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 {

  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);
  }
}