Console.WriteLine("Listening for {0} prefixes...", listener.Prefixes.Count);
HttpListenerContext context = listener.GetContext();
HttpListenerRequest request = context.Request;
Console.WriteLine("Received a request.");
// This server requires a valid client certificate
// for requests that are not sent from the local computer.

// Did the client omit the certificate or send an invalid certificate?
if (request.IsAuthenticated &&
    request.GetClientCertificate() == null || 
    request.ClientCertificateError != 0)
{
    // Send a 403 response.
    HttpListenerResponse badCertificateResponse = context.Response ;
    SendBadCertificateResponse(badCertificateResponse);
    Console.WriteLine("Client has invalid certificate.");
    continue;
}