arc4random.c revision 50476 
1/* $FreeBSD: head/lib/libc/gen/arc4random.c 50476 1999-08-28 00:22:10Z peter $ */
2
3/*
4 * Arc4 random number generator for OpenBSD.
5 * Copyright 1996 David Mazieres <dm@lcs.mit.edu>.
6 *
7 * Modification and redistribution in source and binary forms is
8 * permitted provided that due credit is given to the author and the
9 * OpenBSD project (for instance by leaving this copyright notice
10 * intact).
11 */
12
13/*
14 * This code is derived from section 17.1 of Applied Cryptography,
15 * second edition, which describes a stream cipher allegedly
16 * compatible with RSA Labs "RC4" cipher (the actual description of
17 * which is a trade secret).  The same algorithm is used as a stream
18 * cipher called "arcfour" in Tatu Ylonen's ssh package.
19 *
20 * Here the stream cipher has been modified always to include the time
21 * when initializing the state.  That makes it impossible to
22 * regenerate the same random sequence twice, so this can't be used
23 * for encryption, but will generate good random numbers.
24 *
25 * RC4 is a registered trademark of RSA Laboratories.
26 */
27
28#include <stdlib.h>
29#include <fcntl.h>
30#include <unistd.h>
31#include <sys/types.h>
32#include <sys/time.h>
33
34struct arc4_stream {
35	u_int8_t i;
36	u_int8_t j;
37	u_int8_t s[256];
38};
39
40static int rs_initialized;
41static struct arc4_stream rs;
42
43static inline void
44arc4_init(as)
45	struct arc4_stream *as;
46{
47	int     n;
48
49	for (n = 0; n < 256; n++)
50		as->s[n] = n;
51	as->i = 0;
52	as->j = 0;
53}
54
55static inline void
56arc4_addrandom(as, dat, datlen)
57	struct arc4_stream *as;
58	u_char *dat;
59	int     datlen;
60{
61	int     n;
62	u_int8_t si;
63
64	as->i--;
65	for (n = 0; n < 256; n++) {
66		as->i = (as->i + 1);
67		si = as->s[as->i];
68		as->j = (as->j + si + dat[n % datlen]);
69		as->s[as->i] = as->s[as->j];
70		as->s[as->j] = si;
71	}
72}
73
74static void
75arc4_stir(as)
76	struct arc4_stream *as;
77{
78	int     fd;
79	struct {
80		struct timeval tv;
81		pid_t pid;
82		u_int8_t rnd[128 - sizeof(struct timeval) - sizeof(pid_t)];
83	}       rdat;
84
85	gettimeofday(&rdat.tv, NULL);
86	rdat.pid = getpid();
87	fd = open("/dev/urandom", O_RDONLY, 0);
88	if (fd >= 0) {
89		(void) read(fd, rdat.rnd, sizeof(rdat.rnd));
90		close(fd);
91	}
92	/* fd < 0?  Ah, what the heck. We'll just take whatever was on the
93	 * stack... */
94
95	arc4_addrandom(as, (void *) &rdat, sizeof(rdat));
96}
97
98static inline u_int8_t
99arc4_getbyte(as)
100	struct arc4_stream *as;
101{
102	u_int8_t si, sj;
103
104	as->i = (as->i + 1);
105	si = as->s[as->i];
106	as->j = (as->j + si);
107	sj = as->s[as->j];
108	as->s[as->i] = sj;
109	as->s[as->j] = si;
110	return (as->s[(si + sj) & 0xff]);
111}
112
113static inline u_int32_t
114arc4_getword(as)
115	struct arc4_stream *as;
116{
117	u_int32_t val;
118	val = arc4_getbyte(as) << 24;
119	val |= arc4_getbyte(as) << 16;
120	val |= arc4_getbyte(as) << 8;
121	val |= arc4_getbyte(as);
122	return val;
123}
124
125void
126arc4random_stir()
127{
128	if (!rs_initialized) {
129		arc4_init(&rs);
130		rs_initialized = 1;
131	}
132	arc4_stir(&rs);
133}
134
135void
136arc4random_addrandom(dat, datlen)
137	u_char *dat;
138	int     datlen;
139{
140	if (!rs_initialized)
141		arc4random_stir();
142	arc4_addrandom(&rs, dat, datlen);
143}
144
145u_int32_t
146arc4random()
147{
148	if (!rs_initialized)
149		arc4random_stir();
150	return arc4_getword(&rs);
151}
152
153#if 0
154/*-------- Test code for i386 --------*/
155#include <stdio.h>
156#include <machine/pctr.h>
157int
158main(int argc, char **argv)
159{
160	const int iter = 1000000;
161	int     i;
162	pctrval v;
163
164	v = rdtsc();
165	for (i = 0; i < iter; i++)
166		arc4random();
167	v = rdtsc() - v;
168	v /= iter;
169
170	printf("%qd cycles\n", v);
171}
172#endif
173