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: src/sys/libkern/arc4random.c,v 1.6.4.2 2000/10/06 22:49:54 alfred Exp $
11 */
12
13 #include <sys/libkern.h>
14
15 #define ARC4_MAXRUNS 64
16
17 static u_int8_t arc4_i, arc4_j;
18 static int arc4_initialized = 0;
19 static int arc4_numruns = 0;
20 static u_int8_t arc4_sbox[256];
21
22 extern u_int read_random (void *, u_int);
23
24 static __inline void
25 arc4_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 */
37 static void
38 arc4_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 */
61 static void
62 arc4_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 */
77 static u_int8_t
78 arc4_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
91 u_int32_t
92 arc4random(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 }
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