Question

Write a program that can encrypt and decrypt using the El-GAMALCipher Please show in Java with message = 100.

Answer 1

/* ElGamalEncryptDecrypt.java */

package dateoperation;

import java.math.*;
import java.util.*;
import java.security.*;
import java.io.*;

public class ElGamalEncryptDecrypt
{
    public static void main(String[] args) throws IOException
    {
        BigInteger secretKey,p, b, c;
        Random random = new SecureRandom();
        secretKey = new BigInteger("12345678901234567890");
        ///getting public key
        System.out.println("Your secretKey = " + secretKey);
        p = BigInteger.probablePrime(64, random);
        b = new BigInteger("3");
        c = b.modPow(secretKey, p);
        System.out.println("p = " + p);
        System.out.println("b = " + b);
        System.out.println("c = " + c);
        //
        // Encryption
        // 100 as plain text
      
        String plaintext ="100";
        BigInteger X = new BigInteger(plaintext);
        BigInteger r = new BigInteger(64, random);
        BigInteger Encryptedmessage = X.multiply(c.modPow(r, p)).mod(p);
        BigInteger brmodp = b.modPow(r, p);
        System.out.println("Plaintext = " + X);
        System.out.println("r = " + r);
        System.out.println("Encrypted Message = " + Encryptedmessage);
        System.out.println("b^r mod p = " + brmodp);
        //
        // Decryption
        //
        BigInteger crmodp = brmodp.modPow(secretKey, p);
        BigInteger d = crmodp.modInverse(p);
        BigInteger DecryptedMessage = d.multiply(Encryptedmessage).mod(p);
        System.out.println("\n\nc^r mod p = " + crmodp);
        System.out.println("d = " + d);
        System.out.println("Decrypted Message: " + DecryptedMessage);

    }
}

/*

output

run:
Your secretKey = 12345678901234567890
p = 15529472551224172823
b = 3
c = 15214339181978182378
Plaintext = 100
r = 1766218395270747804
Encrypted Message = 8737472744694909418
b^r mod p = 9030210705495725744

c^r mod p = 5367395394863167854
d = 13554491082901739413
Decrypted Message: 100
BUILD SUCCESSFUL (total time: 0 seconds)

*/