arc4random.c revision 180657 
1/*
2 * Arc4 random number generator for OpenBSD.
3 * Copyright 1996 David Mazieres <dm@lcs.mit.edu>.
4 *
5 * Modification and redistribution in source and binary forms is
6 * permitted provided that due credit is given to the author and the
7 * OpenBSD project (for instance by leaving this copyright notice
8 * intact).
9 */
10
11/*
12 * This code is derived from section 17.1 of Applied Cryptography,
13 * second edition, which describes a stream cipher allegedly
14 * compatible with RSA Labs "RC4" cipher (the actual description of
15 * which is a trade secret).  The same algorithm is used as a stream
16 * cipher called "arcfour" in Tatu Ylonen's ssh package.
17 *
18 * Here the stream cipher has been modified always to include the time
19 * when initializing the state.  That makes it impossible to
20 * regenerate the same random sequence twice, so this can't be used
21 * for encryption, but will generate good random numbers.
22 *
23 * RC4 is a registered trademark of RSA Laboratories.
24 */
25
26#include <sys/cdefs.h>
27__FBSDID("$FreeBSD: head/lib/libc/gen/arc4random.c 180657 2008-07-21 13:52:06Z ache $");
28
29#include "namespace.h"
30#include <sys/types.h>
31#include <sys/time.h>
32#include <stdlib.h>
33#include <fcntl.h>
34#include <unistd.h>
35#include <pthread.h>
36
37#include "libc_private.h"
38#include "un-namespace.h"
39
40struct arc4_stream {
41	u_int8_t i;
42	u_int8_t j;
43	u_int8_t s[256];
44};
45
46static pthread_mutex_t	arc4random_mtx = PTHREAD_MUTEX_INITIALIZER;
47
48#define	RANDOMDEV	"/dev/urandom"
49#define	THREAD_LOCK()						\
50	do {							\
51		if (__isthreaded)				\
52			_pthread_mutex_lock(&arc4random_mtx);	\
53	} while (0)
54
55#define	THREAD_UNLOCK()						\
56	do {							\
57		if (__isthreaded)				\
58			_pthread_mutex_unlock(&arc4random_mtx);	\
59	} while (0)
60
61static struct arc4_stream rs;
62static int rs_initialized;
63static int rs_stired;
64static int arc4_count;
65
66static inline u_int8_t arc4_getbyte(struct arc4_stream *);
67static void arc4_stir(struct arc4_stream *);
68
69static inline void
70arc4_init(struct arc4_stream *as)
71{
72	int     n;
73
74	for (n = 0; n < 256; n++)
75		as->s[n] = n;
76	as->i = 0;
77	as->j = 0;
78}
79
80static inline void
81arc4_addrandom(struct arc4_stream *as, u_char *dat, int datlen)
82{
83	int     n;
84	u_int8_t si;
85
86	as->i--;
87	for (n = 0; n < 256; n++) {
88		as->i = (as->i + 1);
89		si = as->s[as->i];
90		as->j = (as->j + si + dat[n % datlen]);
91		as->s[as->i] = as->s[as->j];
92		as->s[as->j] = si;
93	}
94}
95
96static void
97arc4_stir(struct arc4_stream *as)
98{
99	int     fd, n;
100	struct {
101		struct timeval tv;
102		pid_t pid;
103		u_int8_t rnd[128 - sizeof(struct timeval) - sizeof(pid_t)];
104	}       rdat;
105
106	gettimeofday(&rdat.tv, NULL);
107	rdat.pid = getpid();
108	fd = _open(RANDOMDEV, O_RDONLY, 0);
109	if (fd >= 0) {
110		(void) _read(fd, rdat.rnd, sizeof(rdat.rnd));
111		_close(fd);
112	}
113	/* fd < 0?  Ah, what the heck. We'll just take whatever was on the
114	 * stack... */
115
116	arc4_addrandom(as, (void *) &rdat, sizeof(rdat));
117
118	/*
119	 * Throw away the first N bytes of output, as suggested in the
120	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
121	 * by Fluher, Mantin, and Shamir.  N=1024 is based on
122	 * suggestions in the paper "(Not So) Random Shuffles of RC4"
123	 * by Ilya Mironov.
124	 */
125	for (n = 0; n < 1024; n++)
126		(void) arc4_getbyte(as);
127	arc4_count = 1600000;
128}
129
130static inline u_int8_t
131arc4_getbyte(struct arc4_stream *as)
132{
133	u_int8_t si, sj;
134
135	as->i = (as->i + 1);
136	si = as->s[as->i];
137	as->j = (as->j + si);
138	sj = as->s[as->j];
139	as->s[as->i] = sj;
140	as->s[as->j] = si;
141
142	return (as->s[(si + sj) & 0xff]);
143}
144
145static inline u_int32_t
146arc4_getword(struct arc4_stream *as)
147{
148	u_int32_t val;
149
150	val = arc4_getbyte(as) << 24;
151	val |= arc4_getbyte(as) << 16;
152	val |= arc4_getbyte(as) << 8;
153	val |= arc4_getbyte(as);
154
155	return (val);
156}
157
158static void
159arc4_check_init(void)
160{
161	if (!rs_initialized) {
162		arc4_init(&rs);
163		rs_initialized = 1;
164	}
165}
166
167static inline void
168arc4_check_stir(void)
169{
170	if (!rs_stired || arc4_count <= 0) {
171		arc4_stir(&rs);
172		rs_stired = 1;
173	}
174}
175
176void
177arc4random_stir(void)
178{
179	THREAD_LOCK();
180	arc4_check_init();
181	arc4_stir(&rs);
182	rs_stired = 1;
183	THREAD_UNLOCK();
184}
185
186void
187arc4random_addrandom(u_char *dat, int datlen)
188{
189	THREAD_LOCK();
190	arc4_check_init();
191	arc4_check_stir();
192	arc4_addrandom(&rs, dat, datlen);
193	THREAD_UNLOCK();
194}
195
196u_int32_t
197arc4random(void)
198{
199	u_int32_t rnd;
200
201	THREAD_LOCK();
202	arc4_check_init();
203	arc4_check_stir();
204	rnd = arc4_getword(&rs);
205	arc4_count -= 4;
206	THREAD_UNLOCK();
207
208	return (rnd);
209}
210
211void
212arc4random_buf(void *_buf, size_t n)
213{
214	u_char *buf = (u_char *)_buf;
215
216	THREAD_LOCK();
217	arc4_check_init();
218	while (n--) {
219		arc4_check_stir();
220		buf[n] = arc4_getbyte(&rs);
221		arc4_count--;
222	}
223	THREAD_UNLOCK();
224}
225
226#if 0
227/*-------- Test code for i386 --------*/
228#include <stdio.h>
229#include <machine/pctr.h>
230int
231main(int argc, char **argv)
232{
233	const int iter = 1000000;
234	int     i;
235	pctrval v;
236
237	v = rdtsc();
238	for (i = 0; i < iter; i++)
239		arc4random();
240	v = rdtsc() - v;
241	v /= iter;
242
243	printf("%qd cycles\n", v);
244}
245#endif
246