Hill Cipher Program in Java

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.

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package com.sanfoundry.setandstring;
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import java.io.BufferedReader;
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import java.io.IOException;
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import java.io.InputStreamReader;
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public class HillCipher
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{
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    int keymatrix[][];
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    int linematrix[];
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    int resultmatrix[];
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    public void divide(String temp, int s)

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    {
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        while (temp.length() > s)
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        {
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            String sub = temp.substring(0, s);

            temp = temp.substring(s, temp.length());

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            perform(sub);
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        }
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        if (temp.length() == s)
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            perform(temp);
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        else if (temp.length() < s)
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        {
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            for (int i = temp.length(); i < s; i++)

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                temp = temp + 'x';
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            perform(temp);
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```
        }
```
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    }
```
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    public void perform(String line)
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    {
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        linetomatrix(line);
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        linemultiplykey(line.length());
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        result(line.length());
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    }
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    public void keytomatrix(String key, int len)

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    {
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        keymatrix = new int[len][len];
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        int c = 0;
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        for (int i = 0; i < len; i++)
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        {
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            for (int j = 0; j < len; j++)

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            {
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                keymatrix[i][j] = ((int) key.charAt(c)) - 97;

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                c++;
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            }
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        }
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    }
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    public void linetomatrix(String line)

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    {
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        linematrix = new int[line.length()];

        for (int i = 0; i < line.length(); i++)

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        {
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            linematrix[i] = ((int) line.charAt(i)) - 97;

```
        }
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    }
```
```
 
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    public void linemultiplykey(int len)

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    {
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        resultmatrix = new int[len];
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        for (int i = 0; i < len; i++)
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        {
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            for (int j = 0; j < len; j++)

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            {
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                resultmatrix[i] += keymatrix[i][j] * linematrix[j];

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            }
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            resultmatrix[i] %= 26;
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        }
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    }
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    public void result(int len)
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    {
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        String result = "";
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        for (int i = 0; i < len; i++)
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        {
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            result += (char) (resultmatrix[i] + 97);

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        }
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        System.out.print(result);
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    }
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    public boolean check(String key, int len)

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    {
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        keytomatrix(key, len);
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        int d = determinant(keymatrix, len);

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        d = d % 26;
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        if (d == 0)
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        {
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            System.out
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                    .println("Invalid key!!! Key is not invertible because determinant=0...");

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            return false;
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        }
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        else if (d % 2 == 0 || d % 13 == 0)

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        {
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            System.out
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                    .println("Invalid key!!! Key is not invertible because determinant has common factor with 26...");

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            return false;
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        }
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        else
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        {
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            return true;
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        }
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    }
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    public int determinant(int A[][], int N)

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    {
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        int res;
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        if (N == 1)
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            res = A[0][0];
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        else if (N == 2)
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        {
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            res = A[0][0] * A[1][1] - A[1][0] * A[0][1];

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        }
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        else
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        {
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            res = 0;
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            for (int j1 = 0; j1 < N; j1++)

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            {
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                int m[][] = new int[N - 1][N - 1];

                for (int i = 1; i < N; i++)

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                {
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                    int j2 = 0;
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                    for (int j = 0; j < N; j++)

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                    {
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                        if (j == j1)
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                            continue;
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                        m[i - 1][j2] = A[i][j];

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                        j2++;
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                    }
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                }
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                res += Math.pow(-1.0, 1.0 + j1 + 1.0) * A[0][j1]

                        * determinant(m, N - 1);

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            }
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        }
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        return res;
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    }
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    public void cofact(int num[][], int f)

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    {
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        int b[][], fac[][];
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        b = new int[f][f];
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        fac = new int[f][f];
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        int p, q, m, n, i, j;
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        for (q = 0; q < f; q++)
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        {
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            for (p = 0; p < f; p++)
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            {
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                m = 0;
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                n = 0;
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                for (i = 0; i < f; i++)
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                {
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                    for (j = 0; j < f; j++)

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                    {
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                        b[i][j] = 0;
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                        if (i != q && j != p)

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                        {
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                            b[m][n] = num[i][j];

                            if (n < (f - 2))

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                                n++;
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                            else
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                            {
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                                n = 0;
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                                m++;
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                            }
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                        }
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                    }
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                }
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                fac[q][p] = (int) Math.pow(-1, q + p) * determinant(b, f - 1);

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            }
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        }
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        trans(fac, f);
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    }
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    void trans(int fac[][], int r)
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    {
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        int i, j;
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        int b[][], inv[][];
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        b = new int[r][r];
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        inv = new int[r][r];
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        int d = determinant(keymatrix, r);

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        int mi = mi(d % 26);
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        mi %= 26;
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        if (mi < 0)
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            mi += 26;
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        for (i = 0; i < r; i++)
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        {
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            for (j = 0; j < r; j++)
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            {
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                b[i][j] = fac[j][i];
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            }
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        }
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        for (i = 0; i < r; i++)
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        {
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            for (j = 0; j < r; j++)
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            {
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                inv[i][j] = b[i][j] % 26;

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                if (inv[i][j] < 0)
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                    inv[i][j] += 26;
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                inv[i][j] *= mi;
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                inv[i][j] %= 26;
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            }
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        }
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        System.out.println("\nInverse key:");

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        matrixtoinvkey(inv, r);
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    }
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    public int mi(int d)
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    {
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        int q, r1, r2, r, t1, t2, t;
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        r1 = 26;
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        r2 = d;
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        t1 = 0;
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        t2 = 1;
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        while (r1 != 1 && r2 != 0)
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        {
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            q = r1 / r2;
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            r = r1 % r2;
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            t = t1 - (t2 * q);
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            r1 = r2;
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            r2 = r;
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            t1 = t2;
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            t2 = t;
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        }
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        return (t1 + t2);
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    }
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    public void matrixtoinvkey(int inv[][], int n)

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    {
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        String invkey = "";
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        for (int i = 0; i < n; i++)
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        {
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            for (int j = 0; j < n; j++)
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            {
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                invkey += (char) (inv[i][j] + 97);

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            }
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        }
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        System.out.print(invkey);
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    }
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```

    public static void main(String args[]) throws IOException

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    {
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        HillCipher obj = new HillCipher();

        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

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        int choice;
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        System.out.println("Menu:\n1: Encryption\n2: Decryption");

        choice = Integer.parseInt(in.readLine());

        System.out.println("Enter the line: ");

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        String line = in.readLine();
```

        System.out.println("Enter the key: ");

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        String key = in.readLine();
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        double sq = Math.sqrt(key.length());

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        if (sq != (long) sq)
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            System.out
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                    .println("Invalid key length!!! Does not form a square matrix...");

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        else
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        {
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            int s = (int) sq;
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            if (obj.check(key, s))
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            {
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                System.out.println("Result:");

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                obj.divide(line, s);
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                obj.cofact(obj.keymatrix, s);

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            }
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        }
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    }
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}
```

Output:

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

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Java Program to Implement the Hill Cipher

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