bsd-arc4random.c revision 181111 
1/*
2 * Copyright (c) 1999,2000,2004 Damien Miller <djm@mindrot.org>
3 *
4 * Permission to use, copy, modify, and distribute this software for any
5 * purpose with or without fee is hereby granted, provided that the above
6 * copyright notice and this permission notice appear in all copies.
7 *
8 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
9 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
11 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
13 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
14 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
15 */
16
17#include "includes.h"
18
19#include <sys/types.h>
20
21#include <string.h>
22#include <stdlib.h>
23#include <stdarg.h>
24
25#include "log.h"
26
27#ifndef HAVE_ARC4RANDOM
28
29#include <openssl/rand.h>
30#include <openssl/rc4.h>
31#include <openssl/err.h>
32
33/* Size of key to use */
34#define SEED_SIZE 20
35
36/* Number of bytes to reseed after */
37#define REKEY_BYTES	(1 << 24)
38
39static int rc4_ready = 0;
40static RC4_KEY rc4;
41
42unsigned int
43arc4random(void)
44{
45	unsigned int r = 0;
46	static int first_time = 1;
47
48	if (rc4_ready <= 0) {
49		if (first_time)
50			seed_rng();
51		first_time = 0;
52		arc4random_stir();
53	}
54
55	RC4(&rc4, sizeof(r), (unsigned char *)&r, (unsigned char *)&r);
56
57	rc4_ready -= sizeof(r);
58
59	return(r);
60}
61
62void
63arc4random_stir(void)
64{
65	unsigned char rand_buf[SEED_SIZE];
66	int i;
67
68	memset(&rc4, 0, sizeof(rc4));
69	if (RAND_bytes(rand_buf, sizeof(rand_buf)) <= 0)
70		fatal("Couldn't obtain random bytes (error %ld)",
71		    ERR_get_error());
72	RC4_set_key(&rc4, sizeof(rand_buf), rand_buf);
73
74	/*
75	 * Discard early keystream, as per recommendations in:
76	 * http://www.wisdom.weizmann.ac.il/~itsik/RC4/Papers/Rc4_ksa.ps
77	 */
78	for(i = 0; i <= 256; i += sizeof(rand_buf))
79		RC4(&rc4, sizeof(rand_buf), rand_buf, rand_buf);
80
81	memset(rand_buf, 0, sizeof(rand_buf));
82
83	rc4_ready = REKEY_BYTES;
84}
85#endif /* !HAVE_ARC4RANDOM */
86
87#ifndef ARC4RANDOM_BUF
88void
89arc4random_buf(void *_buf, size_t n)
90{
91	size_t i;
92	u_int32_t r = 0;
93	char *buf = (char *)_buf;
94
95	for (i = 0; i < n; i++) {
96		if (i % 4 == 0)
97			r = arc4random();
98		buf[i] = r & 0xff;
99		r >>= 8;
100	}
101	i = r = 0;
102}
103#endif /* !HAVE_ARC4RANDOM_BUF */
104
105#ifndef ARC4RANDOM_UNIFORM
106/*
107 * Calculate a uniformly distributed random number less than upper_bound
108 * avoiding "modulo bias".
109 *
110 * Uniformity is achieved by generating new random numbers until the one
111 * returned is outside the range [0, 2**32 % upper_bound).  This
112 * guarantees the selected random number will be inside
113 * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
114 * after reduction modulo upper_bound.
115 */
116u_int32_t
117arc4random_uniform(u_int32_t upper_bound)
118{
119	u_int32_t r, min;
120
121	if (upper_bound < 2)
122		return 0;
123
124#if (ULONG_MAX > 0xffffffffUL)
125	min = 0x100000000UL % upper_bound;
126#else
127	/* Calculate (2**32 % upper_bound) avoiding 64-bit math */
128	if (upper_bound > 0x80000000)
129		min = 1 + ~upper_bound;		/* 2**32 - upper_bound */
130	else {
131		/* (2**32 - (x * 2)) % x == 2**32 % x when x <= 2**31 */
132		min = ((0xffffffff - (upper_bound * 2)) + 1) % upper_bound;
133	}
134#endif
135
136	/*
137	 * This could theoretically loop forever but each retry has
138	 * p > 0.5 (worst case, usually far better) of selecting a
139	 * number inside the range we need, so it should rarely need
140	 * to re-roll.
141	 */
142	for (;;) {
143		r = arc4random();
144		if (r >= min)
145			break;
146	}
147
148	return r % upper_bound;
149}
150#endif /* !HAVE_ARC4RANDOM_UNIFORM */
151