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 * $FreeBSD: src/sys/netinet/ip_id.c,v 1.1.2.1 2001/07/19 06:37:26 kris Exp $ 38 * $DragonFly: src/sys/netinet/ip_id.c,v 1.6 2006/01/14 11:33:50 swildner Exp $ 39 */ 40 41 /* 42 * seed = random 15bit 43 * n = prime, g0 = generator to n, 44 * j = random so that gcd(j,n-1) == 1 45 * g = g0^j mod n will be a generator again. 46 * 47 * X[0] = random seed. 48 * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator 49 * with a = 7^(even random) mod m, 50 * b = random with gcd(b,m) == 1 51 * m = 31104 and a maximal period of m-1. 52 * 53 * The transaction id is determined by: 54 * id[n] = seed xor (g^X[n] mod n) 55 * 56 * Effectivly the id is restricted to the lower 15 bits, thus 57 * yielding two different cycles by toggling the msb on and off. 58 * This avoids reuse issues caused by reseeding. 59 */ 60 61 #include <sys/param.h> 62 #include <sys/time.h> 63 #include <sys/kernel.h> 64 #include <sys/random.h> 65 66 #define RU_OUT 180 /* Time after wich will be reseeded */ 67 #define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */ 68 #define RU_GEN 2 /* Starting generator */ 69 #define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */ 70 #define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */ 71 #define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */ 72 73 #define PFAC_N 3 74 const static u_int16_t pfacts[PFAC_N] = { 75 2, 76 3, 77 2729 78 }; 79 80 static u_int16_t ru_x; 81 static u_int16_t ru_seed, ru_seed2; 82 static u_int16_t ru_a, ru_b; 83 static u_int16_t ru_g; 84 static u_int16_t ru_counter = 0; 85 static u_int16_t ru_msb = 0; 86 static long ru_reseed; 87 static u_int32_t tmp; /* Storage for unused random */ 88 89 static u_int16_t pmod (u_int16_t, u_int16_t, u_int16_t); 90 static void ip_initid (void); 91 u_int16_t ip_randomid (void); 92 93 /* 94 * Do a fast modular exponation, returned value will be in the range 95 * of 0 - (mod-1) 96 */ 97 static u_int16_t 98 pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod) 99 { 100 u_int16_t s, t, u; 101 102 s = 1; 103 t = gen; 104 u = exp; 105 106 while (u) { 107 if (u & 1) 108 s = (s*t) % mod; 109 u >>= 1; 110 t = (t*t) % mod; 111 } 112 return (s); 113 } 114 115 /* 116 * Initalizes the seed and chooses a suitable generator. Also toggles 117 * the msb flag. The msb flag is used to generate two distinct 118 * cycles of random numbers and thus avoiding reuse of ids. 119 * 120 * This function is called from id_randomid() when needed, an 121 * application does not have to worry about it. 122 */ 123 static void 124 ip_initid(void) 125 { 126 u_int16_t j, i; 127 int noprime = 1; 128 struct timeval time; 129 130 getmicrotime(&time); 131 read_random((void *) &tmp, sizeof(tmp)); 132 ru_x = (tmp & 0xFFFF) % RU_M; 133 134 /* 15 bits of random seed */ 135 ru_seed = (tmp >> 16) & 0x7FFF; 136 read_random((void *) &tmp, sizeof(tmp)); 137 ru_seed2 = tmp & 0x7FFF; 138 139 read_random((void *) &tmp, sizeof(tmp)); 140 141 /* Determine the LCG we use */ 142 ru_b = (tmp & 0xfffe) | 1; 143 ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M); 144 while (ru_b % 3 == 0) 145 ru_b += 2; 146 147 read_random((void *) &tmp, sizeof(tmp)); 148 j = tmp % RU_N; 149 tmp = tmp >> 16; 150 151 /* 152 * Do a fast gcd(j,RU_N-1), so we can find a j with 153 * gcd(j, RU_N-1) == 1, giving a new generator for 154 * RU_GEN^j mod RU_N 155 */ 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 getmicrotime(&time); 182 if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed) 183 ip_initid(); 184 185 if (!tmp) 186 read_random((void *) &tmp, sizeof(tmp)); 187 188 /* Skip a random number of ids */ 189 n = tmp & 0x3; tmp = tmp >> 2; 190 if (ru_counter + n >= RU_MAX) 191 ip_initid(); 192 193 for (i = 0; i <= n; i++) 194 /* Linear Congruential Generator */ 195 ru_x = (ru_a*ru_x + ru_b) % RU_M; 196 197 ru_counter += i; 198 199 return (ru_seed ^ pmod(ru_g,ru_seed2 ^ ru_x,RU_N)) | ru_msb; 200 } 201