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Compute normalized edit distance over the whole ciphertext

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Dimitri Lozeve 2020-06-09 17:11:30 +02:00
parent 0bce806ec6
commit bbfa502c91
1 changed files with 14 additions and 16 deletions
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30
src/ex06/main.c
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@ -7,24 +7,22 @@
#define BUF_SIZE 4096 #define BUF_SIZE 4096
double normalized_edit_distance(unsigned int keysize, double normalized_edit_distance(unsigned int keysize,
const unsigned char buf[static keysize * 4]) { const unsigned char buf[static keysize * 4],
char block1[keysize]; unsigned int blocks_count) {
char block2[keysize]; char blocks[blocks_count][keysize];
char block3[keysize];
char block4[keysize];
for (size_t i = 0; i < keysize; ++i) { for (size_t i = 0; i < blocks_count; ++i) {
block1[i] = buf[i]; for (size_t j = 0; j < keysize; ++j) {
block2[i] = buf[keysize + i]; blocks[i][j] = buf[i * keysize + j];
block3[i] = buf[2 * keysize + i]; }
block4[i] = buf[3 * keysize + i];
} }
double dist1 = hamming(block1, block2) / (double)keysize; double avg_dist = 0;
double dist2 = hamming(block2, block3) / (double)keysize; for (size_t i = 0; i < blocks_count - 1; ++i) {
double dist3 = hamming(block3, block4) / (double)keysize; avg_dist += hamming(blocks[i], blocks[i + 1]) / (double)keysize;
}
return (dist1 + dist2 + dist3) / 3; return avg_dist / ((double)blocks_count - 1.0);
} }
int main(int argc, char *argv[]) { int main(int argc, char *argv[]) {
@ -62,8 +60,8 @@ int main(int argc, char *argv[]) {
unsigned int keysize = 40; unsigned int keysize = 40;
double min_edit_dist = INFINITY; double min_edit_dist = INFINITY;
for (unsigned int i = 40; i > 1; --i) { for (unsigned int i = 30; i > 1; --i) {
double edit_dist = normalized_edit_distance(i, buf); double edit_dist = normalized_edit_distance(i, buf, len / i);
if (edit_dist < min_edit_dist) { if (edit_dist < min_edit_dist) {
min_edit_dist = edit_dist; min_edit_dist = edit_dist;
keysize = i; keysize = i;