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
34 struct 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
48 static 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
57 static void
arc4_addrandom(struct arc4_stream * as,unsigned char * dat,int datlen)58 arc4_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
74 static uint8_t
arc4_getbyte(struct arc4_stream * as)75 arc4_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
88 static uint32_t
arc4_getword(struct arc4_stream * as)89 arc4_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
100 static void
arc4_stir(struct arc4_stream * as)101 arc4_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
132 static void
arc4_stir_if_needed(struct arc4_stream * as)133 arc4_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
145 uint32_t
arc4random()146 arc4random()
147 {
148
149 arc4_stir_if_needed(&rs);
150 return arc4_getword(&rs);
151 }
152