// Arup Guha
// 6/26/06
// This program illustrates the steps in the Diffie-Hellman Key Exchange
// and utilizes the big integer class so we can see the real value of the
// key exchange.
import java.util.*;
import java.math.BigInteger;

public class DiffieHellmanBigInt {
	
	final static BigInteger one = new BigInteger("1");
	
	public static void main(String args[]) {
		
		Scanner stdin = new Scanner(System.in);
		BigInteger p;
		
		// Get a start spot to pick a prime from the user.
		System.out.println("Enter the approximate value of p you want.");
		String ans = stdin.next();
		p = getNextPrime(ans);
		System.out.println("Your prime is "+p+".");
		
		// Get the base for exponentiation from the user.
		System.out.println("Now, enter a number in between 2 and p-1.");
		BigInteger g = new BigInteger(stdin.next());
		
		// Get A's secret number.
		System.out.println("Person A: enter your secret number now.");
		BigInteger a = new BigInteger(stdin.next());
		
		// Make A's calculation.
		BigInteger resulta = g.modPow(a,p);
		
		// This is the value that will get sent from A to B.
		// This value does NOT compromise the value of a easily.
		System.out.println("Person A sends to person B "+resulta+".");
		
		// Get B's secret number.
		System.out.println("Person B: enter your secret number now.");
		BigInteger b = new BigInteger(stdin.next());
		
		// Make B's calculation.
		BigInteger resultb = g.modPow(b,p);
		
		// This is the value that will get sent from B to A.
		// This value does NOT compromise the value of b easily.
		System.out.println("Person B sends to person A "+resultb+".");
		
		// Once A and B receive their values, they make their new calculations.
		// This involved getting their new numbers and raising them to the 
		// same power as before, their secret number.
		BigInteger KeyACalculates = resultb.modPow(a,p);
		BigInteger KeyBCalculates = resulta.modPow(b,p);
		
		// Print out the Key A calculates.
		System.out.println("A takes "+resultb+" raises it to the power "+a+" mod "+p);
		System.out.println("The Key A calculates is "+KeyACalculates+".");
		
		// Print out the Key B calculates.
		System.out.println("B takes "+resulta+" raises it to the power "+b+" mod "+p);
		System.out.println("The Key B calculates is "+KeyBCalculates+".");
		
	}
	
	public static BigInteger getNextPrime(String ans) {
		
		BigInteger test = new BigInteger(ans);
		while (!test.isProbablePrime(99))
			test = test.add(one);
		return test;		
	}
	
}