63 lines
1.6 KiB
C
63 lines
1.6 KiB
C
#include "utils.h"
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#include <ctype.h>
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#include <math.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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double frequency_score(unsigned char *buf, size_t len) {
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static const double english_freqs[26] = {
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0.08167, 0.01492, 0.02782, 0.04253, 0.12702, 0.02228, 0.02015, // A-G
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0.06094, 0.06966, 0.00153, 0.00772, 0.04025, 0.02406, 0.06749, // H-N
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0.07507, 0.01929, 0.00095, 0.05987, 0.06327, 0.09056, 0.02758, // O-U
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0.00978, 0.02360, 0.00150, 0.01974, 0.00074 // V-Z
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};
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unsigned int counts[26] = {0};
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for (size_t i = 0; i < len; ++i) {
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unsigned char c = tolower(buf[i]) - 'a';
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if (c < 26) {
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counts[c]++;
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}
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}
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double chi2 = 0;
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for (size_t i = 0; i < 26; ++i) {
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double expected = len * english_freqs[i];
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chi2 += pow(counts[i] - expected, 2) / expected;
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}
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return chi2;
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}
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int main(int argc, char *argv[]) {
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if (argc < 2) {
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printf("Usage: %s <hex1>\n", argv[0]);
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return EXIT_FAILURE;
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}
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unsigned char buf[512] = {0};
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size_t len = hex_to_bytes(buf, argv[1]);
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unsigned char cur[512];
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double min_score = INFINITY;
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unsigned char key;
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unsigned char best[512];
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// Test for keys from A to z
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for (char c = 65; c < 127; ++c) {
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for (size_t i = 0; i < len; ++i) {
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cur[i] = buf[i] ^ c;
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}
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double score = frequency_score(cur, len);
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if (score < min_score) {
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min_score = score;
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key = c;
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memcpy(best, cur, len);
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}
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}
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for (size_t i = 0; i < len; ++i) {
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printf("%c", best[i]);
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}
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printf(" (key = %c, score = %.2f)\n", key, min_score);
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return EXIT_SUCCESS;
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}
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