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1/* $OpenBSD: res_random.c,v 1.27 2024/09/03 18:21:55 op 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 * Effectively 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 which 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
81struct 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
90static const u_int16_t pfacts[PFAC_N] = {
91 2,
92 3,
93 2729
94};
95
96static u_int16_t ru_x;
97static u_int16_t ru_seed, ru_seed2;
98static u_int16_t ru_a, ru_b;
99static u_int16_t ru_g;
100static u_int16_t ru_counter = 0;
101static u_int16_t ru_msb = 0;
102static struct prf_ctx *ru_prf = NULL;
103static time_t ru_reseed;
104static pid_t ru_pid;
105
106static u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t);
107static void res_initid(void);
108
109/*
110 * Do a fast modular exponation, returned value will be in the range
111 * of 0 - (mod-1)
112 */
113static u_int16_t
114pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod)
115{
116 u_int16_t s, t, u;
117
118 s = 1;
119 t = gen;
120 u = exp;
121
122 while (u) {
123 if (u & 1)
124 s = (s * t) % mod;
125 u >>= 1;
126 t = (t * t) % mod;
127 }
128 return (s);
129}
130
131/*
132 * 15-bit permutation based on Luby-Rackoff block cipher
133 */
134static u_int
135permute15(u_int in)
136{
137 int i;
138 u_int left, right, tmp;
139
140 if (ru_prf == NULL)
141 return in;
142
143 left = (in >> 8) & 0x7f;
144 right = in & 0xff;
145
146 /*
147 * Each round swaps the width of left and right. Even rounds have
148 * a 7-bit left, odd rounds have an 8-bit left. Since this uses an
149 * odd number of rounds, left is always 8 bits wide at the end.
150 */
151 for (i = 0; i < RU_ROUNDS; i++) {
152 if ((i & 1) == 0)
153 tmp = ru_prf->prf8[(i << (8 - 1)) | right] & 0x7f;
154 else
155 tmp = ru_prf->prf7[((i - 1) << (7 - 1)) | right];
156 tmp ^= left;
157 left = right;
158 right = tmp;
159 }
160
161 return (right << 8) | left;
162}
163
164/*
165 * Initializes the seed and chooses a suitable generator. Also toggles
166 * the msb flag. The msb flag is used to generate two distinct
167 * cycles of random numbers and thus avoiding reuse of ids.
168 *
169 * This function is called from res_randomid() when needed, an
170 * application does not have to worry about it.
171 */
172static void
173res_initid(void)
174{
175 u_int16_t j, i;
176 u_int32_t tmp;
177 int noprime = 1;
178 struct timespec ts;
179
180 ru_x = arc4random_uniform(RU_M);
181
182 /* 15 bits of random seed */
183 tmp = arc4random();
184 ru_seed = (tmp >> 16) & 0x7FFF;
185 ru_seed2 = tmp & 0x7FFF;
186
187 /* Determine the LCG we use */
188 tmp = arc4random();
189 ru_b = (tmp & 0xfffe) | 1;
190 ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
191 while (ru_b % 3 == 0)
192 ru_b += 2;
193
194 j = arc4random_uniform(RU_N);
195
196 /*
197 * Do a fast gcd(j,RU_N-1), so we can find a j with
198 * gcd(j, RU_N-1) == 1, giving a new generator for
199 * RU_GEN^j mod RU_N
200 */
201
202 while (noprime) {
203 for (i = 0; i < PFAC_N; i++)
204 if (j % pfacts[i] == 0)
205 break;
206
207 if (i >= PFAC_N)
208 noprime = 0;
209 else
210 j = (j + 1) % RU_N;
211 }
212
213 ru_g = pmod(RU_GEN, j, RU_N);
214 ru_counter = 0;
215
216 /* Initialise PRF for Luby-Rackoff permutation */
217 if (ru_prf == NULL)
218 ru_prf = malloc(sizeof(*ru_prf));
219 if (ru_prf != NULL)
220 arc4random_buf(ru_prf, sizeof(*ru_prf));
221
222 WRAP(clock_gettime)(CLOCK_MONOTONIC, &ts);
223 ru_reseed = ts.tv_sec + RU_OUT;
224 ru_msb = ru_msb == 0x8000 ? 0 : 0x8000;
225}
226
227u_int
228__res_randomid(void)
229{
230 struct timespec ts;
231 pid_t pid;
232 u_int r;
233 static void *randomid_mutex;
234
235 WRAP(clock_gettime)(CLOCK_MONOTONIC, &ts);
236 pid = getpid();
237
238 _MUTEX_LOCK(&randomid_mutex);
239
240 if (ru_counter >= RU_MAX || ts.tv_sec > ru_reseed || pid != ru_pid) {
241 res_initid();
242 ru_pid = pid;
243 }
244
245 /* Linear Congruential Generator */
246 ru_x = (ru_a * ru_x + ru_b) % RU_M;
247 ru_counter++;
248
249 r = permute15(ru_seed ^ pmod(ru_g, ru_seed2 + ru_x, RU_N)) | ru_msb;
250
251 _MUTEX_UNLOCK(&randomid_mutex);
252
253 return (r);
254}
255DEF_STRONG(__res_randomid);
256
257#if 0
258int
259main(int argc, char **argv)
260{
261 int i, n;
262 u_int16_t wert;
263
264 res_initid();
265
266 printf("Generator: %u\n", ru_g);
267 printf("Seed: %u\n", ru_seed);
268 printf("Reseed at %ld\n", ru_reseed);
269 printf("Ru_X: %u\n", ru_x);
270 printf("Ru_A: %u\n", ru_a);
271 printf("Ru_B: %u\n", ru_b);
272
273 n = argc > 1 ? atoi(argv[1]) : 60001;
274 for (i=0;i<n;i++) {
275 wert = res_randomid();
276 printf("%u\n", wert);
277 }
278 return 0;
279}
280#endif
281
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