1 /* $OpenBSD: ip_id.c,v 1.2 1999/08/26 13:37:01 provos Exp $ */ 2 3 /*- 4 * Copyright 1998 Niels Provos <provos@citi.umich.edu> 5 * All rights reserved. 6 * 7 * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using 8 * such a mathematical system to generate more random (yet non-repeating) 9 * ids to solve the resolver/named problem. But Niels designed the 10 * actual system based on the constraints. 11 * 12 * Redistribution and use in source and binary forms, with or without 13 * modification, are permitted provided that the following conditions 14 * are met: 15 * 1. Redistributions of source code must retain the above copyright 16 * notice, this list of conditions and the following disclaimer. 17 * 2. Redistributions in binary form must reproduce the above copyright 18 * notice, this list of conditions and the following disclaimer in the 19 * documentation and/or other materials provided with the distribution. 20 * 3. All advertising materials mentioning features or use of this software 21 * must display the following acknowledgement: 22 * This product includes software developed by Niels Provos. 23 * 4. The name of the author may not be used to endorse or promote products 24 * derived from this software without specific prior written permission. 25 * 26 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 27 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 28 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 29 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 30 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 31 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 32 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 33 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 34 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 35 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 36 */ 37 38 /*- 39 * seed = random 15bit 40 * n = prime, g0 = generator to n, 41 * j = random so that gcd(j,n-1) == 1 42 * g = g0^j mod n will be a generator again. 43 * 44 * X[0] = random seed. 45 * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator 46 * with a = 7^(even random) mod m, 47 * b = random with gcd(b,m) == 1 48 * m = 31104 and a maximal period of m-1. 49 * 50 * The transaction id is determined by: 51 * id[n] = seed xor (g^X[n] mod n) 52 * 53 * Effectivly the id is restricted to the lower 15 bits, thus yielding two 54 * different cycles by toggling the msb on and off. This avoids reuse issues 55 * caused by reseeding. 56 */ 57 58 #include <sys/cdefs.h> 59 __FBSDID("$FreeBSD$"); 60 61 #include "opt_pf.h" 62 #include <sys/param.h> 63 #include <sys/time.h> 64 #include <sys/kernel.h> 65 #include <sys/random.h> 66 67 #define RU_OUT 180 /* Time after wich will be reseeded */ 68 #define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */ 69 #define RU_GEN 2 /* Starting generator */ 70 #define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */ 71 #define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */ 72 #define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */ 73 74 #define PFAC_N 3 75 const static u_int16_t pfacts[PFAC_N] = { 76 2, 77 3, 78 2729 79 }; 80 81 static u_int16_t ru_x; 82 static u_int16_t ru_seed, ru_seed2; 83 static u_int16_t ru_a, ru_b; 84 static u_int16_t ru_g; 85 static u_int16_t ru_counter = 0; 86 static u_int16_t ru_msb = 0; 87 static long ru_reseed; 88 static u_int32_t tmp; /* Storage for unused random */ 89 90 static u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t); 91 static void ip_initid(void); 92 u_int16_t ip_randomid(void); 93 94 /* 95 * Do a fast modular exponation, returned value will be in the range of 0 - 96 * (mod-1). 97 */ 98 99 static u_int16_t 100 pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod) 101 { 102 u_int16_t s, t, u; 103 104 s = 1; 105 t = gen; 106 u = exp; 107 108 while (u) { 109 if (u & 1) 110 s = (s*t) % mod; 111 u >>= 1; 112 t = (t*t) % mod; 113 } 114 return (s); 115 } 116 117 /* 118 * Initalizes the seed and chooses a suitable generator. Also toggles the 119 * msb flag. The msb flag is used to generate two distinct cycles of random 120 * numbers and thus avoiding reuse of ids. 121 * 122 * This function is called from id_randomid() when needed, an application 123 * does not have to worry about it. 124 */ 125 static void 126 ip_initid(void) 127 { 128 u_int16_t j, i; 129 int noprime = 1; 130 struct timeval time; 131 132 getmicrotime(&time); 133 read_random((void *) &tmp, sizeof(tmp)); 134 ru_x = (tmp & 0xFFFF) % RU_M; 135 136 /* 15 bits of random seed. */ 137 ru_seed = (tmp >> 16) & 0x7FFF; 138 read_random((void *) &tmp, sizeof(tmp)); 139 ru_seed2 = tmp & 0x7FFF; 140 141 read_random((void *) &tmp, sizeof(tmp)); 142 143 /* Determine the LCG we use. */ 144 ru_b = (tmp & 0xfffe) | 1; 145 ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M); 146 while (ru_b % 3 == 0) 147 ru_b += 2; 148 149 read_random((void *) &tmp, sizeof(tmp)); 150 j = tmp % RU_N; 151 tmp = tmp >> 16; 152 153 /* 154 * Do a fast gcd(j,RU_N-1), so we can find a j with gcd(j, RU_N-1) == 155 * 1, giving a new generator for RU_GEN^j mod RU_N. 156 */ 157 while (noprime) { 158 for (i=0; i<PFAC_N; i++) 159 if (j%pfacts[i] == 0) 160 break; 161 162 if (i>=PFAC_N) 163 noprime = 0; 164 else 165 j = (j+1) % RU_N; 166 } 167 168 ru_g = pmod(RU_GEN,j,RU_N); 169 ru_counter = 0; 170 171 ru_reseed = time.tv_sec + RU_OUT; 172 ru_msb = ru_msb == 0x8000 ? 0 : 0x8000; 173 } 174 175 u_int16_t 176 ip_randomid(void) 177 { 178 int i, n; 179 struct timeval time; 180 181 /* XXX not reentrant */ 182 getmicrotime(&time); 183 if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed) 184 ip_initid(); 185 186 if (!tmp) 187 read_random((void *) &tmp, sizeof(tmp)); 188 189 /* Skip a random number of ids. */ 190 n = tmp & 0x3; tmp = tmp >> 2; 191 if (ru_counter + n >= RU_MAX) 192 ip_initid(); 193 194 for (i = 0; i <= n; i++) 195 /* Linear Congruential Generator. */ 196 ru_x = (ru_a*ru_x + ru_b) % RU_M; 197 198 ru_counter += i; 199 200 return (ru_seed ^ pmod(ru_g,ru_seed2 ^ ru_x,RU_N)) | ru_msb; 201 } 202