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Find the most probable keysize using normalized edit distance

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This commit is contained in:
Dimitri Lozeve 2020-06-05 15:28:55 +02:00
parent 5b0ec4d3d8
commit 2c26f4bbcc
1 changed files with 49 additions and 1 deletions
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50
ex06/main.c
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@ -1,8 +1,32 @@
#include "utils.h"
#include <ctype.h>
#include <math.h>
#include <stdio.h>
#include <string.h>
#define BUF_SIZE 4096
double normalized_edit_distance(const unsigned char *buf,
unsigned int keysize) {
char block1[keysize];
char block2[keysize];
char block3[keysize];
char block4[keysize];
for (size_t i = 0; i < keysize; ++i) {
block1[i] = buf[i];
block2[i] = buf[keysize + i];
block3[i] = buf[2 * keysize + i];
block4[i] = buf[3 * keysize + i];
}
double dist1 = hamming(block1, block2) / (double)keysize;
double dist2 = hamming(block2, block3) / (double)keysize;
double dist3 = hamming(block3, block4) / (double)keysize;
return (dist1 + dist2 + dist3) / 3;
}
int main(int argc, char *argv[]) {
if (argc < 2) {
printf("Usage: %s <filename>\n", argv[0]);
@ -17,7 +41,31 @@ int main(int argc, char *argv[]) {
return EXIT_FAILURE;
}
printf("%u\n", hamming("this is a test", "wokka wokka!!!"));
char input[BUF_SIZE] = {'\0'};
size_t nread = fread(input, 1, BUF_SIZE, fp);
if (nread == 0) {
printf("Cannot read any character from file %s\n", filename);
return EXIT_FAILURE;
}
unsigned char buf[BUF_SIZE] = {0};
size_t len = base64_to_bytes(buf, input);
if (len == 0) {
printf("Could not decode base64\n");
return EXIT_FAILURE;
}
unsigned int keysize = 40;
double min_edit_dist = INFINITY;
for (unsigned int i = 40; i > 1; --i) {
double edit_dist = normalized_edit_distance(buf, i);
if (edit_dist < min_edit_dist) {
min_edit_dist = edit_dist;
keysize = i;
}
}
printf("keysize = %u, min edit distance = %f\n", keysize, min_edit_dist);
if (fclose(fp)) {
printf("Error closing file %s\n", filename);