arc4random.c revision 69520 
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 69520 2000-12-02 16:23:54Z markm $
11 */
12
13#include <sys/types.h>
14#include <sys/random.h>
15#include <sys/libkern.h>
16
17#define	ARC4_MAXRUNS 64
18
19static u_int8_t arc4_i, arc4_j;
20static int arc4_initialized = 0;
21static int arc4_numruns = 0;
22static u_int8_t arc4_sbox[256];
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	/* XXX read_random() returns unsafe numbers if the entropy
44	 * devce is not loaded - MarkM
45	 */
46	r = read_random(key, sizeof(key));
47	/* if r == 0 || -1, just use what was on the stack */
48	if (r > 0)
49	{
50		for (n = r; n < sizeof(key); n++)
51			key[n] = key[n % r];
52	}
53
54	for (n = 0; n < 256; n++)
55	{
56		arc4_j = (arc4_j + arc4_sbox[n] + key[n]) % 256;
57		arc4_swap(&arc4_sbox[n], &arc4_sbox[arc4_j]);
58	}
59}
60
61/*
62 * Initialize our S-box to its beginning defaults.
63 */
64static void
65arc4_init(void)
66{
67	int n;
68
69	arc4_i = arc4_j = 0;
70	for (n = 0; n < 256; n++)
71		arc4_sbox[n] = (u_int8_t) n;
72
73	arc4_randomstir();
74	arc4_initialized = 1;
75}
76
77/*
78 * Generate a random byte.
79 */
80static u_int8_t
81arc4_randbyte(void)
82{
83	u_int8_t arc4_t;
84
85	arc4_i = (arc4_i + 1) % 256;
86	arc4_j = (arc4_j + arc4_sbox[arc4_i]) % 256;
87
88	arc4_swap(&arc4_sbox[arc4_i], &arc4_sbox[arc4_j]);
89
90	arc4_t = (arc4_sbox[arc4_i] + arc4_sbox[arc4_j]) % 256;
91	return arc4_sbox[arc4_t];
92}
93
94u_int32_t
95arc4random(void)
96{
97	u_int32_t ret;
98
99	/* Initialize array if needed. */
100	if (!arc4_initialized)
101		arc4_init();
102	if (++arc4_numruns > ARC4_MAXRUNS)
103	{
104		arc4_randomstir();
105		arc4_numruns = 0;
106	}
107
108	ret = arc4_randbyte();
109	ret |= arc4_randbyte() << 8;
110	ret |= arc4_randbyte() << 16;
111	ret |= arc4_randbyte() << 24;
112
113	return ret;
114}
115