Java Program to Implement the Hill Cypher

This is a java program to implement hill cipher. In classical cryptography, the Hill cipher is a polygraphic substitution cipher based on linear algebra. Invented by Lester S. Hill in 1929, it was the first polygraphic cipher in which it was practical (though barely) to operate on more than three symbols at once. The following discussion assumes an elementary knowledge of matrices.

Here is the source code of the Java Program to Implement the Hill Cypher. The Java program is successfully compiled and run on a Windows system. The program output is also shown below.

package com.maixuanviet.setandstring;
  
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
  
public class HillCipher
{
    int keymatrix[][];
    int linematrix[];
    int resultmatrix[];
  
    public void divide(String temp, int s)
    {
        while (temp.length() > s)
        {
            String sub = temp.substring(0, s);
            temp = temp.substring(s, temp.length());
            perform(sub);
        }
        if (temp.length() == s)
            perform(temp);
        else if (temp.length() < s)
        {
            for (int i = temp.length(); i < s; i++)
                temp = temp + 'x';
            perform(temp);
        }
    }
  
    public void perform(String line)
    {
        linetomatrix(line);
        linemultiplykey(line.length());
        result(line.length());
    }
  
    public void keytomatrix(String key, int len)
    {
        keymatrix = new int[len][len];
        int c = 0;
        for (int i = 0; i < len; i++)
        {
            for (int j = 0; j < len; j++)
            {
                keymatrix[i][j] = ((int) key.charAt(c)) - 97;
                c++;
            }
        }
    }
  
    public void linetomatrix(String line)
    {
        linematrix = new int[line.length()];
        for (int i = 0; i < line.length(); i++)
        {
            linematrix[i] = ((int) line.charAt(i)) - 97;
        }
    }
  
    public void linemultiplykey(int len)
    {
        resultmatrix = new int[len];
        for (int i = 0; i < len; i++)
        {
            for (int j = 0; j < len; j++)
            {
                resultmatrix[i] += keymatrix[i][j] * linematrix[j];
            }
            resultmatrix[i] %= 26;
        }
    }
  
    public void result(int len)
    {
        String result = "";
        for (int i = 0; i < len; i++)
        {
            result += (char) (resultmatrix[i] + 97);
        }
        System.out.print(result);
    }
  
    public boolean check(String key, int len)
    {
        keytomatrix(key, len);
        int d = determinant(keymatrix, len);
        d = d % 26;
        if (d == 0)
        {
            System.out
                    .println("Invalid key!!! Key is not invertible because determinant=0...");
            return false;
        }
        else if (d % 2 == 0 || d % 13 == 0)
        {
            System.out
                    .println("Invalid key!!! Key is not invertible because determinant has common factor with 26...");
            return false;
        }
        else
        {
            return true;
        }
    }
  
    public int determinant(int A[][], int N)
    {
        int res;
        if (N == 1)
            res = A[0][0];
        else if (N == 2)
        {
            res = A[0][0] * A[1][1] - A[1][0] * A[0][1];
        }
        else
        {
            res = 0;
            for (int j1 = 0; j1 < N; j1++)
            {
                int m[][] = new int[N - 1][N - 1];
                for (int i = 1; i < N; i++)
                {
                    int j2 = 0;
                    for (int j = 0; j < N; j++)
                    {
                        if (j == j1)
                            continue;
                        m[i - 1][j2] = A[i][j];
                        j2++;
                    }
                }
                res += Math.pow(-1.0, 1.0 + j1 + 1.0) * A[0][j1]
                        * determinant(m, N - 1);
            }
        }
        return res;
    }
  
    public void cofact(int num[][], int f)
    {
        int b[][], fac[][];
        b = new int[f][f];
        fac = new int[f][f];
        int p, q, m, n, i, j;
        for (q = 0; q < f; q++)
        {
            for (p = 0; p < f; p++)
            {
                m = 0;
                n = 0;
                for (i = 0; i < f; i++)
                {
                    for (j = 0; j < f; j++)
                    {
                        b[i][j] = 0;
                        if (i != q && j != p)
                        {
                            b[m][n] = num[i][j];
                            if (n < (f - 2))
                                n++;
                            else
                            {
                                n = 0;
                                m++;
                            }
                        }
                    }
                }
                fac[q][p] = (int) Math.pow(-1, q + p) * determinant(b, f - 1);
            }
        }
        trans(fac, f);
    }
  
    void trans(int fac[][], int r)
    {
        int i, j;
        int b[][], inv[][];
        b = new int[r][r];
        inv = new int[r][r];
        int d = determinant(keymatrix, r);
        int mi = mi(d % 26);
        mi %= 26;
        if (mi < 0)
            mi += 26;
        for (i = 0; i < r; i++)
        {
            for (j = 0; j < r; j++)
            {
                b[i][j] = fac[j][i];
            }
        }
        for (i = 0; i < r; i++)
        {
            for (j = 0; j < r; j++)
            {
                inv[i][j] = b[i][j] % 26;
                if (inv[i][j] < 0)
                    inv[i][j] += 26;
                inv[i][j] *= mi;
                inv[i][j] %= 26;
            }
        }
        System.out.println("\nInverse key:");
        matrixtoinvkey(inv, r);
    }
  
    public int mi(int d)
    {
        int q, r1, r2, r, t1, t2, t;
        r1 = 26;
        r2 = d;
        t1 = 0;
        t2 = 1;
        while (r1 != 1 && r2 != 0)
        {
            q = r1 / r2;
            r = r1 % r2;
            t = t1 - (t2 * q);
            r1 = r2;
            r2 = r;
            t1 = t2;
            t2 = t;
        }
        return (t1 + t2);
    }
  
    public void matrixtoinvkey(int inv[][], int n)
    {
        String invkey = "";
        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < n; j++)
            {
                invkey += (char) (inv[i][j] + 97);
            }
        }
        System.out.print(invkey);
    }
  
    public static void main(String args[]) throws IOException
    {
        HillCipher obj = new HillCipher();
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        int choice;
        System.out.println("Menu:\n1: Encryption\n2: Decryption");
        choice = Integer.parseInt(in.readLine());
        System.out.println("Enter the line: ");
        String line = in.readLine();
        System.out.println("Enter the key: ");
        String key = in.readLine();
        double sq = Math.sqrt(key.length());
        if (sq != (long) sq)
            System.out
                    .println("Invalid key length!!! Does not form a square matrix...");
        else
        {
            int s = (int) sq;
            if (obj.check(key, s))
            {
                System.out.println("Result:");
                obj.divide(line, s);
                obj.cofact(obj.keymatrix, s);
            }
        }
    }
}

Output:

$ javac HillCipher.java
$ java HillCipher
  
Menu:
1: Encryption
2: Decryption
1
Enter the line: 
maixuanviet
Enter the key: 
maixuanvi
Result:
zmnmxfnfzdss
Inverse key:
inabzfjeq
  
Menu:
1: Encryption
2: Decryption
2
Enter the line: 
zmnmxfnfzdss
Enter the key: 
inabzfjeq
Result:
maixuanviet
Inverse key:
maixuanvi