arc4random.c revision 53893 
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 53893 1999-11-29 19:23:35Z 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	/* if r == 0 || -1, just use what was on the stack */
45	if (r > 0)
46	{
47		for (n = r; n < sizeof(key); n++)
48			key[n] = key[n % r];
49	}
50
51	for (n = 0; n < 256; n++)
52	{
53		arc4_j = (arc4_j + arc4_sbox[n] + key[n]) % 256;
54		arc4_swap(&arc4_sbox[n], &arc4_sbox[arc4_j]);
55	}
56}
57
58/*
59 * Initialize our S-box to its beginning defaults.
60 */
61static void
62arc4_init(void)
63{
64	int n;
65
66	arc4_i = arc4_j = 0;
67	for (n = 0; n < 256; n++)
68		arc4_sbox[n] = (u_int8_t) n;
69
70	arc4_randomstir();
71	arc4_initialized = 1;
72}
73
74/*
75 * Generate a random byte.
76 */
77static u_int8_t
78arc4_randbyte(void)
79{
80	u_int8_t arc4_t;
81
82	arc4_i = (arc4_i + 1) % 256;
83	arc4_j = (arc4_j + arc4_sbox[arc4_i]) % 256;
84
85	arc4_swap(&arc4_sbox[arc4_i], &arc4_sbox[arc4_j]);
86
87	arc4_t = (arc4_sbox[arc4_i] + arc4_sbox[arc4_j]) % 256;
88	return arc4_sbox[arc4_t];
89}
90
91u_int32_t
92arc4random(void)
93{
94	u_int32_t ret;
95
96	/* Initialize array if needed. */
97	if (!arc4_initialized)
98		arc4_init();
99	if (++arc4_numruns > ARC4_MAXRUNS)
100	{
101		arc4_randomstir();
102		arc4_numruns = 0;
103	}
104
105	ret = arc4_randbyte();
106	ret |= arc4_randbyte() << 8;
107	ret |= arc4_randbyte() << 16;
108	ret |= arc4_randbyte() << 24;
109
110	return ret;
111}
112