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/* vi: set sw=4 ts=4: */
/*
* $RANDOM support.
*
* Copyright (C) 2009 Denys Vlasenko
*
* Licensed under GPLv2, see file LICENSE in this source tree.
*/
/* For testing against dieharder, you need only random.{c,h}
* Howto:
* gcc -O2 -Wall -DRANDTEST random.c -o random
* ./random | dieharder -g 200 -a
*/
#if !defined RANDTEST
# include "libbb.h"
# include "random.h"
# define RAND_BASH_MASK 0x7fff
#else
# include <stdint.h>
# include <unistd.h>
# include <stdio.h>
# include <time.h>
# define FAST_FUNC /* nothing */
# define PUSH_AND_SET_FUNCTION_VISIBILITY_TO_HIDDEN /* nothing */
# define POP_SAVED_FUNCTION_VISIBILITY /* nothing */
# define monotonic_us() time(NULL)
# include "random.h"
# define RAND_BASH_MASK 0xffffffff /* off */
#endif
uint32_t FAST_FUNC
next_random(random_t *rnd)
{
/* Galois LFSR parameter:
* Taps at 32 31 29 1:
*/
enum { MASK = 0x8000000b };
/* Another example - taps at 32 31 30 10: */
/* enum { MASK = 0x00400007 }; */
/* Xorshift parameters:
* Choices for a,b,c: 10,13,10; 8,9,22; 2,7,3; 23,3,24
* (given by algorithm author)
*/
enum {
a = 2,
b = 7,
c = 3,
};
uint32_t t;
if (UNINITED_RANDOM_T(rnd)) {
/* Can use monotonic_ns() for better randomness but for now
* it is not used anywhere else in busybox... so avoid bloat
*/
INIT_RANDOM_T(rnd, getpid(), monotonic_us());
}
/* LCG: period of 2^32, but quite weak:
* bit 0 alternates beetween 0 and 1 (pattern of length 2)
* bit 1 has a repeating pattern of length 4
* bit 2 has a repeating pattern of length 8
* etc...
*/
rnd->LCG = 1664525 * rnd->LCG + 1013904223;
/* Galois LFSR:
* period of 2^32-1 = 3 * 5 * 17 * 257 * 65537.
* Successive values are right-shifted one bit
* and possibly xored with a sparse constant.
*/
t = (rnd->galois_LFSR << 1);
if (rnd->galois_LFSR < 0) /* if we just shifted 1 out of msb... */
t ^= MASK;
rnd->galois_LFSR = t;
/* http://en.wikipedia.org/wiki/Xorshift
* Moderately good statistical properties:
* fails the following "dieharder -g 200 -a" tests:
* diehard_operm5| 0
* diehard_oqso| 0
* diehard_count_1s_byt| 0
* diehard_3dsphere| 3
* diehard_squeeze| 0
* diehard_runs| 0
* diehard_runs| 0
* diehard_craps| 0
* diehard_craps| 0
* rgb_minimum_distance| 3
* rgb_minimum_distance| 4
* rgb_minimum_distance| 5
* rgb_permutations| 3
* rgb_permutations| 4
* rgb_permutations| 5
* dab_filltree| 32
* dab_filltree| 32
* dab_monobit2| 12
*/
again:
t = rnd->xs64_x ^ (rnd->xs64_x << a);
rnd->xs64_x = rnd->xs64_y;
rnd->xs64_y = rnd->xs64_y ^ (rnd->xs64_y >> c) ^ t ^ (t >> b);
/*
* Period 2^64-1 = 2^32+1 * 2^32-1 has a common divisor with Galois LFSR.
* By skipping two possible states (0x1 and 0x2) we reduce period to
* 2^64-3 = 13 * 3889 * 364870227143809 which has no common divisors:
*/
if (rnd->xs64_y == 0 && rnd->xs64_x <= 2)
goto again;
/* Combined LCG + Galois LFSR rng has 2^32 * 2^32-1 period.
* Strength:
* individually, both are extremely weak cryptographycally;
* when combined, they fail the following "dieharder -g 200 -a" tests:
* diehard_rank_6x8| 0
* diehard_oqso| 0
* diehard_dna| 0
* diehard_count_1s_byt| 0
* rgb_bitdist| 2
* dab_monobit2| 12
*
* Combining them with xorshift-64 increases period to
* 2^32 * 2^32-1 * 2^64-3
* which is about 2^128, or in base 10 ~3.40*10^38.
* Strength of the combination:
* passes all "dieharder -g 200 -a" tests.
*
* Combining with subtraction and addition is just for fun.
* It does not add meaningful strength, could use xor operation instead.
*/
t = rnd->galois_LFSR - rnd->LCG + rnd->xs64_y;
/* bash compat $RANDOM range: */
return t & RAND_BASH_MASK;
}
#ifdef RANDTEST
static random_t rnd;
int main(int argc, char **argv)
{
int i;
uint32_t buf[4096];
for (;;) {
for (i = 0; i < sizeof(buf) / sizeof(buf[0]); i++) {
buf[i] = next_random(&rnd);
}
write(1, buf, sizeof(buf));
}
return 0;
}
#endif
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