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Add Tests for HillCipher (#5562)
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Benjamin Burstein
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@ -1,178 +1,103 @@
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package com.thealgorithms.ciphers;
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import java.util.Scanner;
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public class HillCipher {
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/*
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* Java Implementation of Hill Cipher
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* Hill cipher is a polyalphabetic substitution cipher. Each letter is represented by a number
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* belonging to the set Z26 where A=0 , B=1, ..... Z=25. To encrypt a message, each block of n
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* letters (since matrix size is n x n) is multiplied by an invertible n × n matrix, against
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* modulus 26. To decrypt the message, each block is multiplied by the inverse of the matrix used
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* for encryption. The cipher key and plaintext/ciphertext are user inputs.
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* @author Ojasva Jain
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*/
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public final class HillCipher {
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private HillCipher() {
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}
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static Scanner userInput = new Scanner(System.in);
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/* Following function encrypts the message
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*/
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static void encrypt(String message) {
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message = message.toUpperCase();
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// Get key matrix
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System.out.println("Enter key matrix size");
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int matrixSize = userInput.nextInt();
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System.out.println("Enter Key/encryptionKey matrix ");
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int[][] keyMatrix = new int[matrixSize][matrixSize];
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for (int i = 0; i < matrixSize; i++) {
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for (int j = 0; j < matrixSize; j++) {
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keyMatrix[i][j] = userInput.nextInt();
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}
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}
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// check if det = 0
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// Encrypts the message using the key matrix
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public String encrypt(String message, int[][] keyMatrix) {
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message = message.toUpperCase().replaceAll("[^A-Z]", "");
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int matrixSize = keyMatrix.length;
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validateDeterminant(keyMatrix, matrixSize);
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int[][] messageVector = new int[matrixSize][1];
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String cipherText = "";
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int[][] cipherMatrix = new int[matrixSize][1];
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int j = 0;
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while (j < message.length()) {
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StringBuilder cipherText = new StringBuilder();
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int[] messageVector = new int[matrixSize];
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int[] cipherVector = new int[matrixSize];
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int index = 0;
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while (index < message.length()) {
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for (int i = 0; i < matrixSize; i++) {
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if (j >= message.length()) {
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messageVector[i][0] = 23;
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if (index < message.length()) {
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messageVector[i] = message.charAt(index++) - 'A';
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} else {
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messageVector[i][0] = (message.charAt(j)) % 65;
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messageVector[i] = 'X' - 'A'; // Padding with 'X' if needed
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}
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System.out.println(messageVector[i][0]);
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j++;
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}
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int x;
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int i;
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for (i = 0; i < matrixSize; i++) {
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cipherMatrix[i][0] = 0;
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for (x = 0; x < matrixSize; x++) {
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cipherMatrix[i][0] += keyMatrix[i][x] * messageVector[x][0];
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for (int i = 0; i < matrixSize; i++) {
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cipherVector[i] = 0;
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for (int j = 0; j < matrixSize; j++) {
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cipherVector[i] += keyMatrix[i][j] * messageVector[j];
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}
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System.out.println(cipherMatrix[i][0]);
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cipherMatrix[i][0] = cipherMatrix[i][0] % 26;
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}
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for (i = 0; i < matrixSize; i++) {
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cipherText += (char) (cipherMatrix[i][0] + 65);
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cipherVector[i] = cipherVector[i] % 26;
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cipherText.append((char) (cipherVector[i] + 'A'));
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}
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}
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System.out.println("Ciphertext: " + cipherText);
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return cipherText.toString();
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}
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// Following function decrypts a message
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static void decrypt(String message) {
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message = message.toUpperCase();
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// Get key matrix
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System.out.println("Enter key matrix size");
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int n = userInput.nextInt();
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System.out.println("Enter inverseKey/decryptionKey matrix ");
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int[][] keyMatrix = new int[n][n];
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for (int i = 0; i < n; i++) {
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for (int j = 0; j < n; j++) {
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keyMatrix[i][j] = userInput.nextInt();
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}
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}
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// check if det = 0
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validateDeterminant(keyMatrix, n);
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// Decrypts the message using the inverse key matrix
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public String decrypt(String message, int[][] inverseKeyMatrix) {
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message = message.toUpperCase().replaceAll("[^A-Z]", "");
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int matrixSize = inverseKeyMatrix.length;
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validateDeterminant(inverseKeyMatrix, matrixSize);
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// solving for the required plaintext message
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int[][] messageVector = new int[n][1];
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String plainText = "";
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int[][] plainMatrix = new int[n][1];
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int j = 0;
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while (j < message.length()) {
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for (int i = 0; i < n; i++) {
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if (j >= message.length()) {
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messageVector[i][0] = 23;
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StringBuilder plainText = new StringBuilder();
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int[] messageVector = new int[matrixSize];
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int[] plainVector = new int[matrixSize];
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int index = 0;
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while (index < message.length()) {
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for (int i = 0; i < matrixSize; i++) {
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if (index < message.length()) {
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messageVector[i] = message.charAt(index++) - 'A';
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} else {
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messageVector[i][0] = (message.charAt(j)) % 65;
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messageVector[i] = 'X' - 'A'; // Padding with 'X' if needed
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}
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System.out.println(messageVector[i][0]);
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j++;
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}
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int x;
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int i;
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for (i = 0; i < n; i++) {
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plainMatrix[i][0] = 0;
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for (x = 0; x < n; x++) {
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plainMatrix[i][0] += keyMatrix[i][x] * messageVector[x][0];
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for (int i = 0; i < matrixSize; i++) {
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plainVector[i] = 0;
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for (int j = 0; j < matrixSize; j++) {
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plainVector[i] += inverseKeyMatrix[i][j] * messageVector[j];
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}
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plainMatrix[i][0] = plainMatrix[i][0] % 26;
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}
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for (i = 0; i < n; i++) {
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plainText += (char) (plainMatrix[i][0] + 65);
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plainVector[i] = plainVector[i] % 26;
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plainText.append((char) (plainVector[i] + 'A'));
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}
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}
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System.out.println("Plaintext: " + plainText);
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return plainText.toString();
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}
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// Determinant calculator
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public static int determinant(int[][] a, int n) {
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// Validates that the determinant of the key matrix is not zero modulo 26
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private void validateDeterminant(int[][] keyMatrix, int n) {
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int det = determinant(keyMatrix, n) % 26;
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if (det == 0) {
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throw new IllegalArgumentException("Invalid key matrix. Determinant is zero modulo 26.");
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}
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}
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// Computes the determinant of a matrix recursively
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private int determinant(int[][] matrix, int n) {
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int det = 0;
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int sign = 1;
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int p = 0;
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int q = 0;
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if (n == 1) {
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det = a[0][0];
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} else {
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int[][] b = new int[n - 1][n - 1];
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for (int x = 0; x < n; x++) {
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p = 0;
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q = 0;
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for (int i = 1; i < n; i++) {
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for (int j = 0; j < n; j++) {
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if (j != x) {
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b[p][q++] = a[i][j];
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if (q % (n - 1) == 0) {
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p++;
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q = 0;
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}
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}
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return matrix[0][0];
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}
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int sign = 1;
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int[][] subMatrix = new int[n - 1][n - 1];
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for (int x = 0; x < n; x++) {
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int subI = 0;
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for (int i = 1; i < n; i++) {
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int subJ = 0;
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for (int j = 0; j < n; j++) {
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if (j != x) {
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subMatrix[subI][subJ++] = matrix[i][j];
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}
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}
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det = det + a[0][x] * determinant(b, n - 1) * sign;
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sign = -sign;
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subI++;
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}
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det += sign * matrix[0][x] * determinant(subMatrix, n - 1);
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sign = -sign;
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}
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return det;
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}
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// Function to implement Hill Cipher
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static void hillCipher(String message) {
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System.out.println("What do you want to process from the message?");
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System.out.println("Press 1: To Encrypt");
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System.out.println("Press 2: To Decrypt");
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short sc = userInput.nextShort();
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if (sc == 1) {
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encrypt(message);
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} else if (sc == 2) {
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decrypt(message);
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} else {
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System.out.println("Invalid input, program terminated.");
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}
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}
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static void validateDeterminant(int[][] keyMatrix, int n) {
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if (determinant(keyMatrix, n) % 26 == 0) {
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System.out.println("Invalid key, as determinant = 0. Program Terminated");
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}
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}
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// Driver code
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public static void main(String[] args) {
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// Get the message to be encrypted
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System.out.println("Enter message");
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String message = userInput.nextLine();
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hillCipher(message);
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}
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}
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