// Java Program to Implement the RSA Algorithm
import java.math.*;
import java.util.*;
 
class RSA {
    public static void main(String args[])
    {
        int p, q, n, z, d = 0, e, i;
 
        // The number to be encrypted and decrypted
        int msg = 12;
        double c;
        BigInteger msgback;
 
        // 1st prime number p
        p = 3;
 
        // 2nd prime number q
        q = 11;
        n = p * q;
        z = (p - 1) * (q - 1);
        System.out.println("the value of z = " + z);
 
        for (e = 2; e < z; e++) {
 
            // e is for public key exponent
            if (gcd(e, z) == 1) {
                break;
            }
        }
        System.out.println("the value of e = " + e);
        for (i = 0; i <= 9; i++) {
            int x = 1 + (i * z);
 
            // d is for private key exponent
            if (x % e == 0) {
                d = x / e;
                break;
            }
        }
        System.out.println("the value of d = " + d);
        c = (Math.pow(msg, e)) % n;
        System.out.println("Encrypted message is : " + c);
 
        // converting int value of n to BigInteger
        BigInteger N = BigInteger.valueOf(n);
 
        // converting float value of c to BigInteger
        BigInteger C = BigDecimal.valueOf(c).toBigInteger();
        msgback = (C.pow(d)).mod(N);
        System.out.println("Decrypted message is : "
                           + msgback);
    }
 
    static int gcd(int e, int z)
    {
        if (e == 0)
            return z;
        else
            return gcd(z % e, e);
    }
}

/**
the value of z = 20
the value of e = 3
the value of d = 7
Encrypted message is : 12.0
Decrypted message is : 12
 */