A simple Java program to illustrate the Deffie-Hellman (DH) key exchange protocol.

           Deffie-Hellman key exchange protocol is a method which allows two parties to establish a shared secret key (session key) over an insecure channel without having to send that key through the channel. This is achieved by using private and public keys.This key can then be used to encrypt the messages sent between those two parties using a symmetric key cipher.

The algorithm is as follows.

This is the complete code written in Java.

import java.math.BigInteger;

public class DH{

public static void main(String[] args){

BigInteger g=new BigInteger("5");    //Let g be 5(a primitive root modulo 23)

BigInteger p=new BigInteger("23");   //Let p be 23(a prime number)

if (args.length==1){

BigInteger privateKey=new BigInteger(args[0]);  // Your Private key 

BigInteger publicKey=g.modPow(privateKey,p);    // Calculating the public key relevant to your private key

System.out.println("Public Key: "+publicKey.toString());

}

else if(args.length==2){

BigInteger privateKey=new BigInteger(args[0]);  // Your Private key

BigInteger publicKey=new BigInteger(args[1]);   // Public key of the other party

BigInteger sessionKey=publicKey.modPow(privateKey,p);  // Calculating the session key

System.out.println("Session Key: "+sessionKey.toString());

}

        else{

            System.out.println("Please provide the valid number of arguments."); //If the correct number of arguments are not provided

        }

}

}

This is a sample output that you get when running the program in command prompt in Windows.

When you give the private key of one party, it will calculate his/her public key.
Then when you give the private key of one party along with the public key of the other party, The session key(shared secret key) will be provided.
Note that this session key is the same for both parties. (It is not sent through the network, instead calculated by the 2 parties themselves.)