arc4random.c revision 53864 
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 * $FreeBSD: head/sys/libkern/arc4random.c 53864 1999-11-28 23:39:49Z dan $
11 */
12
13#include <sys/libkern.h>
14
15#define	ARC4_MAXRUNS 64
16
17static u_int8_t arc4_i, arc4_j;
18static int arc4_initialized = 0;
19static int arc4_numruns = 0;
20static u_int8_t arc4_sbox[256];
21
22extern u_int read_random (void *, u_int);
23
24static __inline void
25arc4_swap(u_int8_t *a, u_int8_t *b)
26{
27	u_int8_t c;
28
29	c = *a;
30	*a = *b;
31	*b = c;
32}
33
34/*
35 * Stir our S-box.
36 */
37static void
38arc4_randomstir (void)
39{
40	u_int8_t key[256];
41	int r, n;
42
43	r = read_random(key, sizeof(key));
44	for (n = r; n < sizeof(key); n++)
45		key[n] = key[n % r];
46
47	for (n = 0; n < 256; n++)
48	{
49		arc4_j = (arc4_j + arc4_sbox[n] + key[n]) % 256;
50		arc4_swap(&arc4_sbox[n], &arc4_sbox[arc4_j]);
51	}
52}
53
54/*
55 * Initialize our S-box to its beginning defaults.
56 */
57static void
58arc4_init(void)
59{
60	int n;
61
62	arc4_i = arc4_j = 0;
63	for (n = 0; n < 256; n++)
64		arc4_sbox[n] = (u_int8_t) n;
65
66	arc4_randomstir();
67	arc4_initialized = 1;
68}
69
70/*
71 * Generate a random byte.
72 */
73static u_int8_t
74arc4_randbyte(void)
75{
76	u_int8_t arc4_t;
77
78	arc4_i = (arc4_i + 1) % 256;
79	arc4_j = (arc4_j + arc4_sbox[arc4_i]) % 256;
80
81	arc4_swap(&arc4_sbox[arc4_i], &arc4_sbox[arc4_j]);
82
83	arc4_t = (arc4_sbox[arc4_i] + arc4_sbox[arc4_j]) % 256;
84	return arc4_sbox[arc4_t];
85}
86
87u_int32_t
88arc4random(void)
89{
90	u_int32_t ret;
91
92	/* Initialize array if needed. */
93	if (!arc4_initialized)
94		arc4_init();
95	if (++arc4_numruns > ARC4_MAXRUNS)
96	{
97		arc4_randomstir();
98		arc4_numruns = 0;
99	}
100
101	ret = arc4_randbyte();
102	ret |= arc4_randbyte() << 8;
103	ret |= arc4_randbyte() << 16;
104	ret |= arc4_randbyte() << 24;
105
106	return ret;
107}
108