arc4random.c revision 65754 
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 65754 2000-09-11 19:34:04Z jhb $
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	/* r = read_random(key, sizeof(key)); */
44	r = 0; /* XXX MarkM - revisit this when /dev/random is done */
45	/* if r == 0 || -1, just use what was on the stack */
46	if (r > 0)
47	{
48		for (n = r; n < sizeof(key); n++)
49			key[n] = key[n % r];
50	}
51
52	for (n = 0; n < 256; n++)
53	{
54		arc4_j = (arc4_j + arc4_sbox[n] + key[n]) % 256;
55		arc4_swap(&arc4_sbox[n], &arc4_sbox[arc4_j]);
56	}
57}
58
59/*
60 * Initialize our S-box to its beginning defaults.
61 */
62static void
63arc4_init(void)
64{
65	int n;
66
67	arc4_i = arc4_j = 0;
68	for (n = 0; n < 256; n++)
69		arc4_sbox[n] = (u_int8_t) n;
70
71	arc4_randomstir();
72	arc4_initialized = 1;
73}
74
75/*
76 * Generate a random byte.
77 */
78static u_int8_t
79arc4_randbyte(void)
80{
81	u_int8_t arc4_t;
82
83	arc4_i = (arc4_i + 1) % 256;
84	arc4_j = (arc4_j + arc4_sbox[arc4_i]) % 256;
85
86	arc4_swap(&arc4_sbox[arc4_i], &arc4_sbox[arc4_j]);
87
88	arc4_t = (arc4_sbox[arc4_i] + arc4_sbox[arc4_j]) % 256;
89	return arc4_sbox[arc4_t];
90}
91
92u_int32_t
93arc4random(void)
94{
95	u_int32_t ret;
96
97	/* Initialize array if needed. */
98	if (!arc4_initialized)
99		arc4_init();
100	if (++arc4_numruns > ARC4_MAXRUNS)
101	{
102		arc4_randomstir();
103		arc4_numruns = 0;
104	}
105
106	ret = arc4_randbyte();
107	ret |= arc4_randbyte() << 8;
108	ret |= arc4_randbyte() << 16;
109	ret |= arc4_randbyte() << 24;
110
111	return ret;
112}
113