Java Program to Implement Affine Cipher

This is a java program to implement Affine Cipher. The affine cipher is a type of monoalphabetic substitution cipher, wherein each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter. The formula used means that each letter encrypts to one other letter, and back again, meaning the cipher is essentially a standard substitution cipher with a rule governing which letter goes to which. As such, it has the weaknesses of all substitution ciphers. Each letter is enciphered with the function (ax+b)mod(26), where b is the magnitude of the shift.

Here is the source code of the Java Program to Implement Affine Cipher. 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.util.Scanner;
 
public class AffineCipher
{
    public static String encryptionMessage(String Msg)
    {
        String CTxt = "";
        int a = 3;
        int b = 6;
        for (int i = 0; i < Msg.length(); i++)
        {
            CTxt = CTxt + (char) ((((a * Msg.charAt(i)) + b) % 26) + 65);
        }
        return CTxt;
    }
 
    public static String decryptionMessage(String CTxt)
    {
        String Msg = "";
        int a = 3;
        int b = 6;
        int a_inv = 0;
        int flag = 0;
        for (int i = 0; i < 26; i++)
        {
            flag = (a * i) % 26;
            if (flag == 1)
            {
                a_inv = i;
                System.out.println(i);
            }
        }
        for (int i = 0; i < CTxt.length(); i++)
        {
            Msg = Msg + (char) (((a_inv * ((CTxt.charAt(i) - b)) % 26)) + 65);
        }
        return Msg;
    }
 
    public static void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the message: ");
        String message = sc.next();
        System.out.println("Message is :" + message);
        System.out.println("Encrypted Message is : "
                + encryptionMessage(message));
        System.out.println("Decrypted Message is: "
                + decryptionMessage(encryptionMessage(message)));
        sc.close();
    }
}

Output:

$ javac AffineCipher.java
$ java AffineCipher
 
Enter the message: 
MAIXUANVIET
Message is :MAIXUANVIET
Encrypted Message is : VTGIJBGCSN
9
Decrypted Message is: MAIXUANVIET