/*

* Author: fullarray

* Description: This program was built using the following Java version:

* java version "1.8.0_141"

* Java(TM) SE Runtime Environment (build 1.8.0_141-b15)

* Java HotSpot(TM) 64-Bit Server VM (build 25.141-b15, mixed mode)

*

*/

import java.math.*;

import java.util.*;

import java.lang.String;

import java.security.*;

import java.io.*;

public class elGamaImplementation

{

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

{

BigInteger privateKey;

Random sc = new SecureRandom();

// Get key size from user.

System.out.print("Enter your key size: E.g. 64. 128, 256, etc.\n");

int keySize = elGamaImplementation.GetInt();

while(keySize != 64 && keySize != 128 && keySize != 256){

System.out.println("Please enter a correct bit size. For example: 64, 128, 256.");

System.out.println("Try again.\n");

System.out.print("Enter your key size: E.g. 64. 128, 256, etc.\n");

keySize = elGamaImplementation.GetInt();

}

// For this example, private key is created randomly

privateKey = new BigInteger(keySize,sc);

System.out.println("Generated private key: \n" + privateKey +"\n");

//ElGama

ProcessAlgorithm(keySize, sc, privateKey);

}

public static void ProcessAlgorithm(int keySize, Random rand, BigInteger privateKey ) throws IOException{

//Variables

BigInteger alpha, beta, c;

/*

* Based on key bit size, define a large prime number

* as well as generate public key.

*/

alpha = BigInteger.probablePrime(keySize, rand);

beta = new BigInteger(keySize, rand);

c = beta.modPow(privateKey, alpha);

System.out.println("alpha: \n" + alpha );

System.out.println("beta: \n" + beta );

System.out.println("c: \n" + c +"\n");

// Get plain text

System.out.print("Enter plain text message: E.g. 2345245245425 \n");

String s = elGamaImplementation.GetString();

// Process encryption

BigInteger x = new BigInteger(s);

BigInteger r = new BigInteger(keySize, rand);

BigInteger EC = x.multiply(c.modPow(r, alpha)).mod(alpha);

BigInteger brmodp = beta.modPow(r, alpha);

System.out.println("Plaintext: \n" + x + "\n\n");

System.out.println("Initializing keys and encrypting.");

System.out.println("-----------------------");

System.out.println("r Value: \n" + r);

System.out.println("EC Value: \n" + EC);

System.out.println("Encrypted: \n" + brmodp +"\n\n");

// Process decryption

BigInteger crmodp = brmodp.modPow(privateKey, alpha);

BigInteger d = crmodp.modInverse(alpha);

BigInteger ad = d.multiply(EC).mod(alpha);

System.out.println("Calculating new keys and performing decryption. ");

System.out.println("-----------------------");

System.out.println("d Value: \n" + crmodp);

System.out.println("Ad Value: \n" + d);

System.out.println("Original plain text Message: \n" + ad + "\n");

}

public static String GetString() throws IOException {

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

return stringIn.readLine();

}

public static int GetInt( ) throws IOException {

String aux = GetString();

return Integer.parseInt(aux);

}

}