RSA

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/**
 *
 * @author user
 */
import java.util.*;
import java.math.*;

public class RSA {

	public static void main(String args[])
	{
		Scanner sc=new Scanner(System.in);
		int p,q,n,z,d=0,e,i;
		System.out.println("Enter the number to be encrypted and decrypted");
		int msg=sc.nextInt();
		double c;
		BigInteger msgback;
		System.out.println("Enter 1st prime number p");
		p=sc.nextInt();
		System.out.println("Enter 2nd prime number q");
		q=sc.nextInt();

		n=p*q;
		z=(p-1)*(q-1);
		System.out.println("the value of z = "+z);

		for(e=2;e<z;e++)
		{
			if(gcd(e,z)==1)            // e is for public key exponent
			{
				break;
			}
		}
		System.out.println("the value of e = "+e);
		for(i=0;i<=9;i++)
		{
			int x=1+(i*z);
			if(x%e==0)      //d is for private key exponent
			{
				d=x/e;
				break;
			}
		}
		System.out.println("the value of d = "+d);
		c=(Math.pow(msg,e))%n;
		System.out.println("Encrypted message is : -");
		System.out.println(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("Derypted message is : -");
		System.out.println(msgback);

	}

	public static int gcd(int e, int z)
	{
		if(e==0)
			return z;
		else
			return gcd(z%e,e);
	}
}