16_t, u_int16_t)); static void ip_initid __P((void)); u_int16_t ip_randomid __P((void)); /* * Do a fast modular exponation, returned value will be in the range * of 0 - (mod-1) */ #ifdef __STDC__ static u_int16_t pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod) #else static u_int16_t pmod(gen, exp, mod) u_int16_t gen, exp, mod; #endif { 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; get_random_bytes((void *) &tmp, sizeof(tmp)); ru_x = (tmp & 0xFFFF) % RU_M; /* 15 bits of random seed */ ru_seed = (tmp >> 16) & 0x7FFF; get_random_bytes((void *) &tmp, sizeof(tmp)); /* 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; get_random_bytes((void *) &tmp, sizeof(tmp)); 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) 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) get_random_bytes((void *) &tmp, sizeof(tmp)); /* 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_x,RU_N)) | ru_msb; }