Java Program to Encode a Message Using Playfair Cipher

The Playfair cipher is one of the traditional ciphers which comes under the category of substitution ciphers. In Playfair Cipher, unlike traditional cipher, we encrypt a pair of alphabets (digraphs) instead of a single alphabet. In Playfair cipher, initially, a key table is created. The key table is a 5×5 matrix consisting of alphabets that acts as the key for encryption of the plaintext. Each of the 25 alphabets must be unique and one letter of the alphabet (usually ‘j’) is omitted from the table, as we need only 25 alphabets instead of 26. If the plaintext contains ‘j’, then it is replaced by ‘i’.

Process for Playfair Cipher:

1. The plaintext message is split into pairs of two letters (digraphs). If the plaintext has an odd number of characters, append ‘z’ to the end to make the message of even length.
2. Identify any double letters placed side by side in the plaintext and replace the second occurrence with an ‘x’ e.g. ‘hello’ -> ‘he lx lo’.
3. Now, locate the letters in the 5×5 key table.
4. Use the following rules for encryption of plaintext:
   • If the letters appear on the same row of your table, replace them with the letters to their immediate right respectively (wrapping around to the left side of the row if a letter in the original pair was on the right side of the row).
   • If the letters appear on the same column of your table, replace them with the letters immediately below respectively (wrapping around to the top side of the column if a letter in the original pair was on the bottom side of the column)
   • If the letters are not on the same row or column, replace them with the letters in their own row but in the same column as the other letter.

The decryption process of Playfair cipher is the same encryption process, but it is applied in a reverse manner. The receiver has the same key and can create the same key table, and he uses it to decrypt the ciphertext messages generated using that key.

Example:

Inputs for playfair cipher

Playfair Cipher Key Matrix

Ciphertext generation using Playfair Cipher

Java

// Java Program to Encode a Message Using Playfair Cipher

import java.io.*;
import java.util.*;

class Playfair {
    String key;
    String plainText;
    char[][] matrix = new char[5][5];

    public Playfair(String key, String plainText)
    {
        // convert all the characters to lowercase
        this.key = key.toLowerCase();

        this.plainText = plainText.toLowerCase();
    }

    // function to remove duplicate characters from the key
    public void cleanPlayFairKey()
    {
        LinkedHashSet<Character> set
            = new LinkedHashSet<Character>();

        String newKey = "";

        for (int i = 0; i < key.length(); i++)
            set.add(key.charAt(i));

        Iterator<Character> it = set.iterator();

        while (it.hasNext())
            newKey += (Character)it.next();

        key = newKey;
    }

    // function to generate playfair cipher key table
    public void generateCipherKey()
    {
        Set<Character> set = new HashSet<Character>();

        for (int i = 0; i < key.length(); i++)
        {
            if (key.charAt(i) == 'j')
                continue;
            set.add(key.charAt(i));
        }

        // remove repeated characters from the cipher key
        String tempKey = new String(key);

        for (int i = 0; i < 26; i++)
        {
            char ch = (char)(i + 97);
            if (ch == 'j')
                continue;

            if (!set.contains(ch))
                tempKey += ch;
        }

        // create cipher key table
        for (int i = 0, idx = 0; i < 5; i++)
            for (int j = 0; j < 5; j++)
                matrix[i][j] = tempKey.charAt(idx++);

        System.out.println("Playfair Cipher Key Matrix:");

        for (int i = 0; i < 5; i++)
            System.out.println(Arrays.toString(matrix[i]));
    }

    // function to preprocess plaintext
    public String formatPlainText()
    {
        String message = "";
        int len = plainText.length();

        for (int i = 0; i < len; i++)
        {
            // if plaintext contains the character 'j',
            // replace it with 'i'
            if (plainText.charAt(i) == 'j')
                message += 'i';
            else
                message += plainText.charAt(i);
        }

        // if two consecutive characters are same, then
        // insert character 'x' in between them
        for (int i = 0; i < message.length(); i += 2)
        {
            if (message.charAt(i) == message.charAt(i + 1))
                message = message.substring(0, i + 1) + 'x'
                          + message.substring(i + 1);
        }

        // make the plaintext of even length
        if (len % 2 == 1)
            message += 'x'; // dummy character

        return message;
    }

    // function to group every two characters
    public String[] formPairs(String message)
    {
        int len = message.length();
        String[] pairs = new String[len / 2];

        for (int i = 0, cnt = 0; i < len / 2; i++)
            pairs[i] = message.substring(cnt, cnt += 2);

        return pairs;
    }

    // function to get position of character in key table
    public int[] getCharPos(char ch)
    {
        int[] keyPos = new int[2];

        for (int i = 0; i < 5; i++)
        {
            for (int j = 0; j < 5; j++)
            {

                if (matrix[i][j] == ch)
                {
                    keyPos[0] = i;
                    keyPos[1] = j;
                    break;
                }
            }
        }
        return keyPos;
    }

    public String encryptMessage()
    {
        String message = formatPlainText();
        String[] msgPairs = formPairs(message);
        String encText = "";

        for (int i = 0; i < msgPairs.length; i++)
        {
            char ch1 = msgPairs[i].charAt(0);
            char ch2 = msgPairs[i].charAt(1);
            int[] ch1Pos = getCharPos(ch1);
            int[] ch2Pos = getCharPos(ch2);

            // if both the characters are in the same row
            if (ch1Pos[0] == ch2Pos[0]) {
                ch1Pos[1] = (ch1Pos[1] + 1) % 5;
                ch2Pos[1] = (ch2Pos[1] + 1) % 5;
            }

            // if both the characters are in the same column
            else if (ch1Pos[1] == ch2Pos[1])
            {
                ch1Pos[0] = (ch1Pos[0] + 1) % 5;
                ch2Pos[0] = (ch2Pos[0] + 1) % 5;
            }

            // if both the characters are in different rows
            // and columns
            else {
                int temp = ch1Pos[1];
                ch1Pos[1] = ch2Pos[1];
                ch2Pos[1] = temp;
            }

            // get the corresponding cipher characters from
            // the key matrix
            encText = encText + matrix[ch1Pos[0]][ch1Pos[1]]
                      + matrix[ch2Pos[0]][ch2Pos[1]];
        }

        return encText;
    }
}

public class GFG {
    public static void main(String[] args)
    {
        System.out.println("Example-1\n");

        String key1 = "Problem";
        String plainText1 = "Playfair";

        System.out.println("Key: " + key1);
        System.out.println("PlainText: " + plainText1);

        Playfair pfc1 = new Playfair(key1, plainText1);
        pfc1.cleanPlayFairKey();
        pfc1.generateCipherKey();

        String encText1 = pfc1.encryptMessage();
        System.out.println("Cipher Text is: " + encText1);

        System.out.println("\nExample-2\n");

        String key2 = "Problem";
        String plainText2 = "Hello";

        System.out.println("Key: " + key2);
        System.out.println("PlainText: " + plainText2);

        Playfair pfc2 = new Playfair(key2, plainText2);
        pfc2.cleanPlayFairKey();
        pfc2.generateCipherKey();

        String encText2 = pfc2.encryptMessage();
        System.out.println("Cipher Text is: " + encText2);
    }
}

Output

Example-1

Key: Problem
PlainText: Playfair
Playfair Cipher Key Matrix:
[p, r, o, b, l]
[e, m, a, c, d]
[f, g, h, i, k]
[n, q, s, t, u]
[v, w, x, y, z]
Cipher Text is: rpcxhegb

Example-2

Key: Problem
PlainText: Hello
Playfair Cipher Key Matrix:
[p, r, o, b, l]
[e, m, a, c, d]
[f, g, h, i, k]
[n, q, s, t, u]
[v, w, x, y, z]
Cipher Text is: faozpb