ServerUdpCrypto

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Net;
using System.Net.Sockets;
using System.Numerics;

namespace WireSharkLesson
{
    public struct Datas
    {
        public BigInteger p;
        public BigInteger g;
        public BigInteger message;
    }
    class Program
    {
        static void Main(string[] args)
        {
            Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
            IPEndPoint ep = new IPEndPoint(IPAddress.Any, 5555);
            socket.Bind(ep);

            Dictionary<IPEndPoint, BigInteger> clients = new Dictionary<IPEndPoint, BigInteger>();
            Random r = new Random();

            //read p, read g, read alice message SHORT PREFIX
            while (true)
            {
                byte[] buffer = new byte[1024];
                EndPoint clientEP = new IPEndPoint(IPAddress.Any, 2000);
                int dataReceived = socket.ReceiveFrom(buffer, ref clientEP);

                if (dataReceived < 0) continue;
                if (!clients.ContainsKey((IPEndPoint)clientEP))
                {

                    Datas d = new Datas();

                    short pLength = BitConverter.ToInt16(buffer, 0);
                    byte[] p = new byte[pLength];
                    Buffer.BlockCopy(buffer, 2, p, 0, pLength);
                    d.p = new BigInteger(p);

                    d.g = buffer[2 + pLength];

                    short messageLength = BitConverter.ToInt16(buffer, 3 + pLength);
                    byte[] message = new byte[messageLength];
                    Buffer.BlockCopy(buffer, 5 + pLength, message, 0, messageLength);
                    d.message = new BigInteger(message);

                    int myRandom = r.Next();        //bob secret number

                    BigInteger secretKey = BigInteger.ModPow(d.message, myRandom, d.p);
                    clients.Add((IPEndPoint)clientEP, secretKey);

                    BigInteger myMessage = (BigInteger.ModPow(d.g, myRandom, d.p));

                    List<byte> toSend = new List<byte>();

                    toSend.AddRange(BitConverter.GetBytes((short)myMessage.ToByteArray().Length));
                    toSend.AddRange(myMessage.ToByteArray());

                    socket.SendTo(toSend.ToArray(), clientEP);
                    Console.WriteLine("data received "+ myMessage);
                }
                else
                {
                    List<byte> decriptedData = new List<byte>();
                    byte[] clientKey = clients[(IPEndPoint)clientEP].ToByteArray();
                    for (int i = 0; i < dataReceived; i++)
                    {
                        decriptedData.Add(buffer[i] ^= clientKey[i]);
                    }

                    string text = Encoding.ASCII.GetString(decriptedData.ToArray());
                    Console.WriteLine(text);
                }
            }


        }
    }
}