ip_id.c revision 133874
177574Skris/* $OpenBSD: ip_id.c,v 1.2 1999/08/26 13:37:01 provos Exp $ */ 277574Skris 377574Skris/* 477574Skris * Copyright 1998 Niels Provos <provos@citi.umich.edu> 577574Skris * All rights reserved. 677574Skris * 777574Skris * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using 877574Skris * such a mathematical system to generate more random (yet non-repeating) 977574Skris * ids to solve the resolver/named problem. But Niels designed the 1077574Skris * actual system based on the constraints. 1177574Skris * 1277574Skris * Redistribution and use in source and binary forms, with or without 1377574Skris * modification, are permitted provided that the following conditions 1477574Skris * are met: 1577574Skris * 1. Redistributions of source code must retain the above copyright 1677574Skris * notice, this list of conditions and the following disclaimer. 1777574Skris * 2. Redistributions in binary form must reproduce the above copyright 1877574Skris * notice, this list of conditions and the following disclaimer in the 1977574Skris * documentation and/or other materials provided with the distribution. 2077574Skris * 3. All advertising materials mentioning features or use of this software 2177574Skris * must display the following acknowledgement: 2277574Skris * This product includes software developed by Niels Provos. 2377574Skris * 4. The name of the author may not be used to endorse or promote products 2477574Skris * derived from this software without specific prior written permission. 2577574Skris * 2677574Skris * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 2777574Skris * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 2877574Skris * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 2977574Skris * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 3077574Skris * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 3177574Skris * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 3277574Skris * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 3377574Skris * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 3477574Skris * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 3577574Skris * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 3677574Skris * 3777574Skris * $FreeBSD: head/sys/netinet/ip_id.c 133874 2004-08-16 18:32:07Z rwatson $ 3877574Skris */ 3977574Skris 40133874Srwatson/* 4177574Skris * seed = random 15bit 4277574Skris * n = prime, g0 = generator to n, 4377574Skris * j = random so that gcd(j,n-1) == 1 4477574Skris * g = g0^j mod n will be a generator again. 4577574Skris * 4677574Skris * X[0] = random seed. 4777574Skris * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator 48133874Srwatson * with a = 7^(even random) mod m, 4977574Skris * b = random with gcd(b,m) == 1 5077574Skris * m = 31104 and a maximal period of m-1. 5177574Skris * 5277574Skris * The transaction id is determined by: 5377574Skris * id[n] = seed xor (g^X[n] mod n) 5477574Skris * 5577574Skris * Effectivly the id is restricted to the lower 15 bits, thus 5677574Skris * yielding two different cycles by toggling the msb on and off. 5777574Skris * This avoids reuse issues caused by reseeding. 5877574Skris */ 5977574Skris 60126263Smlaier#include "opt_pf.h" 6177574Skris#include <sys/param.h> 6277574Skris#include <sys/time.h> 6377574Skris#include <sys/kernel.h> 6477574Skris#include <sys/random.h> 6577574Skris 6677574Skris#define RU_OUT 180 /* Time after wich will be reseeded */ 6777574Skris#define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */ 6877574Skris#define RU_GEN 2 /* Starting generator */ 6977574Skris#define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */ 7077574Skris#define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */ 7177574Skris#define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */ 7277574Skris 7377574Skris#define PFAC_N 3 7477574Skrisconst static u_int16_t pfacts[PFAC_N] = { 75133874Srwatson 2, 7677574Skris 3, 7777574Skris 2729 7877574Skris}; 7977574Skris 8077574Skrisstatic u_int16_t ru_x; 8177574Skrisstatic u_int16_t ru_seed, ru_seed2; 8277574Skrisstatic u_int16_t ru_a, ru_b; 8377574Skrisstatic u_int16_t ru_g; 8477574Skrisstatic u_int16_t ru_counter = 0; 8577574Skrisstatic u_int16_t ru_msb = 0; 8677574Skrisstatic long ru_reseed; 8777574Skrisstatic u_int32_t tmp; /* Storage for unused random */ 8877574Skris 8992723Salfredstatic u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t); 9092723Salfredstatic void ip_initid(void); 9192723Salfredu_int16_t ip_randomid(void); 9277574Skris 9377574Skris/* 9477574Skris * Do a fast modular exponation, returned value will be in the range 9577574Skris * of 0 - (mod-1) 9677574Skris */ 9777574Skris 9877574Skris#ifdef __STDC__ 9977574Skrisstatic u_int16_t 10077574Skrispmod(u_int16_t gen, u_int16_t exp, u_int16_t mod) 10177574Skris#else 10277574Skrisstatic u_int16_t 10377574Skrispmod(gen, exp, mod) 10477574Skris u_int16_t gen, exp, mod; 10577574Skris#endif 10677574Skris{ 10777574Skris u_int16_t s, t, u; 10877574Skris 10977574Skris s = 1; 11077574Skris t = gen; 11177574Skris u = exp; 11277574Skris 11377574Skris while (u) { 11477574Skris if (u & 1) 11577574Skris s = (s*t) % mod; 11677574Skris u >>= 1; 11777574Skris t = (t*t) % mod; 11877574Skris } 11977574Skris return (s); 12077574Skris} 12177574Skris 122133874Srwatson/* 123133874Srwatson * Initalizes the seed and chooses a suitable generator. Also toggles 12477574Skris * the msb flag. The msb flag is used to generate two distinct 12577574Skris * cycles of random numbers and thus avoiding reuse of ids. 12677574Skris * 127133874Srwatson * This function is called from id_randomid() when needed, an 12877574Skris * application does not have to worry about it. 12977574Skris */ 130133874Srwatsonstatic void 13177574Skrisip_initid(void) 13277574Skris{ 13377574Skris u_int16_t j, i; 13477574Skris int noprime = 1; 13577574Skris struct timeval time; 13677574Skris 13777574Skris getmicrotime(&time); 13877574Skris read_random((void *) &tmp, sizeof(tmp)); 13977574Skris ru_x = (tmp & 0xFFFF) % RU_M; 14077574Skris 14177574Skris /* 15 bits of random seed */ 14277574Skris ru_seed = (tmp >> 16) & 0x7FFF; 14377574Skris read_random((void *) &tmp, sizeof(tmp)); 14477574Skris ru_seed2 = tmp & 0x7FFF; 14577574Skris 14677574Skris read_random((void *) &tmp, sizeof(tmp)); 14777574Skris 14877574Skris /* Determine the LCG we use */ 14977574Skris ru_b = (tmp & 0xfffe) | 1; 15077574Skris ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M); 15177574Skris while (ru_b % 3 == 0) 15277574Skris ru_b += 2; 153133874Srwatson 15477574Skris read_random((void *) &tmp, sizeof(tmp)); 15577574Skris j = tmp % RU_N; 15677574Skris tmp = tmp >> 16; 15777574Skris 158133874Srwatson /* 15977574Skris * Do a fast gcd(j,RU_N-1), so we can find a j with 16077574Skris * gcd(j, RU_N-1) == 1, giving a new generator for 16177574Skris * RU_GEN^j mod RU_N 16277574Skris */ 16377574Skris 16477574Skris while (noprime) { 16577574Skris for (i=0; i<PFAC_N; i++) 16677574Skris if (j%pfacts[i] == 0) 16777574Skris break; 16877574Skris 16977574Skris if (i>=PFAC_N) 17077574Skris noprime = 0; 171133874Srwatson else 17277574Skris j = (j+1) % RU_N; 17377574Skris } 17477574Skris 17577574Skris ru_g = pmod(RU_GEN,j,RU_N); 17677574Skris ru_counter = 0; 17777574Skris 17877574Skris ru_reseed = time.tv_sec + RU_OUT; 179133874Srwatson ru_msb = ru_msb == 0x8000 ? 0 : 0x8000; 18077574Skris} 18177574Skris 18277574Skrisu_int16_t 18377574Skrisip_randomid(void) 18477574Skris{ 18577574Skris int i, n; 18677574Skris struct timeval time; 18777574Skris 188122267Ssam /* XXX not reentrant */ 18977574Skris getmicrotime(&time); 19077574Skris if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed) 19177574Skris ip_initid(); 19277574Skris 19377574Skris if (!tmp) 19477574Skris read_random((void *) &tmp, sizeof(tmp)); 19577574Skris 19677574Skris /* Skip a random number of ids */ 19777574Skris n = tmp & 0x3; tmp = tmp >> 2; 19877574Skris if (ru_counter + n >= RU_MAX) 19977574Skris ip_initid(); 20077574Skris 20177574Skris for (i = 0; i <= n; i++) 20277574Skris /* Linear Congruential Generator */ 20377574Skris ru_x = (ru_a*ru_x + ru_b) % RU_M; 20477574Skris 20577574Skris ru_counter += i; 20677574Skris 20777574Skris return (ru_seed ^ pmod(ru_g,ru_seed2 ^ ru_x,RU_N)) | ru_msb; 20877574Skris} 20977574Skris 210