package ciphers;

import java.math.BigInteger;
import java.security.SecureRandom;
import javax.swing.JOptionPane;

/**
 * @author Nguyen Duy Tiep on 23-Oct-17.
 */
public final class RSA {

 /**
 * Trivial test program.
 *
 * @param args
 * @deprecated TODO remove main and make JUnit Testing or any other
 * methodology
 */
 public static void main(String[] args) {

 RSA rsa = new RSA(1024);
 String text1 = JOptionPane.showInputDialog("Enter a message to encrypt :");

 String ciphertext = rsa.encrypt(text1);
 JOptionPane.showMessageDialog(null, "Your encrypted message : " + ciphertext);

 JOptionPane.showMessageDialog(null, "Your message after decrypt : " + rsa.decrypt(ciphertext));
 }

 private BigInteger modulus, privateKey, publicKey;

 /**
 *
 * @param bits
 */
 public RSA(int bits) {
 generateKeys(bits);
 }

 /**
 *
 * @param message
 * @return encrypted message
 */
 public synchronized String encrypt(String message) {
 return (new BigInteger(message.getBytes())).modPow(publicKey, modulus).toString();
 }

 /**
 *
 * @param message
 * @return encrypted message as big integer
 */
 public synchronized BigInteger encrypt(BigInteger message) {
 return message.modPow(publicKey, modulus);
 }

 /**
 *
 * @param encryptedMessage
 * @return plain message
 */
 public synchronized String decrypt(String encryptedMessage) {
 return new String((new BigInteger(encryptedMessage)).modPow(privateKey, modulus).toByteArray());
 }

 /**
 *
 * @param encryptedMessage
 * @return plain message as big integer
 */
 public synchronized BigInteger decrypt(BigInteger encryptedMessage) {
 return encryptedMessage.modPow(privateKey, modulus);
 }

 /**
 * Generate a new public and private key set.
 *
 * @param bits
 */
 public synchronized void generateKeys(int bits) {
 SecureRandom r = new SecureRandom();
 BigInteger p = new BigInteger(bits / 2, 100, r);
 BigInteger q = new BigInteger(bits / 2, 100, r);
 modulus = p.multiply(q);

 BigInteger m = (p.subtract(BigInteger.ONE)).multiply(q.subtract(BigInteger.ONE));

 publicKey = new BigInteger("3");

 while (m.gcd(publicKey).intValue() > 1) {
 publicKey = publicKey.add(new BigInteger("2"));
 }

 privateKey = publicKey.modInverse(m);
 }

}