online_security_project/server/Program.cs

using System;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Security.Cryptography;

namespace Server;

public class Program
{
    static void Main(string[] args)
    {
        // Generally this key would be static but since its not production yet we can generate it every time to make sure
        // the users has the key and could load it from file
        RSA key = RSA.Create(2048);
        File.WriteAllText("server_key.pem", key.ExportRSAPublicKeyPem());

        int port = 12345;
        TcpListener server = new(IPAddress.Parse("0.0.0.0"), port);
        try {
            server.Start();
            byte[] buffer = new byte[256];

            while(true) {
                // Currently, every time it gets a block, it will simply send it back but ToUpper
                using TcpClient client = server.AcceptTcpClient();
                Console.WriteLine("Got a client!");

                var stream = client.GetStream();
                int readLen;
                while((readLen = stream.Read(buffer, 0, buffer.Length)) != 0) {
                    // for now, lets just read it as an ascii string
                    string input = System.Text.Encoding.ASCII.GetString(buffer, 0, readLen); 
                    Console.WriteLine($"Got block: {input}");

                    byte[] ret = System.Text.Encoding.ASCII.GetBytes(input.ToUpper());
                    stream.Write(ret, 0, ret.Length);
                }


            }
        }
        catch(Exception ex) {
            Console.WriteLine($"Server error: {ex.Message}");
            Console.WriteLine("Trace: " + ex.StackTrace);
        }
        finally {
            server.Stop();
        }
    }
}