84 lines
1.6 KiB
C
Executable File
84 lines
1.6 KiB
C
Executable File
#include "random.h"
|
|
#include <stdbool.h>
|
|
#include <stdint.h>
|
|
#include "debug.h"
|
|
|
|
/* RC4-based pseudo-random number generator (PRNG).
|
|
|
|
RC4 is a stream cipher. We're not using it here for its
|
|
cryptographic properties, but because it is easy to implement
|
|
and its output is plenty random for non-cryptographic
|
|
purposes.
|
|
|
|
See http://en.wikipedia.org/wiki/RC4_(cipher) for information
|
|
on RC4.*/
|
|
|
|
/* RC4 state. */
|
|
static uint8_t s[256]; /* S[]. */
|
|
static uint8_t s_i, s_j; /* i, j. */
|
|
|
|
/* Already initialized? */
|
|
static bool inited;
|
|
|
|
/* Swaps the bytes pointed to by A and B. */
|
|
static inline void
|
|
swap_byte (uint8_t *a, uint8_t *b)
|
|
{
|
|
uint8_t t = *a;
|
|
*a = *b;
|
|
*b = t;
|
|
}
|
|
|
|
/* Initializes or reinitializes the PRNG with the given SEED. */
|
|
void
|
|
random_init (unsigned seed)
|
|
{
|
|
uint8_t *seedp = (uint8_t *) &seed;
|
|
int i;
|
|
uint8_t j;
|
|
|
|
for (i = 0; i < 256; i++)
|
|
s[i] = i;
|
|
for (i = j = 0; i < 256; i++)
|
|
{
|
|
j += s[i] + seedp[i % sizeof seed];
|
|
swap_byte (s + i, s + j);
|
|
}
|
|
|
|
s_i = s_j = 0;
|
|
inited = true;
|
|
}
|
|
|
|
/* Writes SIZE random bytes into BUF. */
|
|
void
|
|
random_bytes (void *buf_, size_t size)
|
|
{
|
|
uint8_t *buf;
|
|
|
|
if (!inited)
|
|
random_init (0);
|
|
|
|
for (buf = buf_; size-- > 0; buf++)
|
|
{
|
|
uint8_t s_k;
|
|
|
|
s_i++;
|
|
s_j += s[s_i];
|
|
swap_byte (s + s_i, s + s_j);
|
|
|
|
s_k = s[s_i] + s[s_j];
|
|
*buf = s[s_k];
|
|
}
|
|
}
|
|
|
|
/* Returns a pseudo-random unsigned long.
|
|
Use random_ulong() % n to obtain a random number in the range
|
|
0...n (exclusive). */
|
|
unsigned long
|
|
random_ulong (void)
|
|
{
|
|
unsigned long ul;
|
|
random_bytes (&ul, sizeof ul);
|
|
return ul;
|
|
}
|