arc4random.c revision 180805 
1/*-
2 * THE BEER-WARE LICENSE
3 *
4 * <dan@FreeBSD.ORG> wrote this file.  As long as you retain this notice you
5 * can do whatever you want with this stuff.  If we meet some day, and you
6 * think this stuff is worth it, you can buy me a beer in return.
7 *
8 * Dan Moschuk
9 */
10
11#include <sys/cdefs.h>
12__FBSDID("$FreeBSD: head/sys/libkern/arc4random.c 180805 2008-07-25 15:53:32Z ache $");
13
14#include <sys/types.h>
15#include <sys/param.h>
16#include <sys/kernel.h>
17#include <sys/random.h>
18#include <sys/libkern.h>
19#include <sys/lock.h>
20#include <sys/mutex.h>
21#include <sys/time.h>
22
23#define	ARC4_RESEED_BYTES 65536
24#define	ARC4_RESEED_SECONDS 300
25#define	ARC4_KEYBYTES (256 / 8)
26
27static u_int8_t arc4_i, arc4_j;
28static int arc4_numruns = 0;
29static u_int8_t arc4_sbox[256];
30static time_t arc4_t_reseed;
31static struct mtx arc4_mtx;
32
33static u_int8_t arc4_randbyte(void);
34
35static __inline void
36arc4_swap(u_int8_t *a, u_int8_t *b)
37{
38	u_int8_t c;
39
40	c = *a;
41	*a = *b;
42	*b = c;
43}
44
45/*
46 * Stir our S-box.
47 */
48static void
49arc4_randomstir (void)
50{
51	u_int8_t key[256];
52	int r, n;
53	struct timeval tv_now;
54
55	/*
56	 * XXX read_random() returns unsafe numbers if the entropy
57	 * device is not loaded -- MarkM.
58	 */
59	r = read_random(key, ARC4_KEYBYTES);
60	getmicrouptime(&tv_now);
61	mtx_lock(&arc4_mtx);
62	/* If r == 0 || -1, just use what was on the stack. */
63	if (r > 0) {
64		for (n = r; n < sizeof(key); n++)
65			key[n] = key[n % r];
66	}
67
68	for (n = 0; n < 256; n++) {
69		arc4_j = (arc4_j + arc4_sbox[n] + key[n]) % 256;
70		arc4_swap(&arc4_sbox[n], &arc4_sbox[arc4_j]);
71	}
72
73	/* Reset for next reseed cycle. */
74	arc4_t_reseed = tv_now.tv_sec + ARC4_RESEED_SECONDS;
75	arc4_numruns = 0;
76
77	/*
78	 * Throw away the first N words of output, as suggested in the
79	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
80	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
81	 */
82	for (n = 0; n < 256*4; n++)
83		arc4_randbyte();
84	mtx_unlock(&arc4_mtx);
85}
86
87/*
88 * Initialize our S-box to its beginning defaults.
89 */
90static void
91arc4_init(void)
92{
93	int n;
94
95	mtx_init(&arc4_mtx, "arc4_mtx", NULL, MTX_DEF);
96	arc4_i = arc4_j = 0;
97	for (n = 0; n < 256; n++)
98		arc4_sbox[n] = (u_int8_t) n;
99
100	arc4_t_reseed = 0;
101}
102
103SYSINIT(arc4_init, SI_SUB_LOCK, SI_ORDER_ANY, arc4_init, NULL);
104
105/*
106 * Generate a random byte.
107 */
108static u_int8_t
109arc4_randbyte(void)
110{
111	u_int8_t arc4_t;
112
113	arc4_i = (arc4_i + 1) % 256;
114	arc4_j = (arc4_j + arc4_sbox[arc4_i]) % 256;
115
116	arc4_swap(&arc4_sbox[arc4_i], &arc4_sbox[arc4_j]);
117
118	arc4_t = (arc4_sbox[arc4_i] + arc4_sbox[arc4_j]) % 256;
119	return arc4_sbox[arc4_t];
120}
121
122/*
123 * MPSAFE
124 */
125void
126arc4rand(void *ptr, u_int len, int reseed)
127{
128	u_char *p;
129	struct timeval tv;
130
131	getmicrouptime(&tv);
132	if (reseed ||
133	   (arc4_numruns > ARC4_RESEED_BYTES) ||
134	   (tv.tv_sec > arc4_t_reseed))
135		arc4_randomstir();
136
137	mtx_lock(&arc4_mtx);
138	arc4_numruns += len;
139	p = ptr;
140	while (len--)
141		*p++ = arc4_randbyte();
142	mtx_unlock(&arc4_mtx);
143}
144
145uint32_t
146arc4random(void)
147{
148	uint32_t ret;
149
150	arc4rand(&ret, sizeof ret, 0);
151	return ret;
152}
153