/**
 * Author: M&K
 */
package Algorithms;

import java.math.BigInteger;
import java.util.Random;
import java.util.Scanner;

public class RSA {


    public static void main(String[] args) {

        // Unless you have Super computer do not set bitLength greater then 10 ( 2^10 upper bound )
        BigInteger pPrime = BigInteger.probablePrime(10,new Random()); //Generate random prime of with 2^10 as upper bound
        BigInteger qPrime = BigInteger.probablePrime(10,new Random());

        RSA(pPrime,qPrime);
    }

    public static void RSA(BigInteger p, BigInteger q){
        System.out.println("First Prime = " + p);
        System.out.println("Second Prime = " + q);
        BigInteger N = p.multiply(q);
        // Z = (p-1)*(q-1)
        BigInteger Z = p.subtract(BigInteger.ONE).multiply(q.subtract(BigInteger.ONE));

        BigInteger e = BigInteger.ZERO; // Public key exponent, 'e' is relative prime to Z

        // GCD(e,Z) == 1 would mean that both do not have any common factors
        for (long i = 2; i < Z.longValue(); i++) {
            if(Z.gcd(BigInteger.valueOf(i)).equals(BigInteger.ONE)){
                e = BigInteger.valueOf(i);
                break;
            }
        }

//        BigInteger e = BigInteger.valueOf(65537); // Often e is chosen to be (2^16) -1 i.e 65537

        System.out.println("totient function Z = " + Z);
        System.out.println("public key e = " + e);


//        int d = modInverse(e, Z); // Private key exponent
        BigInteger d = e.modInverse(Z);
        System.out.println("private key d = " + d);

        Scanner read = new Scanner(System.in);
        int plainText = read.nextInt();

        // ct = pt^2 mode N  {ct = cipher text, pt = plain text}
//        long cipherText = (long)Math.pow(plainText, e) % N;
        BigInteger cipherText = BigInteger.valueOf(plainText).pow(e.intValue()).mod(N);
        System.out.println("cipher Text " + cipherText.longValue());

        BigInteger decryptText = cipherText.pow(d.intValue()).mod(N); // pt = ct^d mod N
        System.out.println("decrypt Text " + decryptText);
    }

}