/// <summary>
        /// Starts the synchronous authentication process.
        /// </summary>
        /// <exception cref="ProxyException">Authentication with the proxy server failed.</exception>
        /// <exception cref="ProtocolViolationException">The proxy server uses an invalid protocol.</exception>
        /// <exception cref="SocketException">An operating system error occurs while accessing the Socket.</exception>
        /// <exception cref="ObjectDisposedException">The Socket has been closed.</exception>
        private void Authenticate()
        {
            Server.Send(new byte [] { 5, 2, 0, 2 });
            byte[] buffer = ReadBytes(2);
            if (buffer[1] == 255)
            {
                throw new ProxyException("No authentication method accepted.");
            }
            AuthMethod authenticate;

            switch (buffer[1])
            {
            case 0:
                authenticate = new AuthNone(Server);
                break;

            case 2:
                authenticate = new AuthUserPass(Server, Username, Password);
                break;

            default:
                throw new ProtocolViolationException();
            }
            authenticate.Authenticate();
        }
        /// <summary>
        /// Called when an authentication reply has been received.
        /// </summary>
        /// <param name="ar">Stores state information for this asynchronous operation as well as any user-defined data.</param>
        private void OnAuthReceive(IAsyncResult ar)
        {
            try {
                Received += Server.EndReceive(ar);
                if (Received <= 0)
                {
                    throw new SocketException();
                }
            } catch (Exception e) {
                ProtocolComplete(e);
                return;
            }
            try {
                if (Received < 2)
                {
                    Server.BeginReceive(Buffer, Received, Buffer.Length - Received, SocketFlags.None, new AsyncCallback(this.OnAuthReceive), Server);
                }
                else
                {
                    AuthMethod authenticate;
                    switch (Buffer[1])
                    {
                    case 0:
                        authenticate = new AuthNone(Server);
                        break;

                    case 2:
                        authenticate = new AuthUserPass(Server, Username, Password);
                        break;

                    default:
                        ProtocolComplete(new SocketException());
                        return;
                    }
                    authenticate.BeginAuthenticate(new HandShakeComplete(this.OnAuthenticated));
                }
            } catch (Exception e) {
                ProtocolComplete(e);
            }
        }
 /// <summary>
 /// Called when an authentication reply has been received.
 /// </summary>
 /// <param name="ar">Stores state information for this asynchronous operation as well as any user-defined data.</param>
 private void OnAuthReceive(IAsyncResult ar)
 {
     try {
         Received += Server.EndReceive(ar);
         if (Received <= 0)
             throw new SocketException();
     } catch (Exception e) {
         ProtocolComplete(e);
         return;
     }
     try {
         if (Received < 2) {
             Server.BeginReceive(Buffer, Received, Buffer.Length - Received, SocketFlags.None, new AsyncCallback(this.OnAuthReceive), Server);
         } else {
             AuthMethod authenticate;
             switch(Buffer[1]) {
                 case 0:
                     authenticate = new AuthNone(Server);
                     break;
                 case 2:
                     authenticate = new AuthUserPass(Server, Username, Password);
                     break;
                 default:
                     ProtocolComplete(new SocketException());
                     return;
             }
             authenticate.BeginAuthenticate(new HandShakeComplete(this.OnAuthenticated));
         }
     } catch (Exception e) {
         ProtocolComplete(e);
     }
 }
 /// <summary>
 /// Starts the synchronous authentication process.
 /// </summary>
 /// <exception cref="ProxyException">Authentication with the proxy server failed.</exception>
 /// <exception cref="ProtocolViolationException">The proxy server uses an invalid protocol.</exception>
 /// <exception cref="SocketException">An operating system error occurs while accessing the Socket.</exception>
 /// <exception cref="ObjectDisposedException">The Socket has been closed.</exception>
 private void Authenticate()
 {
     Server.Send(new byte [] {5, 2, 0, 2});
     byte[] buffer = ReadBytes(2);
     if (buffer[1] == 255)
         throw new ProxyException("No authentication method accepted.");
     AuthMethod authenticate;
     switch (buffer[1]) {
         case 0:
             authenticate = new AuthNone(Server);
             break;
         case 2:
             authenticate = new AuthUserPass(Server, Username, Password);
             break;
         default:
             throw new ProtocolViolationException();
     }
     authenticate.Authenticate();
 }