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