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 by leaving this copyright notice intact.
8 */
9
10/*
11 * This code is derived from section 17.1 of Applied Cryptography,
12 * second edition, which describes a stream cipher allegedly
13 * compatible with RSA Labs "RC4" cipher (the actual description of
14 * which is a trade secret).  The same algorithm is used as a stream
15 * cipher called "arcfour" in Tatu Ylonen's ssh package.
16 *
17 * Here the stream cipher has been modified always to include the time
18 * when initializing the state.  That makes it impossible to
19 * regenerate the same random sequence twice, so this can't be used
20 * for encryption, but will generate good random numbers.
21 *
22 * RC4 is a registered trademark of RSA Laboratories.
23 */
24
25#include <sys/time.h>
26
27#include <fcntl.h>
28#include <stdint.h>
29#include <stdlib.h>
30#include <unistd.h>
31
32#include "arc4random.h"
33
34struct arc4_stream {
35	uint8_t i;
36	uint8_t j;
37	uint8_t s[256];
38	size_t count;
39	pid_t stir_pid;
40};
41
42#define S(n) (n)
43#define S4(n) S(n), S(n + 1), S(n + 2), S(n + 3)
44#define S16(n) S4(n), S4(n + 4), S4(n + 8), S4(n + 12)
45#define S64(n) S16(n), S16(n + 16), S16(n + 32), S16(n + 48)
46#define S256 S64(0), S64(64), S64(128), S64(192)
47
48static struct arc4_stream rs = { .i = 0xff, .j = 0, .s = { S256 },
49                    .count = 0, .stir_pid = 0 };
50
51#undef S
52#undef S4
53#undef S16
54#undef S64
55#undef S256
56
57static void
58arc4_addrandom(struct arc4_stream *as, unsigned char *dat, int datlen)
59{
60	int n;
61	uint8_t si;
62
63	as->i--;
64	for (n = 0; n < 256; n++) {
65		as->i = (uint8_t)(as->i + 1);
66		si = as->s[as->i];
67		as->j = (uint8_t)(as->j + si + dat[n % datlen]);
68		as->s[as->i] = as->s[as->j];
69		as->s[as->j] = si;
70	}
71	as->j = as->i;
72}
73
74static uint8_t
75arc4_getbyte(struct arc4_stream *as)
76{
77	uint8_t si, sj;
78
79	as->i = (uint8_t)(as->i + 1);
80	si = as->s[as->i];
81	as->j = (uint8_t)(as->j + si);
82	sj = as->s[as->j];
83	as->s[as->i] = sj;
84	as->s[as->j] = si;
85	return (as->s[(si + sj) & 0xff]);
86}
87
88static uint32_t
89arc4_getword(struct arc4_stream *as)
90{
91	int val;
92
93	val = arc4_getbyte(as) << 24;
94	val |= arc4_getbyte(as) << 16;
95	val |= arc4_getbyte(as) << 8;
96	val |= arc4_getbyte(as);
97	return (uint32_t)val;
98}
99
100static void
101arc4_stir(struct arc4_stream *as)
102{
103	int fd;
104	struct {
105		struct timeval tv;
106		unsigned int rnd[(128 - sizeof(struct timeval)) /
107			sizeof(unsigned int)];
108	}       rdat;
109	size_t n;
110
111	gettimeofday(&rdat.tv, NULL);
112	fd = open("/dev/urandom", O_RDONLY);
113	if (fd != -1) {
114		(void)read(fd, rdat.rnd, sizeof(rdat.rnd));
115		close(fd);
116	}
117
118	/* fd < 0?  Ah, what the heck. We'll just take
119	 * whatever was on the stack... */
120	arc4_addrandom(as, (void *) &rdat, sizeof(rdat));
121
122	/*
123	 * Throw away the first N words of output, as suggested in the
124	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
125	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
126	 */
127	for (n = 0; n < 256 * sizeof(uint32_t); n++)
128		arc4_getbyte(as);
129	as->count = 1600000;
130}
131
132static void
133arc4_stir_if_needed(struct arc4_stream *as)
134{
135	pid_t pid;
136
137	pid = getpid();
138	if (as->count <= sizeof(uint32_t) || !as->stir_pid != pid) {
139		as->stir_pid = pid;
140		arc4_stir(as);
141	} else
142		as->count -= sizeof(uint32_t);
143}
144
145uint32_t
146arc4random()
147{
148
149	arc4_stir_if_needed(&rs);
150	return arc4_getword(&rs);
151}
152