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package ciphers;

import java.math.BigInteger;

import java.security.SecureRandom;

/**

* Created by Nguyen Duy Tiep on 23-Oct-17.

*/

public class RSA {

private BigInteger modulus, privateKey, publicKey;

public RSA(int bits) {

generateKeys(bits);

}

public synchronized String encrypt(String message) {

return (new BigInteger(message.getBytes())).modPow(publicKey, modulus).toString();

}

public synchronized BigInteger encrypt(BigInteger message) {

return message.modPow(publicKey, modulus);

}

public synchronized String decrypt(String message) {

return new String((new BigInteger(message)).modPow(privateKey, modulus).toByteArray());

}

public synchronized BigInteger decrypt(BigInteger message) {

return message.modPow(privateKey, modulus);

}

/** Generate a new public and private key set. */

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);

}

/** Trivial test program. */

public static void main(String[] args) {

RSA rsa = new RSA(1024);

String text1 = "This is a message";

System.out.println("Plaintext: " + text1);

String ciphertext = rsa.encrypt(text1);

System.out.println("Ciphertext: " + ciphertext);

System.out.println("Plaintext: " + rsa.decrypt(ciphertext));

}

}