Java Implementation of Diffie-Hellman Algorithm between Client and Server

Program to implement Diffie-Hellman Algorithm in Client-Server Fashion.

Prerequisite: Server Socket Programming, Diffie-Hellman algorithm

The Diffie Hellman Algorithm is being used to establish a shared secret that can be used for secret communications while exchanging data over a public network.

In the below program, the client will share the value of , , and public key . Whereas, the server will accept the values and calculate its public key and send it to the client.

Both Client and Server will calculate the secret key for symmetric encryption by using the public key.

Program 1: Server Program

import java.net.*;
import java.io.*;

public class GreetingServer {
    public static void main(String[] args) throws IOException
    {
        try {
            int port = 8088;

            // Server Key
            int b = 3;

            // Client p, g, and key
            double clientP, clientG, clientA, B, Bdash;
            String Bstr;

            // Established the Connection
            ServerSocket serverSocket = new ServerSocket(port);
            System.out.println("Waiting for client on port " + serverSocket.getLocalPort() + "...");
            Socket server = serverSocket.accept();
            System.out.println("Just connected to " + server.getRemoteSocketAddress());

            // Server's Private Key
            System.out.println("From Server : Private Key = " + b);

            // Accepts the data from client
            DataInputStream in = new DataInputStream(server.getInputStream());

            clientP = Integer.parseInt(in.readUTF()); // to accept p
            System.out.println("From Client : P = " + clientP);

            clientG = Integer.parseInt(in.readUTF()); // to accept g
            System.out.println("From Client : G = " + clientG);

            clientA = Double.parseDouble(in.readUTF()); // to accept A
            System.out.println("From Client : Public Key = " + clientA);

            B = ((Math.pow(clientG, b)) % clientP); // calculation of B
            Bstr = Double.toString(B);

            // Sends data to client
            // Value of B
            OutputStream outToclient = server.getOutputStream();
            DataOutputStream out = new DataOutputStream(outToclient);

            out.writeUTF(Bstr); // Sending B

            Bdash = ((Math.pow(clientA, b)) % clientP); // calculation of Bdash

            System.out.println("Secret Key to perform Symmetric Encryption = "
                               + Bdash);
            server.close();
        }

        catch (SocketTimeoutException s) {
            System.out.println("Socket timed out!");
        }
        catch (IOException e) {
        }
    }
}

Program 2: Client Program

import java.net.*;
import java.io.*;

public class GreetingClient {
    public static void main(String[] args)
    {
        try {
            String pstr, gstr, Astr;
            String serverName = "localhost";
            int port = 8088;

            // Declare p, g, and Key of client
            int p = 23;
            int g = 9;
            int a = 4;
            double Adash, serverB;

            // Established the connection
            System.out.println("Connecting to " + serverName
                               + " on port " + port);
            Socket client = new Socket(serverName, port);
            System.out.println("Just connected to "
                               + client.getRemoteSocketAddress());

            // Sends the data to client
            OutputStream outToServer = client.getOutputStream();
            DataOutputStream out = new DataOutputStream(outToServer);

            pstr = Integer.toString(p);
            out.writeUTF(pstr); // Sending p

            gstr = Integer.toString(g);
            out.writeUTF(gstr); // Sending g

            double A = ((Math.pow(g, a)) % p); // calculation of A
            Astr = Double.toString(A);
            out.writeUTF(Astr); // Sending A

            // Client's Private Key
            System.out.println("From Client : Private Key = " + a);

            // Accepts the data
            DataInputStream in = new DataInputStream(client.getInputStream());

            serverB = Double.parseDouble(in.readUTF());
            System.out.println("From Server : Public Key = " + serverB);

            Adash = ((Math.pow(serverB, a)) % p); // calculation of Adash

            System.out.println("Secret Key to perform Symmetric Encryption = "
                               + Adash);
            client.close();
        }
        catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Use javac to Compile the programs, and open two console/terminal to run the system

Output:
In the first console run the server program, it will wait for the client’s connection. As soon as client is connected results will popup

In the second console, run the client’s program