ftp server with diffie-hellman in java

Question

i have only server side code in java and i want to use diffie-hellman encryption for my server so can anyone help me with this I'm new to programming and I'm just starting to learn so it's my homework and deadline is very close so can anyone help me that would be great... so far i got this code and i have no idea how i can merge it with my server code

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);

    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);

    System.out.println("Person B sends to person A "+resultb+".");

    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;        
}

}

Answer 1

As the comment got too long...

Diffie-hellman is just for key exchange over an unprotected connection, using a prime number math problem. It is used to initialize asynchronous encryption with public and private keys (as used early for SSL/TLS).

Unless required as an exercise (for which you should add a homework or exercise tag), DO NOT implement such security stuff yourself, use existing, well tested libraries. Java has builtin support for SSL (perhaps using other key exchange methods): SSLContext

Despite that, you could use those numbers to derive public and private keys, however, you must assure, that they're really large (and not just isProbablePrime) prime numbers, what can get slow with BigInteger.

To connect client and server, start with Socket. But again a disclaimer, if this is not an exercise, you WILL most probably be attacked (hacked, DDOSed,...), you're best using an existing, well tested and hardened server.