blob: 5d3620516a329fad26e45227138f6cb775464fbf
1 | /* vi: set sw=4 ts=4: */ |
2 | /* |
3 | * $RANDOM support. |
4 | * |
5 | * Copyright (C) 2009 Denys Vlasenko |
6 | * |
7 | * Licensed under GPLv2, see file LICENSE in this source tree. |
8 | */ |
9 | |
10 | /* For testing against dieharder, you need only random.{c,h} |
11 | * Howto: |
12 | * gcc -O2 -Wall -DRANDTEST random.c -o random |
13 | * ./random | dieharder -g 200 -a |
14 | */ |
15 | |
16 | #if !defined RANDTEST |
17 | |
18 | # include "libbb.h" |
19 | # include "random.h" |
20 | # define RAND_BASH_MASK 0x7fff |
21 | |
22 | #else |
23 | # include <stdint.h> |
24 | # include <unistd.h> |
25 | # include <stdio.h> |
26 | # include <time.h> |
27 | # define FAST_FUNC /* nothing */ |
28 | # define PUSH_AND_SET_FUNCTION_VISIBILITY_TO_HIDDEN /* nothing */ |
29 | # define POP_SAVED_FUNCTION_VISIBILITY /* nothing */ |
30 | # define monotonic_us() time(NULL) |
31 | # include "random.h" |
32 | # define RAND_BASH_MASK 0xffffffff /* off */ |
33 | #endif |
34 | |
35 | uint32_t FAST_FUNC |
36 | next_random(random_t *rnd) |
37 | { |
38 | /* Galois LFSR parameter: |
39 | * Taps at 32 31 29 1: |
40 | */ |
41 | enum { MASK = 0x8000000b }; |
42 | /* Another example - taps at 32 31 30 10: */ |
43 | /* enum { MASK = 0x00400007 }; */ |
44 | |
45 | /* Xorshift parameters: |
46 | * Choices for a,b,c: 10,13,10; 8,9,22; 2,7,3; 23,3,24 |
47 | * (given by algorithm author) |
48 | */ |
49 | enum { |
50 | a = 2, |
51 | b = 7, |
52 | c = 3, |
53 | }; |
54 | |
55 | uint32_t t; |
56 | |
57 | if (UNINITED_RANDOM_T(rnd)) { |
58 | /* Can use monotonic_ns() for better randomness but for now |
59 | * it is not used anywhere else in busybox... so avoid bloat |
60 | */ |
61 | INIT_RANDOM_T(rnd, getpid(), monotonic_us()); |
62 | } |
63 | |
64 | /* LCG: period of 2^32, but quite weak: |
65 | * bit 0 alternates beetween 0 and 1 (pattern of length 2) |
66 | * bit 1 has a repeating pattern of length 4 |
67 | * bit 2 has a repeating pattern of length 8 |
68 | * etc... |
69 | */ |
70 | rnd->LCG = 1664525 * rnd->LCG + 1013904223; |
71 | |
72 | /* Galois LFSR: |
73 | * period of 2^32-1 = 3 * 5 * 17 * 257 * 65537. |
74 | * Successive values are right-shifted one bit |
75 | * and possibly xored with a sparse constant. |
76 | */ |
77 | t = (rnd->galois_LFSR << 1); |
78 | if (rnd->galois_LFSR < 0) /* if we just shifted 1 out of msb... */ |
79 | t ^= MASK; |
80 | rnd->galois_LFSR = t; |
81 | |
82 | /* http://en.wikipedia.org/wiki/Xorshift |
83 | * Moderately good statistical properties: |
84 | * fails the following "dieharder -g 200 -a" tests: |
85 | * diehard_operm5| 0 |
86 | * diehard_oqso| 0 |
87 | * diehard_count_1s_byt| 0 |
88 | * diehard_3dsphere| 3 |
89 | * diehard_squeeze| 0 |
90 | * diehard_runs| 0 |
91 | * diehard_runs| 0 |
92 | * diehard_craps| 0 |
93 | * diehard_craps| 0 |
94 | * rgb_minimum_distance| 3 |
95 | * rgb_minimum_distance| 4 |
96 | * rgb_minimum_distance| 5 |
97 | * rgb_permutations| 3 |
98 | * rgb_permutations| 4 |
99 | * rgb_permutations| 5 |
100 | * dab_filltree| 32 |
101 | * dab_filltree| 32 |
102 | * dab_monobit2| 12 |
103 | */ |
104 | again: |
105 | t = rnd->xs64_x ^ (rnd->xs64_x << a); |
106 | rnd->xs64_x = rnd->xs64_y; |
107 | rnd->xs64_y = rnd->xs64_y ^ (rnd->xs64_y >> c) ^ t ^ (t >> b); |
108 | /* |
109 | * Period 2^64-1 = 2^32+1 * 2^32-1 has a common divisor with Galois LFSR. |
110 | * By skipping two possible states (0x1 and 0x2) we reduce period to |
111 | * 2^64-3 = 13 * 3889 * 364870227143809 which has no common divisors: |
112 | */ |
113 | if (rnd->xs64_y == 0 && rnd->xs64_x <= 2) |
114 | goto again; |
115 | |
116 | /* Combined LCG + Galois LFSR rng has 2^32 * 2^32-1 period. |
117 | * Strength: |
118 | * individually, both are extremely weak cryptographycally; |
119 | * when combined, they fail the following "dieharder -g 200 -a" tests: |
120 | * diehard_rank_6x8| 0 |
121 | * diehard_oqso| 0 |
122 | * diehard_dna| 0 |
123 | * diehard_count_1s_byt| 0 |
124 | * rgb_bitdist| 2 |
125 | * dab_monobit2| 12 |
126 | * |
127 | * Combining them with xorshift-64 increases period to |
128 | * 2^32 * 2^32-1 * 2^64-3 |
129 | * which is about 2^128, or in base 10 ~3.40*10^38. |
130 | * Strength of the combination: |
131 | * passes all "dieharder -g 200 -a" tests. |
132 | * |
133 | * Combining with subtraction and addition is just for fun. |
134 | * It does not add meaningful strength, could use xor operation instead. |
135 | */ |
136 | t = rnd->galois_LFSR - rnd->LCG + rnd->xs64_y; |
137 | |
138 | /* bash compat $RANDOM range: */ |
139 | return t & RAND_BASH_MASK; |
140 | } |
141 | |
142 | #ifdef RANDTEST |
143 | static random_t rnd; |
144 | |
145 | int main(int argc, char **argv) |
146 | { |
147 | int i; |
148 | uint32_t buf[4096]; |
149 | |
150 | for (;;) { |
151 | for (i = 0; i < sizeof(buf) / sizeof(buf[0]); i++) { |
152 | buf[i] = next_random(&rnd); |
153 | } |
154 | write(1, buf, sizeof(buf)); |
155 | } |
156 | |
157 | return 0; |
158 | } |
159 | |
160 | #endif |
161 |