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