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 */
73static 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
86struct 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
95const static u_int16_t pfacts[PFAC_N] = {
96	2,
97	3,
98	2729
99};
100
101static u_int16_t ru_x;
102static u_int16_t ru_seed, ru_seed2;
103static u_int16_t ru_a, ru_b;
104static u_int16_t ru_g;
105static u_int16_t ru_counter = 0;
106static u_int16_t ru_msb = 0;
107static struct prf_ctx *ru_prf = NULL;
108static long ru_reseed;
109
110static u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t);
111static 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 */
117static u_int16_t
118pmod(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 */
138u_int
139permute15(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 */
176static void
177res_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
231u_int
232res_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
252int
253main(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