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Add Hill Cipher (Hacktoberfest) (TheAlgorithms#2463)
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‎Ciphers/HillCipher.java‎

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package Ciphers;
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import java.util.*;
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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 belonging to the set Z26 where A=0 , B=1, ..... Z=25.
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* To encrypt a message, each block of n letters (since matrix size is n x n) is multiplied by an invertible n × n matrix, against modulus 26.
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* To decrypt the message, each block is multiplied by the inverse of the matrix used for encryption.
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* 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 class HillCipher{
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static Scanner in = 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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{
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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 = in.nextInt();
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System.out.println("Enter Key/encryptionKey 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] = in.nextInt();
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}
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}
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//check if det = 0
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if(determinant(keyMatrix,n)%26 == 0)
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{
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System.out.println("Invalid key, as determinant = 0. Program Terminated");
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return;
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}
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int [][]messageVector = new int[n][1];
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String CipherText="";
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int cipherMatrix [][] = 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()){ messageVector[i][0] = 23;}
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else
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messageVector[i][0] = (message.charAt(j))%65;
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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, i;
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for (i = 0; i < n; i++)
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{
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cipherMatrix[i][0] = 0;
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for (x = 0; x < n; x++)
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{
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cipherMatrix[i][0] += keyMatrix[i][x] * messageVector[x][0];
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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 < n; i++)
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CipherText += (char)(cipherMatrix[i][0] + 65);
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}
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System.out.println("Ciphertext: "+ CipherText);
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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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{
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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 = in.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] = in.nextInt();
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}
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}
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//check if det = 0
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if(determinant(keyMatrix,n)%26 == 0)
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{
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System.out.println("Invalid key, as determinant = 0. Program Terminated");
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return;
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}
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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()){ messageVector[i][0] = 23;}
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else
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messageVector[i][0] = (message.charAt(j))%65;
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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, i;
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for (i = 0; i < n; i++)
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{
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plainMatrix[i][0] = 0;
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for (x = 0; x < n; x++)
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{
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plainMatrix[i][0] += keyMatrix[i][x] * messageVector[x][0];
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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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}
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System.out.println("Plaintext: "+PlainText);
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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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int det = 0, sign = 1, p = 0, q = 0;
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if(n==1){
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det = a[0][0];
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}
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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;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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}
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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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}
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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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{
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message.toUpperCase();
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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 = in.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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// Driver code
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public static void main(String[] args)
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{
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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 = in.nextLine();
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hillcipher(message);
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}
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}

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