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fc81021fcf
A so-called "generalized CRC" is a form of hash function based on a table similar to the conventional bytewise software implementation of CRC. For each byte in each data set, it contains a random byte permutation table. Signed-off-by: H. Peter Anvin <hpa@zytor.com>
81 lines
1.3 KiB
C
81 lines
1.3 KiB
C
/*
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* Produce a "generalized CRC" table. Assumes a platform with
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* /dev/urandom -- otherwise reimplement get_random_byte().
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*/
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#include <errno.h>
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#include <fcntl.h>
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#include <inttypes.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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static uint8_t get_random_byte(void)
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{
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static int fd = -1;
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uint8_t buf;
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int rv;
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if (fd < 0)
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fd = open("/dev/urandom", O_RDONLY);
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do {
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errno = 0;
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rv = read(fd, &buf, 1);
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if (rv < 1 && errno != EAGAIN)
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abort();
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} while (rv < 1);
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return buf;
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}
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static void random_permute(uint8_t *buf)
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{
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int i, j, k;
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int m;
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for (i = 0; i < 256; i++)
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buf[i] = i;
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m = 255;
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for (i = 255; i > 0; i--) {
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if (i <= (m >> 1))
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m >>= 1;
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do {
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j = get_random_byte() & m;
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} while (j > i);
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k = buf[i];
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buf[i] = buf[j];
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buf[j] = k;
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}
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}
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static void xcrc_table(uint64_t *buf)
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{
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uint8_t perm[256];
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int i, j;
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memset(buf, 0, 8*256); /* Make static checkers happy */
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for (i = 0; i < 8; i++) {
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random_permute(perm);
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for (j = 0; j < 256; j++)
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buf[j] = (buf[j] << 8) | perm[j];
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}
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}
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int main(void)
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{
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int i;
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uint64_t buf[256];
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xcrc_table(buf);
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for (i = 0; i < 256; i++) {
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printf("%016"PRIx64"\n", buf[i]);
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}
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return 0;
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}
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