import java.math.BigInteger;
import java.security.SecureRandom;
import java.util.Scanner;

public class elgamal {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        SecureRandom random = new SecureRandom();

        BigInteger p = BigInteger.valueOf(467);
        BigInteger g = BigInteger.valueOf(2);
        BigInteger x = BigInteger.valueOf(127);
        BigInteger y = g.modPow(x, p);

        System.out.println("ElGamal Cryptosystem");
        System.out.println("--------------------");
        System.out.println("Prime (p) = " + p);
        System.out.println("Generator (g) = " + g);
        System.out.println("Private key (x) = " + x);
        System.out.println("Public key (y) = " + y);

        System.out.print("\nEnter message (less than p): ");
        BigInteger m = sc.nextBigInteger();

        BigInteger k;
        do {
            k = new BigInteger(p.bitLength(), random);
        } while (k.compareTo(BigInteger.ONE) <= 0
                || k.compareTo(p.subtract(BigInteger.ONE)) >= 0);

        // Encryption
        BigInteger c1 = g.modPow(k, p);
        BigInteger c2 = m.multiply(y.modPow(k, p)).mod(p);

        System.out.println("\nEphemeral key (k) = " + k);
        System.out.println("Ciphertext (c1) = " + c1);
        System.out.println("Ciphertext (c2) = " + c2);

        // Decryption
        BigInteger s = c1.modPow(x, p);
        BigInteger decrypted = c2.multiply(s.modInverse(p)).mod(p);

        System.out.println("\nOriginal Message = " + m);
        System.out.println("Decrypted Message = " + decrypted);

        sc.close();
    }
}