1 /* $OpenBSD: res_random.c,v 1.17 2008/04/13 00:28:35 djm Exp $ */ 2 3 /* 4 * Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de> 5 * Copyright 2008 Damien Miller <djm@openbsd.org> 6 * Copyright 2008 Android Open Source Project (thread-safety) 7 * All rights reserved. 8 * 9 * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using 10 * such a mathematical system to generate more random (yet non-repeating) 11 * ids to solve the resolver/named problem. But Niels designed the 12 * actual system based on the constraints. 13 * 14 * Later modified by Damien Miller to wrap the LCG output in a 15-bit 15 * permutation generator based on a Luby-Rackoff block cipher. This 16 * ensures the output is non-repeating and preserves the MSB twiddle 17 * trick, but makes it more resistant to LCG prediction. 18 * 19 * Redistribution and use in source and binary forms, with or without 20 * modification, are permitted provided that the following conditions 21 * are met: 22 * 1. Redistributions of source code must retain the above copyright 23 * notice, this list of conditions and the following disclaimer. 24 * 2. Redistributions in binary form must reproduce the above copyright 25 * notice, this list of conditions and the following disclaimer in the 26 * documentation and/or other materials provided with the distribution. 27 * 28 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 29 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 30 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 31 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 32 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 33 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 34 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 35 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 36 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 37 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 38 */ 39 40 /* 41 * seed = random 15bit 42 * n = prime, g0 = generator to n, 43 * j = random so that gcd(j,n-1) == 1 44 * g = g0^j mod n will be a generator again. 45 * 46 * X[0] = random seed. 47 * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator 48 * with a = 7^(even random) mod m, 49 * b = random with gcd(b,m) == 1 50 * m = 31104 and a maximal period of m-1. 51 * 52 * The transaction id is determined by: 53 * id[n] = seed xor (g^X[n] mod n) 54 * 55 * Effectivly the id is restricted to the lower 15 bits, thus 56 * yielding two different cycles by toggling the msb on and off. 57 * This avoids reuse issues caused by reseeding. 58 * 59 * The output of this generator is then randomly permuted though a 60 * custom 15 bit Luby-Rackoff block cipher. 61 */ 62 63 #include <sys/types.h> 64 #include <netinet/in.h> 65 #include <sys/time.h> 66 #include "resolv_private.h" 67 68 #include <unistd.h> 69 #include <stdlib.h> 70 #include <string.h> 71 72 /* BIONIC-BEGIN */ 73 static pthread_mutex_t _res_random_lock = PTHREAD_MUTEX_INITIALIZER; 74 #define _RES_RANDOM_LOCK() pthread_mutex_lock(&_res_random_lock) 75 #define _RES_RANDOM_UNLOCK() pthread_mutex_unlock(&_res_random_lock) 76 /* BIONIC-END */ 77 78 #define RU_OUT 180 /* Time after wich will be reseeded */ 79 #define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */ 80 #define RU_GEN 2 /* Starting generator */ 81 #define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */ 82 #define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */ 83 #define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */ 84 #define RU_ROUNDS 11 /* Number of rounds for permute (odd) */ 85 86 struct prf_ctx { 87 /* PRF lookup table for odd rounds (7 bits input to 8 bits output) */ 88 u_char prf7[(RU_ROUNDS / 2) * (1 << 7)]; 89 90 /* PRF lookup table for even rounds (8 bits input to 7 bits output) */ 91 u_char prf8[((RU_ROUNDS + 1) / 2) * (1 << 8)]; 92 }; 93 94 #define PFAC_N 3 95 const static u_int16_t pfacts[PFAC_N] = { 96 2, 97 3, 98 2729 99 }; 100 101 static u_int16_t ru_x; 102 static u_int16_t ru_seed, ru_seed2; 103 static u_int16_t ru_a, ru_b; 104 static u_int16_t ru_g; 105 static u_int16_t ru_counter = 0; 106 static u_int16_t ru_msb = 0; 107 static struct prf_ctx *ru_prf = NULL; 108 static long ru_reseed; 109 110 static u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t); 111 static void res_initid(void); 112 113 /* 114 * Do a fast modular exponation, returned value will be in the range 115 * of 0 - (mod-1) 116 */ 117 static u_int16_t pmod(u_int16_t gen,u_int16_t exp,u_int16_t mod)118 pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod) 119 { 120 u_int16_t s, t, u; 121 122 s = 1; 123 t = gen; 124 u = exp; 125 126 while (u) { 127 if (u & 1) 128 s = (s * t) % mod; 129 u >>= 1; 130 t = (t * t) % mod; 131 } 132 return (s); 133 } 134 135 /* 136 * 15-bit permutation based on Luby-Rackoff block cipher 137 */ 138 u_int permute15(u_int in)139 permute15(u_int in) 140 { 141 int i; 142 u_int left, right, tmp; 143 144 if (ru_prf == NULL) 145 return in; 146 147 left = (in >> 8) & 0x7f; 148 right = in & 0xff; 149 150 /* 151 * Each round swaps the width of left and right. Even rounds have 152 * a 7-bit left, odd rounds have an 8-bit left. Since this uses an 153 * odd number of rounds, left is always 8 bits wide at the end. 154 */ 155 for (i = 0; i < RU_ROUNDS; i++) { 156 if ((i & 1) == 0) 157 tmp = ru_prf->prf8[(i << (8 - 1)) | right] & 0x7f; 158 else 159 tmp = ru_prf->prf7[((i - 1) << (7 - 1)) | right]; 160 tmp ^= left; 161 left = right; 162 right = tmp; 163 } 164 165 return (right << 8) | left; 166 } 167 168 /* 169 * Initializes the seed and chooses a suitable generator. Also toggles 170 * the msb flag. The msb flag is used to generate two distinct 171 * cycles of random numbers and thus avoiding reuse of ids. 172 * 173 * This function is called from res_randomid() when needed, an 174 * application does not have to worry about it. 175 */ 176 static void res_initid(void)177 res_initid(void) 178 { 179 u_int16_t j, i; 180 u_int32_t tmp; 181 int noprime = 1; 182 struct timeval tv; 183 184 ru_x = arc4random_uniform(RU_M); 185 186 /* 15 bits of random seed */ 187 tmp = arc4random(); 188 ru_seed = (tmp >> 16) & 0x7FFF; 189 ru_seed2 = tmp & 0x7FFF; 190 191 /* Determine the LCG we use */ 192 tmp = arc4random(); 193 ru_b = (tmp & 0xfffe) | 1; 194 ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M); 195 while (ru_b % 3 == 0) 196 ru_b += 2; 197 198 j = arc4random_uniform(RU_N); 199 200 /* 201 * Do a fast gcd(j,RU_N-1), so we can find a j with 202 * gcd(j, RU_N-1) == 1, giving a new generator for 203 * RU_GEN^j mod RU_N 204 */ 205 206 while (noprime) { 207 for (i = 0; i < PFAC_N; i++) 208 if (j % pfacts[i] == 0) 209 break; 210 211 if (i >= PFAC_N) 212 noprime = 0; 213 else 214 j = (j + 1) % RU_N; 215 } 216 217 ru_g = pmod(RU_GEN, j, RU_N); 218 ru_counter = 0; 219 220 /* Initialise PRF for Luby-Rackoff permutation */ 221 if (ru_prf == NULL) 222 ru_prf = malloc(sizeof(*ru_prf)); 223 if (ru_prf != NULL) 224 arc4random_buf(ru_prf, sizeof(*ru_prf)); 225 226 gettimeofday(&tv, NULL); 227 ru_reseed = tv.tv_sec + RU_OUT; 228 ru_msb = ru_msb == 0x8000 ? 0 : 0x8000; 229 } 230 231 u_int res_randomid(void)232 res_randomid(void) 233 { 234 struct timeval tv; 235 u_int result; 236 237 _RES_RANDOM_LOCK() 238 gettimeofday(&tv, NULL); 239 if (ru_counter >= RU_MAX || tv.tv_sec > ru_reseed) 240 res_initid(); 241 242 /* Linear Congruential Generator */ 243 ru_x = (ru_a * ru_x + ru_b) % RU_M; 244 ru_counter++; 245 246 result = permute15(ru_seed ^ pmod(ru_g, ru_seed2 + ru_x, RU_N)) | ru_msb; 247 _RES_RANDOM_UNLOCK() 248 return result; 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