JavaAlgorithms/Ciphers/HillCipher.java

package Ciphers;

import java.util.*;

/*
* Java Implementation of Hill Cipher
* 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.
* 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.
* To decrypt the message, each block is multiplied by the inverse of the matrix used for encryption.
* The cipher key and plaintext/ciphertext are user inputs.
* @author Ojasva Jain
*/

public class HillCipher{
static Scanner in = new Scanner (System.in);

/* Following function encrypts the message
*/
static void encrypt(String message)
{
    message = message.toUpperCase();
    // Get key matrix
    System.out.println("Enter key matrix size");
    int n = in.nextInt();
    System.out.println("Enter Key/encryptionKey matrix ");
    int keyMatrix[][] = new int [n][n];
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            keyMatrix[i][j] = in.nextInt();
        }
    }
    //check if det = 0
    if(determinant(keyMatrix,n)%26 == 0)
    {
        System.out.println("Invalid key, as determinant = 0. Program Terminated");
        return;
    }

    int [][]messageVector = new int[n][1];
    String CipherText="";
    int cipherMatrix [][] = new int [n][1];
    int j = 0;
    while(j<message.length()){
    for (int i = 0; i < n; i++){
        if(j>=message.length()){ messageVector[i][0] = 23;}
        else
        messageVector[i][0] = (message.charAt(j))%65;
        System.out.println(messageVector[i][0]);
        j++; 
    }
    int x, i;
    for (i = 0; i < n; i++)
    {
        cipherMatrix[i][0] = 0;

            for (x = 0; x < n; x++)
            {
                cipherMatrix[i][0] += keyMatrix[i][x] * messageVector[x][0];
            }
            System.out.println(cipherMatrix[i][0]);
            cipherMatrix[i][0] = cipherMatrix[i][0] % 26;
        }
    for (i = 0; i < n; i++)
    CipherText += (char)(cipherMatrix[i][0] + 65);
}
System.out.println("Ciphertext: "+ CipherText);
}
//Following function decrypts a message
static void decrypt(String message)
{
    message = message.toUpperCase();
    // Get key matrix
    System.out.println("Enter key matrix size");
    int n = in.nextInt();
    System.out.println("Enter inverseKey/decryptionKey matrix ");
    int keyMatrix[][] = new int [n][n];
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            keyMatrix[i][j] = in.nextInt();
        }
    }
    //check if det = 0
    if(determinant(keyMatrix,n)%26 == 0)
    {
        System.out.println("Invalid key, as determinant = 0. Program Terminated");
        return;
    }
    //solving for the required plaintext message
    int [][]messageVector = new int[n][1];
    String PlainText="";
    int plainMatrix [][] = new int [n][1];
    int j = 0;
    while(j<message.length()){
        for (int i = 0; i < n; i++){
            if(j>=message.length()){ messageVector[i][0] = 23;}
            else
            messageVector[i][0] = (message.charAt(j))%65;
            System.out.println(messageVector[i][0]);
            j++; 
        }
        int x, i;
        for (i = 0; i < n; i++)
        {
            plainMatrix[i][0] = 0;
    
                for (x = 0; x < n; x++)
                {
                    plainMatrix[i][0] += keyMatrix[i][x] * messageVector[x][0];
                }
    
                plainMatrix[i][0] = plainMatrix[i][0] % 26;
            }
        for (i = 0; i < n; i++)
        PlainText += (char)(plainMatrix[i][0] + 65);
    }
    System.out.println("Plaintext: "+PlainText);
}

// Determinant calculator
public static int determinant(int a[][], int n){
    int det = 0, sign = 1, p = 0, q = 0;

    if(n==1){
        det = a[0][0];
    }
    else{
        int b[][] = new int[n-1][n-1];
        for(int x = 0 ; x < n ; x++){
            p=0;q=0;
            for(int i = 1;i < n; i++){
                for(int j = 0; j < n;j++){
                    if(j != x){
                        b[p][q++] = a[i][j];
                        if(q % (n-1) == 0){
                            p++;
                            q=0;
                        }
                    }
                }
            }
            det = det + a[0][x] *determinant(b, n-1) * sign;
            sign = -sign;
        }
    }
    return det;
}

// Function to implement Hill Cipher
static void hillcipher(String message)
{
    message.toUpperCase();
    System.out.println("What do you want to process from the message?");
    System.out.println("Press 1: To Encrypt");
    System.out.println("Press 2: To Decrypt");
    short sc = in.nextShort();
    if(sc == 1)
    encrypt(message);
    else if(sc == 2)
    decrypt(message);
    else
    System.out.println("Invalid input, program terminated.");
}

// Driver code
public static void main(String[] args)
   {
    // Get the message to be encrypted
    System.out.println("Enter message");
    String message = in.nextLine();
    hillcipher(message);
    }
}