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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);

}

}