1 /* $OpenBSD: ip_id.c,v 1.4 2001/06/08 03:53:46 angelos 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 54 * yielding two different cycles by toggling the msb on and off. 55 * This avoids reuse issues caused by reseeding. 56 */ 57 58 #include <sys/param.h> 59 #include <sys/kernel.h> 60 61 #include <dev/rndvar.h> 62 63 #define RU_OUT 180 /* Time after wich will be reseeded */ 64 #define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */ 65 #define RU_GEN 2 /* Starting generator */ 66 #define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */ 67 #define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */ 68 #define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */ 69 70 #define PFAC_N 3 71 const static u_int16_t pfacts[PFAC_N] = { 72 2, 73 3, 74 2729 75 }; 76 77 static u_int16_t ru_x; 78 static u_int16_t ru_seed, ru_seed2; 79 static u_int16_t ru_a, ru_b; 80 static u_int16_t ru_g; 81 static u_int16_t ru_counter = 0; 82 static u_int16_t ru_msb = 0; 83 static long ru_reseed; 84 static u_int32_t tmp; /* Storage for unused random */ 85 86 static u_int16_t pmod __P((u_int16_t, u_int16_t, u_int16_t)); 87 static void ip_initid __P((void)); 88 u_int16_t ip_randomid __P((void)); 89 90 /* 91 * Do a fast modular exponation, returned value will be in the range 92 * of 0 - (mod-1) 93 */ 94 95 #ifdef __STDC__ 96 static u_int16_t 97 pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod) 98 #else 99 static u_int16_t 100 pmod(gen, exp, mod) 101 u_int16_t gen, exp, mod; 102 #endif 103 { 104 u_int16_t s, t, u; 105 106 s = 1; 107 t = gen; 108 u = exp; 109 110 while (u) { 111 if (u & 1) 112 s = (s*t) % mod; 113 u >>= 1; 114 t = (t*t) % mod; 115 } 116 return (s); 117 } 118 119 /* 120 * Initalizes the seed and chooses a suitable generator. Also toggles 121 * the msb flag. The msb flag is used to generate two distinct 122 * cycles of random numbers and thus avoiding reuse of ids. 123 * 124 * This function is called from id_randomid() when needed, an 125 * application does not have to worry about it. 126 */ 127 static void 128 ip_initid(void) 129 { 130 u_int16_t j, i; 131 int noprime = 1; 132 133 ru_x = ((tmp = arc4random()) & 0xFFFF) % RU_M; 134 135 /* 15 bits of random seed */ 136 ru_seed = (tmp >> 16) & 0x7FFF; 137 ru_seed2 = arc4random() & 0x7FFF; 138 139 /* Determine the LCG we use */ 140 ru_b = ((tmp = arc4random()) & 0xfffe) | 1; 141 ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M); 142 while (ru_b % 3 == 0) 143 ru_b += 2; 144 145 j = (tmp = arc4random()) % RU_N; 146 tmp = tmp >> 16; 147 148 /* 149 * Do a fast gcd(j,RU_N-1), so we can find a j with 150 * gcd(j, RU_N-1) == 1, giving a new generator for 151 * RU_GEN^j mod RU_N 152 */ 153 154 while (noprime) { 155 for (i = 0; i < PFAC_N; i++) 156 if (j % pfacts[i] == 0) 157 break; 158 159 if (i >= PFAC_N) 160 noprime = 0; 161 else 162 j = (j+1) % RU_N; 163 } 164 165 ru_g = pmod(RU_GEN,j,RU_N); 166 ru_counter = 0; 167 168 ru_reseed = time.tv_sec + RU_OUT; 169 ru_msb = ru_msb == 0x8000 ? 0 : 0x8000; 170 } 171 172 u_int16_t 173 ip_randomid(void) 174 { 175 int i, n; 176 177 if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed) 178 ip_initid(); 179 180 if (!tmp) 181 tmp = arc4random(); 182 183 /* Skip a random number of ids */ 184 n = tmp & 0x3; tmp = tmp >> 2; 185 if (ru_counter + n >= RU_MAX) 186 ip_initid(); 187 188 for (i = 0; i <= n; i++) 189 /* Linear Congruential Generator */ 190 ru_x = (ru_a * ru_x + ru_b) % RU_M; 191 192 ru_counter += i; 193 194 return (ru_seed ^ pmod(ru_g,ru_seed2 ^ ru_x,RU_N)) | ru_msb; 195 } 196