Question

I saw examples programs on MSDN: Asynchronous Socket as below. I’ve tried the program and run normally. Is it possible Asynchronous Socket to be modified in order to support TLS/SSL? How to do it?

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;

// State object for reading client data asynchronously
public class StateObject {
    // Client  socket.
    public Socket workSocket = null;

    // Size of receive buffer.
    public const int BufferSize = 1024;

    // Receive buffer.
    public byte[] buffer = new byte[BufferSize];

    // Received data string.
    public StringBuilder sb = new StringBuilder();  
}

public class AsynchronousSocketListener {
    // Thread signal.
    public static ManualResetEvent allDone = new ManualResetEvent(false);

    public static void StartListening() {
        // Data buffer for incoming data.
        byte[] bytes = new Byte[1024];

        // Establish the local endpoint for the socket.
        // The DNS name of the computer
        // running the listener is "host.contoso.com".
        IPHostEntry ipHostInfo = Dns.Resolve(Dns.GetHostName());
        IPAddress ipAddress = ipHostInfo.AddressList[0];
        IPEndPoint localEndPoint = new IPEndPoint(ipAddress, 11000);

        // Create a TCP/IP socket.
        Socket listener = new Socket(AddressFamily.InterNetwork,
        SocketType.Stream, ProtocolType.Tcp );

        // Bind the socket to the local endpoint and listen for incoming connections.
        try {
            listener.Bind(localEndPoint);
            listener.Listen(100);

            while (true) {
                // Set the event to nonsignaled state.
                allDone.Reset();

                // Start an asynchronous socket to listen for connections.
                Console.WriteLine("Waiting for a connection...");

                listener.BeginAccept(new AsyncCallback(AcceptCallback), listener );

                // Wait until a connection is made before continuing.
                allDone.WaitOne();
            }
        } 
        catch (Exception e) {
            Console.WriteLine(e.ToString());
        }

        Console.WriteLine("\nPress ENTER to continue...");
        Console.Read();    
    }

    public static void AcceptCallback(IAsyncResult ar) {
        // Signal the main thread to continue.
        allDone.Set();

        // Get the socket that handles the client request.
        Socket listener = (Socket) ar.AsyncState;
        Socket handler = listener.EndAccept(ar);

        // Create the state object.
        StateObject state = new StateObject();
        state.workSocket = handler;
        handler.BeginReceive( state.buffer, 0, StateObject.BufferSize, 0,
        new AsyncCallback(ReadCallback), state);
    }

    public static void ReadCallback(IAsyncResult ar) {
        String content = String.Empty;

        // Retrieve the state object and the handler socket
        // from the asynchronous state object.
        StateObject state = (StateObject) ar.AsyncState;
        Socket handler = state.workSocket;

        // Read data from the client socket. 
        int bytesRead = handler.EndReceive(ar);

        if (bytesRead > 0) {
            // There  might be more data, so store the data received so far.
            state.sb.Append(Encoding.ASCII.GetString(
            state.buffer,0,bytesRead));

            // Check for end-of-file tag. If it is not there, read 
            // more data.
            content = state.sb.ToString();
            if (content.IndexOf("<EOF>") > -1) {
                // All the data has been read from the 
                // client. Display it on the console.
                Console.WriteLine("Read {0} bytes from socket. \n Data : {1}",
                content.Length, content );
                // Echo the data back to the client.
                Send(handler, content);
            } 
            else {
                // Not all data received. Get more.
                handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0,
                new AsyncCallback(ReadCallback), state);
            }
        }
    }

    private static void Send(Socket handler, String data) {
        // Convert the string data to byte data using ASCII encoding.
        byte[] byteData = Encoding.ASCII.GetBytes(data);

        // Begin sending the data to the remote device.
        handler.BeginSend(byteData, 0, byteData.Length, 0,
        new AsyncCallback(SendCallback), handler);
    }

    private static void SendCallback(IAsyncResult ar) {
        try {
            // Retrieve the socket from the state object.
            Socket handler = (Socket) ar.AsyncState;

            // Complete sending the data to the remote device.
            int bytesSent = handler.EndSend(ar);
            Console.WriteLine("Sent {0} bytes to client.", bytesSent);

            handler.Shutdown(SocketShutdown.Both);
            handler.Close();

        } 
        catch (Exception e) {
            Console.WriteLine(e.ToString());
        }
    }


    public static int Main(String[] args) {
        StartListening();
        return 0;
    }
}

Answer 1

Not sure how to do it in C#, but it can be done in Java using the SSLEngine. This API is generally acknowledged as rather difficult to program. So, yes, it’s possible to use asynchronous sockets for SSL/TLS, but I’m not sure what the equivalent of Java’s SSLEngine would be. Perhaps there is another (better?) API for this in C#.

There are a few problems that you will almost inevitably encounter along this path (based on Java experience, but this would apply similarly in C#):

While SSLSockets tend to do a fair job at behaving similarly to normal Sockets, there are slight edge cases due to the nature of SSL/TLS. The effect of these differences is even more important with asynchronous I/O. Some of these problems are described in this (rather long) answer to “Properly closing SSLSocket” (in Java).

In addition, some SSL/TLS behaviours are already ill-defined with respect to the application layer, and get a bit messier with asynchronous behaviour. I have client-certificate renegotiation (or renegotiation in general) in mind. Using SSL/TLS, either party can in principle initiate a renegotiation handshake. This is done for example if you protect only one directory with client-certificates in Apache Httpd, or if only a portion of a web-app requires CLIENT-CERT in a Java container. When using client-certificate authentication, even IIS uses re-negotation by default. This consists of doing a second handshake during the SSL/TLS connection (effectively to get more information from the client here: its client-certificate).

When it works (usually with blocking I/O), the traffic looks like this (here both SSL and HTTP layers):

C->S SSL Client Hello
S->C SSL Server Hello, Certificate, Server Hello Done
C->S SSL Client Key Exchange, Change Cipher Spec, Finished
S->C SSL Change Cipher Spec
(then encrypted)
C->S SSL Finished
C->S HTTP GET /.../
S->C SSL Hello Request
C->S SSL Client Hello
S->C SSL Server Hello, Certificate, Certificate Request, Server Hello Done
C->S SSL Certificate, Client Key Exchange, Certificate Verify, Change Cipher
Spec, Finished
S->C SSL Change Cipher Spec
C->S SSL Finished
S->C HTTP 200 OK

Re-negotiating in asynchronous mode is quite tricky, since the re-negotiation should apply to both sides of the traffic at the same time. Therefore, the underlying properties of the SSL/TLS session may change during its use by the application layer (which isn’t normally expected to handle this). One side could still be sending data assuming certain SSL/TLS settings, while the re-negotiation occurs, thereby affecting both sides.

The implementation of all this can be difficult, as shown in this Grizzly issue, for example.