1 /* $OpenBSD: res_random.c,v 1.20 2013/11/12 07:00:24 deraadt Exp $ */ 2 3 /* 4 * Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de> 5 * Copyright 2008 Damien Miller <djm@openbsd.org> 6 * All rights reserved. 7 * 8 * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using 9 * such a mathematical system to generate more random (yet non-repeating) 10 * ids to solve the resolver/named problem. But Niels designed the 11 * actual system based on the constraints. 12 * 13 * Later modified by Damien Miller to wrap the LCG output in a 15-bit 14 * permutation generator based on a Luby-Rackoff block cipher. This 15 * ensures the output is non-repeating and preserves the MSB twiddle 16 * trick, but makes it more resistant to LCG prediction. 17 * 18 * Redistribution and use in source and binary forms, with or without 19 * modification, are permitted provided that the following conditions 20 * are met: 21 * 1. Redistributions of source code must retain the above copyright 22 * notice, this list of conditions and the following disclaimer. 23 * 2. Redistributions in binary form must reproduce the above copyright 24 * notice, this list of conditions and the following disclaimer in the 25 * documentation and/or other materials provided with the distribution. 26 * 27 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 28 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 29 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 30 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 31 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 32 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 33 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 34 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 35 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 36 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 37 */ 38 39 /* 40 * seed = random 15bit 41 * n = prime, g0 = generator to n, 42 * j = random so that gcd(j,n-1) == 1 43 * g = g0^j mod n will be a generator again. 44 * 45 * X[0] = random seed. 46 * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator 47 * with a = 7^(even random) mod m, 48 * b = random with gcd(b,m) == 1 49 * m = 31104 and a maximal period of m-1. 50 * 51 * The transaction id is determined by: 52 * id[n] = seed xor (g^X[n] mod n) 53 * 54 * Effectivly the id is restricted to the lower 15 bits, thus 55 * yielding two different cycles by toggling the msb on and off. 56 * This avoids reuse issues caused by reseeding. 57 * 58 * The output of this generator is then randomly permuted though a 59 * custom 15 bit Luby-Rackoff block cipher. 60 */ 61 62 #include <sys/types.h> 63 #include <netinet/in.h> 64 #include <sys/time.h> 65 #include <resolv.h> 66 67 #include <unistd.h> 68 #include <stdlib.h> 69 #include <string.h> 70 71 #include "thread_private.h" 72 73 #define RU_OUT 180 /* Time after wich will be reseeded */ 74 #define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */ 75 #define RU_GEN 2 /* Starting generator */ 76 #define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */ 77 #define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */ 78 #define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */ 79 #define RU_ROUNDS 11 /* Number of rounds for permute (odd) */ 80 81 struct prf_ctx { 82 /* PRF lookup table for odd rounds (7 bits input to 8 bits output) */ 83 u_char prf7[(RU_ROUNDS / 2) * (1 << 7)]; 84 85 /* PRF lookup table for even rounds (8 bits input to 7 bits output) */ 86 u_char prf8[((RU_ROUNDS + 1) / 2) * (1 << 8)]; 87 }; 88 89 #define PFAC_N 3 90 const static u_int16_t pfacts[PFAC_N] = { 91 2, 92 3, 93 2729 94 }; 95 96 static u_int16_t ru_x; 97 static u_int16_t ru_seed, ru_seed2; 98 static u_int16_t ru_a, ru_b; 99 static u_int16_t ru_g; 100 static u_int16_t ru_counter = 0; 101 static u_int16_t ru_msb = 0; 102 static struct prf_ctx *ru_prf = NULL; 103 static time_t ru_reseed; 104 105 static u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t); 106 static void res_initid(void); 107 108 /* 109 * Do a fast modular exponation, returned value will be in the range 110 * of 0 - (mod-1) 111 */ 112 static u_int16_t 113 pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod) 114 { 115 u_int16_t s, t, u; 116 117 s = 1; 118 t = gen; 119 u = exp; 120 121 while (u) { 122 if (u & 1) 123 s = (s * t) % mod; 124 u >>= 1; 125 t = (t * t) % mod; 126 } 127 return (s); 128 } 129 130 /* 131 * 15-bit permutation based on Luby-Rackoff block cipher 132 */ 133 static u_int 134 permute15(u_int in) 135 { 136 int i; 137 u_int left, right, tmp; 138 139 if (ru_prf == NULL) 140 return in; 141 142 left = (in >> 8) & 0x7f; 143 right = in & 0xff; 144 145 /* 146 * Each round swaps the width of left and right. Even rounds have 147 * a 7-bit left, odd rounds have an 8-bit left. Since this uses an 148 * odd number of rounds, left is always 8 bits wide at the end. 149 */ 150 for (i = 0; i < RU_ROUNDS; i++) { 151 if ((i & 1) == 0) 152 tmp = ru_prf->prf8[(i << (8 - 1)) | right] & 0x7f; 153 else 154 tmp = ru_prf->prf7[((i - 1) << (7 - 1)) | right]; 155 tmp ^= left; 156 left = right; 157 right = tmp; 158 } 159 160 return (right << 8) | left; 161 } 162 163 /* 164 * Initializes the seed and chooses a suitable generator. Also toggles 165 * the msb flag. The msb flag is used to generate two distinct 166 * cycles of random numbers and thus avoiding reuse of ids. 167 * 168 * This function is called from res_randomid() when needed, an 169 * application does not have to worry about it. 170 */ 171 static void 172 res_initid(void) 173 { 174 u_int16_t j, i; 175 u_int32_t tmp; 176 int noprime = 1; 177 struct timespec ts; 178 179 ru_x = arc4random_uniform(RU_M); 180 181 /* 15 bits of random seed */ 182 tmp = arc4random(); 183 ru_seed = (tmp >> 16) & 0x7FFF; 184 ru_seed2 = tmp & 0x7FFF; 185 186 /* Determine the LCG we use */ 187 tmp = arc4random(); 188 ru_b = (tmp & 0xfffe) | 1; 189 ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M); 190 while (ru_b % 3 == 0) 191 ru_b += 2; 192 193 j = arc4random_uniform(RU_N); 194 195 /* 196 * Do a fast gcd(j,RU_N-1), so we can find a j with 197 * gcd(j, RU_N-1) == 1, giving a new generator for 198 * RU_GEN^j mod RU_N 199 */ 200 201 while (noprime) { 202 for (i = 0; i < PFAC_N; i++) 203 if (j % pfacts[i] == 0) 204 break; 205 206 if (i >= PFAC_N) 207 noprime = 0; 208 else 209 j = (j + 1) % RU_N; 210 } 211 212 ru_g = pmod(RU_GEN, j, RU_N); 213 ru_counter = 0; 214 215 /* Initialise PRF for Luby-Rackoff permutation */ 216 if (ru_prf == NULL) 217 ru_prf = malloc(sizeof(*ru_prf)); 218 if (ru_prf != NULL) 219 arc4random_buf(ru_prf, sizeof(*ru_prf)); 220 221 clock_gettime(CLOCK_MONOTONIC, &ts); 222 ru_reseed = ts.tv_sec + RU_OUT; 223 ru_msb = ru_msb == 0x8000 ? 0 : 0x8000; 224 } 225 226 u_int 227 res_randomid(void) 228 { 229 struct timespec ts; 230 u_int r; 231 _THREAD_PRIVATE_MUTEX(random); 232 233 clock_gettime(CLOCK_MONOTONIC, &ts); 234 235 _THREAD_PRIVATE_MUTEX_LOCK(random); 236 237 if (ru_counter >= RU_MAX || ts.tv_sec > ru_reseed) 238 res_initid(); 239 240 /* Linear Congruential Generator */ 241 ru_x = (ru_a * ru_x + ru_b) % RU_M; 242 ru_counter++; 243 244 r = permute15(ru_seed ^ pmod(ru_g, ru_seed2 + ru_x, RU_N)) | ru_msb; 245 246 _THREAD_PRIVATE_MUTEX_UNLOCK(random); 247 248 return (r); 249 } 250 251 #if 0 252 int 253 main(int argc, char **argv) 254 { 255 int i, n; 256 u_int16_t wert; 257 258 res_initid(); 259 260 printf("Generator: %u\n", ru_g); 261 printf("Seed: %u\n", ru_seed); 262 printf("Reseed at %ld\n", ru_reseed); 263 printf("Ru_X: %u\n", ru_x); 264 printf("Ru_A: %u\n", ru_a); 265 printf("Ru_B: %u\n", ru_b); 266 267 n = argc > 1 ? atoi(argv[1]) : 60001; 268 for (i=0;i<n;i++) { 269 wert = res_randomid(); 270 printf("%u\n", wert); 271 } 272 return 0; 273 } 274 #endif 275 276