Computer Penetration Tutorial: Deciphering multiple plaintexts encryped with the same key and finding the key

SameKey.c

#include <stdio.h>
#include <stdbool.h>

#define NO_OF_WORDS 24
#define LENGTH 6

int main() {
    unsigned char ctxt[NO_OF_WORDS][LENGTH] = { // Ciphertext
        {0xd1, 0x0d, 0x01, 0x90, 0xaa, 0x80},
        {0xd9, 0x11, 0x11, 0x87, 0xb6, 0x9b},
        {0xc2, 0x0a, 0x02, 0x9a, 0xb2, 0x8a},
        {0xd0, 0x0a, 0x02, 0x9a, 0xbb, 0x9d},
        {0xc8, 0x16, 0x1f, 0x9a, 0xbf, 0x88},
        {0xc1, 0x16, 0x02, 0x9a, 0xb2, 0x8a},
        {0xda, 0x16, 0x12, 0x81, 0xbd, 0x84},
        {0xd8, 0x10, 0x13, 0x89, 0xb0, 0x88},
        {0xd3, 0x1d, 0x17, 0x81, 0xac, 0x8b},
        {0xcb, 0x1e, 0x1b, 0x88, 0xaa, 0x9c},
        {0xc5, 0x1e, 0x1e, 0x86, 0xb2, 0x8a},
        {0xd6, 0x1e, 0x1d, 0x8d, 0xb1, 0x81},
        {0xd1, 0x1e, 0x1b, 0x94, 0xab, 0x9c},
        {0xc4, 0x1e, 0x1b, 0x95, 0xab, 0x82},
        {0xd3, 0x1c, 0x1b, 0x85, 0xae, 0x9b},
        {0xd4, 0x10, 0x14, 0x8c, 0xb1, 0x98},
        {0xd6, 0x10, 0x0d, 0x82, 0xb2, 0x8a},
        {0xcb, 0x1a, 0x14, 0x8c, 0xb1, 0x98},
        {0xd0, 0x1a, 0x0c, 0x94, 0xbb, 0x9d},
        {0xd3, 0x1c, 0x1b, 0x85, 0xad, 0x9c},
        {0xd7, 0x11, 0x1c, 0x89, 0xb0, 0x88},
        {0xc4, 0x16, 0x0b, 0x95, 0xbf, 0x83},
        {0xd1, 0x1e, 0x16, 0x83, 0xbb, 0x9d},
        {0xd3, 0x1c, 0x0c, 0x89, 0xa8, 0x8a},
    };

	unsigned char ptxt[NO_OF_WORDS][LENGTH]; // Plaintext
    unsigned char key[LENGTH];

	// Guessing algorithm
	for(int i = 0; i < LENGTH; i++) { // Character iteration
		unsigned char xorval[NO_OF_WORDS];
		for(int j = 0; j < NO_OF_WORDS; j++) { // XOR value calculation
			xorval[j] = ctxt[0][i] ^ ctxt[j][i];
		}
		bool ptxtfound = false;
		int notyet = 0;
		for(unsigned char j = 0x61; j <= 0x7a; j++) {
			for(int k = 0; k < NO_OF_WORDS; k++){ // Word iteration
				if((j ^ xorval[k]) >= 0x61 && (j ^ xorval[k]) <= 0x7a) { // Guess and check
					ptxt[k][i] = (j ^ xorval[k]);
					if(k==NO_OF_WORDS-1){
						if(i > 3 && notyet < i) {
							/*
							 * Special case for the fifth and sixth character:
							 * The correct guess is not the first guess and
							 * coincidentally is the same number of guesses
							 * as the character's position in the word.
							 */
							notyet++;
							break;
						}
						else {
							ptxtfound = true;
							break;
						}
					}
				}
				else {
					break; // Use next letter if not in range
				}
			}
			if(i!=0 && notyet == i || ptxtfound)
				break;
		}
	}

	// Finding the key (Brute force method)
	for (int i = 0; i < LENGTH; i++) {
		for (unsigned char j = 0x00; j < 0xff; j++){
			for (int k=0; k < NO_OF_WORDS; k++) {
				if((ptxt[k][i] ^ j) == ctxt[k][i]){
					key[i] = j;
				}
				else {
					break;
				}
			}
		}
	}

	//Output
	printf("\nCiphertext:\n");
	for (int i=0; i < NO_OF_WORDS; i++) {
		for (int j = 0; j < LENGTH; j++) {
			printf("%02x ", ctxt[i][j]);
		}
		printf("\n");
	}
	printf("\nPlaintext:\n");
	for (int i = 0; i < NO_OF_WORDS; i++) {
		for (int j = 0; j < LENGTH; j++) {
			printf("%02x ", ptxt[i][j]);
		}
		printf("( ");
		for (int j=0; j < LENGTH; j++) {
			printf("%c ", ptxt[i][j]);
		}
		printf(")\n");
	}
	printf("\nKey: ");
		for (int i = 0; i < LENGTH; i++)
			printf("%02x ", key[i]);
	printf("\n");

    return 0;
}