SSH.NET/src/Renci.SshNet/Session.cs

using System;
using System.Collections.Generic;
using System.Net;
using System.Net.Sockets;
using System.Security.Cryptography;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading;
using Renci.SshNet.Channels;
using Renci.SshNet.Common;
using Renci.SshNet.Compression;
using Renci.SshNet.Messages;
using Renci.SshNet.Messages.Authentication;
using Renci.SshNet.Messages.Connection;
using Renci.SshNet.Messages.Transport;
using Renci.SshNet.Security;
using System.Globalization;
using System.Linq;
using Renci.SshNet.Abstractions;
using Renci.SshNet.Security.Cryptography;

namespace Renci.SshNet
{
    /// <summary>
    /// Provides functionality to connect and interact with SSH server.
    /// </summary>
    public class Session : ISession
    {
        private const byte Null = 0x00;
        private const byte CarriageReturn = 0x0d;
        private const byte LineFeed = 0x0a;

        /// <summary>
        /// Specifies an infinite waiting period.
        /// </summary>
        /// <remarks>
        /// The value of this field is <c>-1</c> millisecond. 
        /// </remarks>
        internal static readonly TimeSpan InfiniteTimeSpan = new TimeSpan(0, 0, 0, 0, -1);

        /// <summary>
        /// Specifies an infinite waiting period.
        /// </summary>
        /// <remarks>
        /// The value of this field is <c>-1</c>.
        /// </remarks>
        internal static readonly int Infinite = -1;

        /// <summary>
        /// Specifies maximum packet size defined by the protocol.
        /// </summary>
        /// <value>
        /// 35000.
        /// </value>
        private const int MaximumSshPacketSize = 35000;

        /// <summary>
        /// Holds the initial local window size for the channels.
        /// </summary>
        /// <value>
        /// 2 MB.
        /// </value>
        private const int InitialLocalWindowSize = LocalChannelDataPacketSize * 32;

        /// <summary>
        /// Holds the maximum size of channel data packets that we receive.
        /// </summary>
        /// <value>
        /// 64 KB.
        /// </value>
        /// <remarks>
        /// <para>
        /// This is the maximum size (in bytes) we support for the data (payload) of a
        /// <c>SSH_MSG_CHANNEL_DATA</c> message we receive.
        /// </para>
        /// <para>
        /// We currently do not enforce this limit.
        /// </para>
        /// </remarks>
        private const int LocalChannelDataPacketSize = 1024*64;

#if FEATURE_REGEX_COMPILE
        private static readonly Regex ServerVersionRe = new Regex("^SSH-(?<protoversion>[^-]+)-(?<softwareversion>.+)( SP.+)?$", RegexOptions.Compiled);
#else
        private static readonly Regex ServerVersionRe = new Regex("^SSH-(?<protoversion>[^-]+)-(?<softwareversion>.+)( SP.+)?$");
#endif

        /// <summary>
        /// Controls how many authentication attempts can take place at the same time.
        /// </summary>
        /// <remarks>
        /// Some server may restrict number to prevent authentication attacks
        /// </remarks>
        private static readonly SemaphoreLight AuthenticationConnection = new SemaphoreLight(3);

        /// <summary>
        /// Holds metada about session messages
        /// </summary>
        private SshMessageFactory _sshMessageFactory;

        /// <summary>
        /// Holds a <see cref="WaitHandle"/> that is signaled when the message listener loop has completed.
        /// </summary>
        private EventWaitHandle _messageListenerCompleted;

        /// <summary>
        /// Specifies outbound packet number
        /// </summary>
        private volatile uint _outboundPacketSequence;

        /// <summary>
        /// Specifies incoming packet number
        /// </summary>
        private uint _inboundPacketSequence;

        /// <summary>
        /// WaitHandle to signal that last service request was accepted
        /// </summary>
        private EventWaitHandle _serviceAccepted = new AutoResetEvent(false);

        /// <summary>
        /// WaitHandle to signal that exception was thrown by another thread.
        /// </summary>
        private EventWaitHandle _exceptionWaitHandle = new ManualResetEvent(false);

        /// <summary>
        /// WaitHandle to signal that key exchange was completed.
        /// </summary>
        private EventWaitHandle _keyExchangeCompletedWaitHandle = new ManualResetEvent(false);

        /// <summary>
        /// WaitHandle to signal that key exchange is in progress.
        /// </summary>
        private bool _keyExchangeInProgress;

        /// <summary>
        /// Exception that need to be thrown by waiting thread
        /// </summary>
        private Exception _exception;

        /// <summary>
        /// Specifies whether connection is authenticated
        /// </summary>
        private bool _isAuthenticated;

        /// <summary>
        /// Specifies whether user issued Disconnect command or not
        /// </summary>
        private bool _isDisconnecting;

        private IKeyExchange _keyExchange;

        private HashAlgorithm _serverMac;

        private HashAlgorithm _clientMac;

        private Cipher _clientCipher;

        private Cipher _serverCipher;

        private Compressor _serverDecompression;

        private Compressor _clientCompression;

        private SemaphoreLight _sessionSemaphore;

        /// <summary>
        /// Holds the factory to use for creating new services.
        /// </summary>
        private readonly IServiceFactory _serviceFactory;

        /// <summary>
        /// Holds connection socket.
        /// </summary>
        private Socket _socket;

#if FEATURE_SOCKET_POLL
        /// <summary>
        /// Holds an object that is used to ensure only a single thread can read from
        /// <see cref="_socket"/> at any given time.
        /// </summary>
        private readonly object _socketReadLock = new object();
#endif // FEATURE_SOCKET_POLL

        /// <summary>
        /// Holds an object that is used to ensure only a single thread can write to
        /// <see cref="_socket"/> at any given time.
        /// </summary>
        /// <remarks>
        /// This is also used to ensure that <see cref="_outboundPacketSequence"/> is
        /// incremented atomatically.
        /// </remarks>
        private readonly object _socketWriteLock = new object();

        /// <summary>
        /// Holds an object that is used to ensure only a single thread can dispose
        /// <see cref="_socket"/> at any given time.
        /// </summary>
        /// <remarks>
        /// This is also used to ensure that <see cref="_socket"/> will not be disposed
        /// while performing a given operation or set of operations on <see cref="_socket"/>.
        /// </remarks>
        private readonly object _socketDisposeLock = new object();

        /// <summary>
        /// Gets the session semaphore that controls session channels.
        /// </summary>
        /// <value>
        /// The session semaphore.
        /// </value>
        public SemaphoreLight SessionSemaphore
        {
            get
            {
                if (_sessionSemaphore == null)
                {
                    lock (this)
                    {
                        if (_sessionSemaphore == null)
                        {
                            _sessionSemaphore = new SemaphoreLight(ConnectionInfo.MaxSessions);
                        }
                    }
                }

                return _sessionSemaphore;
            }
        }

        private bool _isDisconnectMessageSent;

        private uint _nextChannelNumber;

        /// <summary>
        /// Gets the next channel number.
        /// </summary>
        /// <value>
        /// The next channel number.
        /// </value>
        private uint NextChannelNumber
        {
            get
            {
                uint result;

                lock (this)
                {
                    result = _nextChannelNumber++;
                }

                return result;
            }
        }

        /// <summary>
        /// Gets a value indicating whether the session is connected.
        /// </summary>
        /// <value>
        /// <c>true</c> if the session is connected; otherwise, <c>false</c>.
        /// </value>
        /// <remarks>
        /// This methods returns <c>true</c> in all but the following cases:
        /// <list type="bullet">
        ///     <item>
        ///         <description>The <see cref="Session"/> is disposed.</description>
        ///     </item>
        ///     <item>
        ///         <description>The SSH_MSG_DISCONNECT message - which is used to disconnect from the server - has been sent.</description>
        ///     </item>
        ///     <item>
        ///         <description>The client has not been authenticated successfully.</description>
        ///     </item>
        ///     <item>
        ///         <description>The listener thread - which is used to receive messages from the server - has stopped.</description>
        ///     </item>
        ///     <item>
        ///         <description>The socket used to communicate with the server is no longer connected.</description>
        ///     </item>
        /// </list>
        /// </remarks>
        public bool IsConnected
        {
            get
            {
                if (_disposed || _isDisconnectMessageSent || !_isAuthenticated)
                    return false;
                if (_messageListenerCompleted == null || _messageListenerCompleted.WaitOne(0))
                    return false;

                return IsSocketConnected();
            }
        }

        /// <summary>
        /// Gets the session id.
        /// </summary>
        /// <value>
        /// The session id, or <c>null</c> if the client has not been authenticated.
        /// </value>
        public byte[] SessionId { get; private set; }

        private Message _clientInitMessage;

        /// <summary>
        /// Gets the client init message.
        /// </summary>
        /// <value>The client init message.</value>
        public Message ClientInitMessage
        {
            get
            {
                if (_clientInitMessage == null)
                {
                    _clientInitMessage = new KeyExchangeInitMessage
                        {
                            KeyExchangeAlgorithms = ConnectionInfo.KeyExchangeAlgorithms.Keys.ToArray(),
                            ServerHostKeyAlgorithms = ConnectionInfo.HostKeyAlgorithms.Keys.ToArray(),
                            EncryptionAlgorithmsClientToServer = ConnectionInfo.Encryptions.Keys.ToArray(),
                            EncryptionAlgorithmsServerToClient = ConnectionInfo.Encryptions.Keys.ToArray(),
                            MacAlgorithmsClientToServer = ConnectionInfo.HmacAlgorithms.Keys.ToArray(),
                            MacAlgorithmsServerToClient = ConnectionInfo.HmacAlgorithms.Keys.ToArray(),
                            CompressionAlgorithmsClientToServer = ConnectionInfo.CompressionAlgorithms.Keys.ToArray(),
                            CompressionAlgorithmsServerToClient = ConnectionInfo.CompressionAlgorithms.Keys.ToArray(),
                            LanguagesClientToServer = new[] {string.Empty},
                            LanguagesServerToClient = new[] {string.Empty},
                            FirstKexPacketFollows = false,
                            Reserved = 0
                        };
                }
                return _clientInitMessage;
            }
        }

        /// <summary>
        /// Gets or sets the server version string.
        /// </summary>
        /// <value>The server version.</value>
        public string ServerVersion { get; private set; }

        /// <summary>
        /// Gets or sets the client version string.
        /// </summary>
        /// <value>The client version.</value>
        public string ClientVersion { get; private set; }

        /// <summary>
        /// Gets or sets the connection info.
        /// </summary>
        /// <value>The connection info.</value>
        public ConnectionInfo ConnectionInfo { get; private set; }

        /// <summary>
        /// Occurs when an error occurred.
        /// </summary>
        public event EventHandler<ExceptionEventArgs> ErrorOccured;

        /// <summary>
        /// Occurs when session has been disconnected from the server.
        /// </summary>
        public event EventHandler<EventArgs> Disconnected;

        /// <summary>
        /// Occurs when host key received.
        /// </summary>
        public event EventHandler<HostKeyEventArgs> HostKeyReceived;

        /// <summary>
        /// Occurs when <see cref="BannerMessage"/> message is received from the server.
        /// </summary>
        public event EventHandler<MessageEventArgs<BannerMessage>> UserAuthenticationBannerReceived;

        /// <summary>
        /// Occurs when <see cref="InformationRequestMessage"/> message is received from the server.
        /// </summary>
        internal event EventHandler<MessageEventArgs<InformationRequestMessage>> UserAuthenticationInformationRequestReceived;

        /// <summary>
        /// Occurs when <see cref="PasswordChangeRequiredMessage"/> message is received from the server.
        /// </summary>
        internal event EventHandler<MessageEventArgs<PasswordChangeRequiredMessage>> UserAuthenticationPasswordChangeRequiredReceived;

        /// <summary>
        /// Occurs when <see cref="PublicKeyMessage"/> message is received from the server.
        /// </summary>
        internal event EventHandler<MessageEventArgs<PublicKeyMessage>> UserAuthenticationPublicKeyReceived;

        /// <summary>
        /// Occurs when <see cref="KeyExchangeDhGroupExchangeGroup"/> message is received from the server.
        /// </summary>
        internal event EventHandler<MessageEventArgs<KeyExchangeDhGroupExchangeGroup>> KeyExchangeDhGroupExchangeGroupReceived;

        /// <summary>
        /// Occurs when <see cref="KeyExchangeDhGroupExchangeReply"/> message is received from the server.
        /// </summary>
        internal event EventHandler<MessageEventArgs<KeyExchangeDhGroupExchangeReply>> KeyExchangeDhGroupExchangeReplyReceived;

        #region Message events

        /// <summary>
        /// Occurs when <see cref="DisconnectMessage"/> message received
        /// </summary>
        internal event EventHandler<MessageEventArgs<DisconnectMessage>> DisconnectReceived;

        /// <summary>
        /// Occurs when <see cref="IgnoreMessage"/> message received
        /// </summary>
        internal event EventHandler<MessageEventArgs<IgnoreMessage>> IgnoreReceived;

        /// <summary>
        /// Occurs when <see cref="UnimplementedMessage"/> message received
        /// </summary>
        internal event EventHandler<MessageEventArgs<UnimplementedMessage>> UnimplementedReceived;

        /// <summary>
        /// Occurs when <see cref="DebugMessage"/> message received
        /// </summary>
        internal event EventHandler<MessageEventArgs<DebugMessage>> DebugReceived;

        /// <summary>
        /// Occurs when <see cref="ServiceRequestMessage"/> message received
        /// </summary>
        internal event EventHandler<MessageEventArgs<ServiceRequestMessage>> ServiceRequestReceived;

        /// <summary>
        /// Occurs when <see cref="ServiceAcceptMessage"/> message received
        /// </summary>
        internal event EventHandler<MessageEventArgs<ServiceAcceptMessage>> ServiceAcceptReceived;

        /// <summary>
        /// Occurs when <see cref="KeyExchangeInitMessage"/> message received
        /// </summary>
        internal event EventHandler<MessageEventArgs<KeyExchangeInitMessage>> KeyExchangeInitReceived;

        /// <summary>
        /// Occurs when a <see cref="KeyExchangeDhReplyMessage"/> message is received from the SSH server.
        /// </summary>
        internal event EventHandler<MessageEventArgs<KeyExchangeDhReplyMessage>> KeyExchangeDhReplyMessageReceived;

        /// <summary>
        /// Occurs when <see cref="NewKeysMessage"/> message received
        /// </summary>
        internal event EventHandler<MessageEventArgs<NewKeysMessage>> NewKeysReceived;

        /// <summary>
        /// Occurs when <see cref="RequestMessage"/> message received
        /// </summary>
        internal event EventHandler<MessageEventArgs<RequestMessage>> UserAuthenticationRequestReceived;

        /// <summary>
        /// Occurs when <see cref="FailureMessage"/> message received
        /// </summary>
        internal event EventHandler<MessageEventArgs<FailureMessage>> UserAuthenticationFailureReceived;

        /// <summary>
        /// Occurs when <see cref="SuccessMessage"/> message received
        /// </summary>
        internal event EventHandler<MessageEventArgs<SuccessMessage>> UserAuthenticationSuccessReceived;

        /// <summary>
        /// Occurs when <see cref="GlobalRequestMessage"/> message received
        /// </summary>
        internal event EventHandler<MessageEventArgs<GlobalRequestMessage>> GlobalRequestReceived;

        /// <summary>
        /// Occurs when <see cref="RequestSuccessMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<RequestSuccessMessage>> RequestSuccessReceived;

        /// <summary>
        /// Occurs when <see cref="RequestFailureMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<RequestFailureMessage>> RequestFailureReceived;

        /// <summary>
        /// Occurs when <see cref="ChannelOpenMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<ChannelOpenMessage>> ChannelOpenReceived;

        /// <summary>
        /// Occurs when <see cref="ChannelOpenConfirmationMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<ChannelOpenConfirmationMessage>> ChannelOpenConfirmationReceived;

        /// <summary>
        /// Occurs when <see cref="ChannelOpenFailureMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<ChannelOpenFailureMessage>> ChannelOpenFailureReceived;

        /// <summary>
        /// Occurs when <see cref="ChannelWindowAdjustMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<ChannelWindowAdjustMessage>> ChannelWindowAdjustReceived;

        /// <summary>
        /// Occurs when <see cref="ChannelDataMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<ChannelDataMessage>> ChannelDataReceived;

        /// <summary>
        /// Occurs when <see cref="ChannelExtendedDataMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<ChannelExtendedDataMessage>> ChannelExtendedDataReceived;

        /// <summary>
        /// Occurs when <see cref="ChannelEofMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<ChannelEofMessage>> ChannelEofReceived;

        /// <summary>
        /// Occurs when <see cref="ChannelCloseMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<ChannelCloseMessage>> ChannelCloseReceived;

        /// <summary>
        /// Occurs when <see cref="ChannelRequestMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<ChannelRequestMessage>> ChannelRequestReceived;

        /// <summary>
        /// Occurs when <see cref="ChannelSuccessMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<ChannelSuccessMessage>> ChannelSuccessReceived;

        /// <summary>
        /// Occurs when <see cref="ChannelFailureMessage"/> message received
        /// </summary>
        public event EventHandler<MessageEventArgs<ChannelFailureMessage>> ChannelFailureReceived;

        #endregion

        /// <summary>
        /// Initializes a new instance of the <see cref="Session"/> class.
        /// </summary>
        /// <param name="connectionInfo">The connection info.</param>
        /// <param name="serviceFactory">The factory to use for creating new services.</param>
        /// <exception cref="ArgumentNullException"><paramref name="connectionInfo"/> is <c>null</c>.</exception>
        /// <exception cref="ArgumentNullException"><paramref name="serviceFactory"/> is <c>null</c>.</exception>
        internal Session(ConnectionInfo connectionInfo, IServiceFactory serviceFactory)
        {
            if (connectionInfo == null)
                throw new ArgumentNullException("connectionInfo");
            if (serviceFactory == null)
                throw new ArgumentNullException("serviceFactory");

            ClientVersion = "SSH-2.0-Renci.SshNet.SshClient.0.0.1";
            ConnectionInfo = connectionInfo;
            _serviceFactory = serviceFactory;
            _messageListenerCompleted = new ManualResetEvent(true);
        }

        /// <summary>
        /// Connects to the server.
        /// </summary>
        /// <exception cref="SocketException">Socket connection to the SSH server or proxy server could not be established, or an error occurred while resolving the hostname.</exception>
        /// <exception cref="SshConnectionException">SSH session could not be established.</exception>
        /// <exception cref="SshAuthenticationException">Authentication of SSH session failed.</exception>
        /// <exception cref="ProxyException">Failed to establish proxy connection.</exception>
        public void Connect()
        {
            if (IsConnected)
                return;

            try
            {
                AuthenticationConnection.Wait();

                if (IsConnected)
                    return;

                lock (this)
                {
                    //  If connected don't connect again
                    if (IsConnected)
                        return;

                    // reset connection specific information
                    Reset();

                    //  Build list of available messages while connecting
                    _sshMessageFactory = new SshMessageFactory();

                    switch (ConnectionInfo.ProxyType)
                    {
                        case ProxyTypes.None:
                            SocketConnect(ConnectionInfo.Host, ConnectionInfo.Port);
                            break;
                        case ProxyTypes.Socks4:
                            SocketConnect(ConnectionInfo.ProxyHost, ConnectionInfo.ProxyPort);
                            ConnectSocks4();
                            break;
                        case ProxyTypes.Socks5:
                            SocketConnect(ConnectionInfo.ProxyHost, ConnectionInfo.ProxyPort);
                            ConnectSocks5();
                            break;
                        case ProxyTypes.Http:
                            SocketConnect(ConnectionInfo.ProxyHost, ConnectionInfo.ProxyPort);
                            ConnectHttp();
                            break;
                    }

                    Match versionMatch;

                    //  Get server version from the server,
                    //  ignore text lines which are sent before if any
                    while (true)
                    {
                        var serverVersion = SocketReadLine(ConnectionInfo.Timeout);
                        if (serverVersion == null)
                            throw new SshConnectionException("Server response does not contain SSH protocol identification.", DisconnectReason.ProtocolError);
                        versionMatch = ServerVersionRe.Match(serverVersion);
                        if (versionMatch.Success)
                        {
                            ServerVersion = serverVersion;
                            break;
                        }
                    }

                    //  Set connection versions
                    ConnectionInfo.ServerVersion = ServerVersion;
                    ConnectionInfo.ClientVersion = ClientVersion;

                    //  Get server SSH version
                    var version = versionMatch.Result("${protoversion}");

                    var softwareName = versionMatch.Result("${softwareversion}");

                    DiagnosticAbstraction.Log(string.Format("Server version '{0}' on '{1}'.", version, softwareName));

                    if (!(version.Equals("2.0") || version.Equals("1.99")))
                    {
                        throw new SshConnectionException(string.Format(CultureInfo.CurrentCulture, "Server version '{0}' is not supported.", version), DisconnectReason.ProtocolVersionNotSupported);
                    }

                    SocketAbstraction.Send(_socket, Encoding.UTF8.GetBytes(string.Format(CultureInfo.InvariantCulture, "{0}\x0D\x0A", ClientVersion)));

                    //  Register Transport response messages
                    RegisterMessage("SSH_MSG_DISCONNECT");
                    RegisterMessage("SSH_MSG_IGNORE");
                    RegisterMessage("SSH_MSG_UNIMPLEMENTED");
                    RegisterMessage("SSH_MSG_DEBUG");
                    RegisterMessage("SSH_MSG_SERVICE_ACCEPT");
                    RegisterMessage("SSH_MSG_KEXINIT");
                    RegisterMessage("SSH_MSG_NEWKEYS");

                    //  Some server implementations might sent this message first, prior establishing encryption algorithm
                    RegisterMessage("SSH_MSG_USERAUTH_BANNER");

                    // mark the message listener threads as started
                    _messageListenerCompleted.Reset();

                    //  Start incoming request listener
                    ThreadAbstraction.ExecuteThread(MessageListener);

                    //  Wait for key exchange to be completed
                    WaitOnHandle(_keyExchangeCompletedWaitHandle);

                    //  If sessionId is not set then its not connected
                    if (SessionId == null)
                    {
                        Disconnect();
                        return;
                    }

                    //  Request user authorization service
                    SendMessage(new ServiceRequestMessage(ServiceName.UserAuthentication));

                    //  Wait for service to be accepted
                    WaitOnHandle(_serviceAccepted);

                    if (string.IsNullOrEmpty(ConnectionInfo.Username))
                    {
                        throw new SshException("Username is not specified.");
                    }

                    // Some servers send a global request immediately after successful authentication
                    // Avoid race condition by already enabling SSH_MSG_GLOBAL_REQUEST before authentication
                    RegisterMessage("SSH_MSG_GLOBAL_REQUEST");

                    ConnectionInfo.Authenticate(this, _serviceFactory);
                    _isAuthenticated = true;

                    //  Register Connection messages
                    RegisterMessage("SSH_MSG_REQUEST_SUCCESS");
                    RegisterMessage("SSH_MSG_REQUEST_FAILURE");
                    RegisterMessage("SSH_MSG_CHANNEL_OPEN_CONFIRMATION");
                    RegisterMessage("SSH_MSG_CHANNEL_OPEN_FAILURE");
                    RegisterMessage("SSH_MSG_CHANNEL_WINDOW_ADJUST");
                    RegisterMessage("SSH_MSG_CHANNEL_EXTENDED_DATA");
                    RegisterMessage("SSH_MSG_CHANNEL_REQUEST");
                    RegisterMessage("SSH_MSG_CHANNEL_SUCCESS");
                    RegisterMessage("SSH_MSG_CHANNEL_FAILURE");
                    RegisterMessage("SSH_MSG_CHANNEL_DATA");
                    RegisterMessage("SSH_MSG_CHANNEL_EOF");
                    RegisterMessage("SSH_MSG_CHANNEL_CLOSE");
                }
            }
            finally
            {
                AuthenticationConnection.Release();
            }
        }

        /// <summary>
        /// Disconnects from the server.
        /// </summary>
        /// <remarks>
        /// This sends a <b>SSH_MSG_DISCONNECT</b> message to the server, waits for the
        /// server to close the socket on its end and subsequently closes the client socket.
        /// </remarks>
        public void Disconnect()
        {
            DiagnosticAbstraction.Log(string.Format("[{0}] Disconnecting session.", ToHex(SessionId)));

            // send SSH_MSG_DISCONNECT message, clear socket read buffer and dispose it
            Disconnect(DisconnectReason.ByApplication, "Connection terminated by the client.");

            // at this point, we are sure that the listener thread will stop as we've
            // disconnected the socket, so lets wait until the message listener thread
            // has completed
            if (_messageListenerCompleted != null)
            {
                _messageListenerCompleted.WaitOne();
            }
        }

        private void Disconnect(DisconnectReason reason, string message)
        {
            // transition to disconnecting state to avoid throwing exceptions while cleaning up, and to
            // ensure any exceptions that are raised do not overwrite the exception that is set
            _isDisconnecting = true;

            // send disconnect message to the server if the connection is still open
            // and the disconnect message has not yet been sent
            //
            // note that this should also cause the listener loop to be interrupted as
            // the server should respond by closing the socket
            if (IsConnected)
            {
                TrySendDisconnect(reason, message);
            }

            // disconnect socket, and dispose it
            SocketDisconnectAndDispose();
        }

        /// <summary>
        /// Waits for the specified handle or the exception handle for the receive thread
        /// to signal within the connection timeout.
        /// </summary>
        /// <param name="waitHandle">The wait handle.</param>
        /// <exception cref="SshConnectionException">A received package was invalid or failed the message integrity check.</exception>
        /// <exception cref="SshOperationTimeoutException">None of the handles are signaled in time and the session is not disconnecting.</exception>
        /// <exception cref="SocketException">A socket error was signaled while receiving messages from the server.</exception>
        /// <remarks>
        /// When neither handles are signaled in time and the session is not closing, then the
        /// session is disconnected.
        /// </remarks>
        void ISession.WaitOnHandle(WaitHandle waitHandle)
        {
            WaitOnHandle(waitHandle, ConnectionInfo.Timeout);
        }

        /// <summary>
        /// Waits for the specified handle or the exception handle for the receive thread
        /// to signal within the specified timeout.
        /// </summary>
        /// <param name="waitHandle">The wait handle.</param>
        /// <param name="timeout">The time to wait for any of the handles to become signaled.</param>
        /// <exception cref="SshConnectionException">A received package was invalid or failed the message integrity check.</exception>
        /// <exception cref="SshOperationTimeoutException">None of the handles are signaled in time and the session is not disconnecting.</exception>
        /// <exception cref="SocketException">A socket error was signaled while receiving messages from the server.</exception>
        /// <remarks>
        /// When neither handles are signaled in time and the session is not closing, then the
        /// session is disconnected.
        /// </remarks>
        void ISession.WaitOnHandle(WaitHandle waitHandle, TimeSpan timeout)
        {
            WaitOnHandle(waitHandle, timeout);
        }

        /// <summary>
        /// Waits for the specified handle or the exception handle for the receive thread
        /// to signal within the connection timeout.
        /// </summary>
        /// <param name="waitHandle">The wait handle.</param>
        /// <exception cref="SshConnectionException">A received package was invalid or failed the message integrity check.</exception>
        /// <exception cref="SshOperationTimeoutException">None of the handles are signaled in time and the session is not disconnecting.</exception>
        /// <exception cref="SocketException">A socket error was signaled while receiving messages from the server.</exception>
        /// <remarks>
        /// When neither handles are signaled in time and the session is not closing, then the
        /// session is disconnected.
        /// </remarks>
        internal void WaitOnHandle(WaitHandle waitHandle)
        {
            WaitOnHandle(waitHandle, ConnectionInfo.Timeout);
        }

        /// <summary>
        /// Waits for the specified handle or the exception handle for the receive thread
        /// to signal within the specified timeout.
        /// </summary>
        /// <param name="waitHandle">The wait handle.</param>
        /// <param name="timeout">The time to wait for any of the handles to become signaled.</param>
        /// <exception cref="SshConnectionException">A received package was invalid or failed the message integrity check.</exception>
        /// <exception cref="SshOperationTimeoutException">None of the handles are signaled in time and the session is not disconnecting.</exception>
        /// <exception cref="SocketException">A socket error was signaled while receiving messages from the server.</exception>
        internal void WaitOnHandle(WaitHandle waitHandle, TimeSpan timeout)
        {
            if (waitHandle == null)
                throw new ArgumentNullException("waitHandle");

            var waitHandles = new[]
                {
                    _exceptionWaitHandle,
                    _messageListenerCompleted,
                    waitHandle
                };

            switch (WaitHandle.WaitAny(waitHandles, timeout))
            {
                case 0:
                    throw _exception;
                case 1:
                    throw new SshConnectionException("Client not connected.");
                case WaitHandle.WaitTimeout:
                    // when the session is disconnecting, a timeout is likely when no
                    // network connectivity is available; depending on the configured
                    // timeout either the WaitAny times out first or a SocketException
                    // detailing a timeout thrown hereby completing the listener thread
                    // (which makes us end up in case 1). Either way, we do not want to
                    // report an exception to the client when we're disconnecting anyway
                    if (!_isDisconnecting)
                    {
                        throw new SshOperationTimeoutException("Session operation has timed out");
                    }
                    break;
            }
        }

        /// <summary>
        /// Sends a message to the server.
        /// </summary>
        /// <param name="message">The message to send.</param>
        /// <exception cref="SshConnectionException">The client is not connected.</exception>
        /// <exception cref="SshOperationTimeoutException">The operation timed out.</exception>
        /// <exception cref="InvalidOperationException">The size of the packet exceeds the maximum size defined by the protocol.</exception>
        internal void SendMessage(Message message)
        {
            if (!_socket.CanWrite())
                throw new SshConnectionException("Client not connected.");

            if (_keyExchangeInProgress && !(message is IKeyExchangedAllowed))
            {
                //  Wait for key exchange to be completed
                WaitOnHandle(_keyExchangeCompletedWaitHandle);
            }

            DiagnosticAbstraction.Log(string.Format("[{0}] Sending message '{1}' to server: '{2}'.", ToHex(SessionId), message.GetType().Name, message));

            var paddingMultiplier = _clientCipher == null ? (byte) 8 : Math.Max((byte) 8, _serverCipher.MinimumSize);
            var packetData = message.GetPacket(paddingMultiplier, _clientCompression);

            // take a write lock to ensure the outbound packet sequence number is incremented
            // atomically, and only after the packet has actually been sent
            lock (_socketWriteLock)
            {
                byte[] hash = null;
                var packetDataOffset = 4; // first four bytes are reserved for outbound packet sequence

                if (_clientMac != null)
                {
                    // write outbound packet sequence to start of packet data
                    Pack.UInt32ToBigEndian(_outboundPacketSequence, packetData);
                    //  calculate packet hash
                    hash = _clientMac.ComputeHash(packetData);
                }

                //  Encrypt packet data
                if (_clientCipher != null)
                {
                    packetData = _clientCipher.Encrypt(packetData, packetDataOffset, (packetData.Length - packetDataOffset));
                    packetDataOffset = 0;
                }

                if (packetData.Length > MaximumSshPacketSize)
                {
                    throw new InvalidOperationException(string.Format(CultureInfo.CurrentCulture, "Packet is too big. Maximum packet size is {0} bytes.", MaximumSshPacketSize));
                }

                var packetLength = packetData.Length - packetDataOffset;
                if (hash == null)
                {
                    SendPacket(packetData, packetDataOffset, packetLength);
                }
                else
                {
                    var data = new byte[packetLength + hash.Length];
                    Buffer.BlockCopy(packetData, packetDataOffset, data, 0, packetLength);
                    Buffer.BlockCopy(hash, 0, data, packetLength, hash.Length);
                    SendPacket(data, 0, data.Length);
                }

                // increment the packet sequence number only after we're sure the packet has
                // been sent; even though it's only used for the MAC, it needs to be incremented
                // for each package sent.
                // 
                // the server will use it to verify the data integrity, and as such the order in
                // which messages are sent must follow the outbound packet sequence number
                _outboundPacketSequence++;
            }
        }

        /// <summary>
        /// Sends an SSH packet to the server.
        /// </summary>
        /// <param name="packet">A byte array containing the packet to send.</param>
        /// <param name="offset">The offset of the packet.</param>
        /// <param name="length">The length of the packet.</param>
        /// <exception cref="SshConnectionException">Client is not connected to the server.</exception>
        /// <remarks>
        /// <para>
        /// The send is performed in a dispose lock to avoid <see cref="NullReferenceException"/>
        /// and/or <see cref="ObjectDisposedException"/> when sending the packet.
        /// </para>
        /// <para>
        /// This method is only to be used when the connection is established, as the locking
        /// overhead is not required while establising the connection.
        /// </para>
        /// </remarks>
        private void SendPacket(byte[] packet, int offset, int length)
        {
            lock (_socketDisposeLock)
            {
                if (!_socket.IsConnected())
                    throw new SshConnectionException("Client not connected.");

                SocketAbstraction.Send(_socket, packet, offset, length);
            }
        }

        /// <summary>
        /// Sends a message to the server.
        /// </summary>
        /// <param name="message">The message to send.</param>
        /// <returns>
        /// <c>true</c> if the message was sent to the server; otherwise, <c>false</c>.
        /// </returns>
        /// <exception cref="InvalidOperationException">The size of the packet exceeds the maximum size defined by the protocol.</exception>
        /// <remarks>
        /// This methods returns <c>false</c> when the attempt to send the message results in a
        /// <see cref="SocketException"/> or a <see cref="SshException"/>.
        /// </remarks>
        private bool TrySendMessage(Message message)
        {
            try
            {
                SendMessage(message);
                return true;
            }
            catch (SshException ex)
            {
                DiagnosticAbstraction.Log(string.Format("Failure sending message '{0}' to server: '{1}' => {2}", message.GetType().Name, message, ex));
                return false;
            }
            catch (SocketException ex)
            {
                DiagnosticAbstraction.Log(string.Format("Failure sending message '{0}' to server: '{1}' => {2}", message.GetType().Name, message, ex));
                return false;
            }
        }

        /// <summary>
        /// Receives the message from the server.
        /// </summary>
        /// <returns>
        /// The incoming SSH message, or <c>null</c> if the connection with the SSH server was closed.
        /// </returns>
        /// <remarks>
        /// We need no locking here since all messages are read by a single thread.
        /// </remarks>
        private Message ReceiveMessage()
        {
            // the length of the packet sequence field in bytes
            const int inboundPacketSequenceLength = 4;
            // The length of the "packet length" field in bytes
            const int packetLengthFieldLength = 4;
            // The length of the "padding length" field in bytes
            const int paddingLengthFieldLength = 1;

            // Determine the size of the first block, which is 8 or cipher block size (whichever is larger) bytes
            var blockSize = _serverCipher == null ? (byte) 8 : Math.Max((byte) 8, _serverCipher.MinimumSize);

            var serverMacLength = _serverMac != null ? _serverMac.HashSize/8 : 0;

            byte[] data;
            uint packetLength;

#if FEATURE_SOCKET_POLL
            // avoid reading from socket while IsSocketConnected is attempting to determine whether the
            // socket is still connected by invoking Socket.Poll(...) and subsequently verifying value of
            // Socket.Available
            lock (_socketReadLock)
            {
#endif // FEATURE_SOCKET_POLL
                //  Read first block - which starts with the packet length
                var firstBlock = new byte[blockSize];
                if (TrySocketRead(firstBlock, 0, blockSize) == 0)
                {
                    // connection with SSH server was closed
                    return null;
                }

                if (_serverCipher != null)
                {
                    firstBlock = _serverCipher.Decrypt(firstBlock);
                }

                packetLength = Pack.BigEndianToUInt32(firstBlock);

                // Test packet minimum and maximum boundaries
                if (packetLength < Math.Max((byte) 16, blockSize) - 4 || packetLength > MaximumSshPacketSize - 4)
                    throw new SshConnectionException(
                        string.Format(CultureInfo.CurrentCulture, "Bad packet length: {0}.", packetLength),
                        DisconnectReason.ProtocolError);

                // Determine the number of bytes left to read; We've already read "blockSize" bytes, but the
                // "packet length" field itself - which is 4 bytes - is not included in the length of the packet
                var bytesToRead = (int) (packetLength - (blockSize - packetLengthFieldLength)) + serverMacLength;

                // Construct buffer for holding the payload and the inbound packet sequence as we need both in order
                // to generate the hash.
                // 
                // The total length of the "data" buffer is an addition of:
                // - inboundPacketSequenceLength (4 bytes)
                // - packetLength
                // - serverMacLength
                // 
                // We include the inbound packet sequence to allow us to have the the full SSH packet in a single
                // byte[] for the purpose of calculating the client hash. Room for the server MAC is foreseen
                // to read the packet including server MAC in a single pass (except for the initial block).
                data = new byte[bytesToRead + blockSize + inboundPacketSequenceLength];
                Pack.UInt32ToBigEndian(_inboundPacketSequence, data);
                Buffer.BlockCopy(firstBlock, 0, data, inboundPacketSequenceLength, firstBlock.Length);

                if (bytesToRead > 0)
                {
                    if (TrySocketRead(data, blockSize + inboundPacketSequenceLength, bytesToRead) == 0)
                    {
                        return null;
                    }
                }
#if FEATURE_SOCKET_POLL
            }
#endif // FEATURE_SOCKET_POLL

            if (_serverCipher != null)
            {
                var numberOfBytesToDecrypt = data.Length - (blockSize + inboundPacketSequenceLength + serverMacLength);
                if (numberOfBytesToDecrypt > 0)
                {
                    var decryptedData = _serverCipher.Decrypt(data, blockSize + inboundPacketSequenceLength, numberOfBytesToDecrypt);
                    Buffer.BlockCopy(decryptedData, 0, data, blockSize + inboundPacketSequenceLength, decryptedData.Length);
                }
            }

            var paddingLength = data[inboundPacketSequenceLength + packetLengthFieldLength];
            var messagePayloadLength = (int) packetLength - paddingLength - paddingLengthFieldLength;
            var messagePayloadOffset = inboundPacketSequenceLength + packetLengthFieldLength + paddingLengthFieldLength;

            // validate message against MAC
            if (_serverMac != null)
            {
                var clientHash = _serverMac.ComputeHash(data, 0, data.Length - serverMacLength);
                var serverHash = data.Take(data.Length - serverMacLength, serverMacLength);

                // TODO add IsEqualTo overload that takes left+right index and number of bytes to compare;
                // TODO that way we can eliminate the extrate allocation of the Take above
                if (!serverHash.IsEqualTo(clientHash))
                {
                    throw new SshConnectionException("MAC error", DisconnectReason.MacError);
                }
            }

            if (_serverDecompression != null)
            {
                data = _serverDecompression.Decompress(data, messagePayloadOffset, messagePayloadLength);

                // data now only contains the decompressed payload, and as such the offset is reset to zero
                messagePayloadOffset = 0;
                // the length of the payload is now the complete decompressed content
                messagePayloadLength = data.Length;
            }

            _inboundPacketSequence++;

            return LoadMessage(data, messagePayloadOffset, messagePayloadLength);
        }

        private void TrySendDisconnect(DisconnectReason reasonCode, string message)
        {
            var disconnectMessage = new DisconnectMessage(reasonCode, message);

            // send the disconnect message, but ignore the outcome
            TrySendMessage(disconnectMessage);

            // mark disconnect message sent regardless of whether the send sctually succeeded
            _isDisconnectMessageSent = true;
        }

#region Handle received message events

        /// <summary>
        /// Called when <see cref="DisconnectMessage"/> received.
        /// </summary>
        /// <param name="message"><see cref="DisconnectMessage"/> message.</param>
        internal void OnDisconnectReceived(DisconnectMessage message)
        {
            DiagnosticAbstraction.Log(string.Format("[{0}] Disconnect received: {1} {2}.", ToHex(SessionId), message.ReasonCode, message.Description));

            // transition to disconnecting state to avoid throwing exceptions while cleaning up, and to
            // ensure any exceptions that are raised do not overwrite the SshConnectionException that we
            // set below
            _isDisconnecting = true;

            _exception = new SshConnectionException(string.Format(CultureInfo.InvariantCulture, "The connection was closed by the server: {0} ({1}).", message.Description, message.ReasonCode), message.ReasonCode);
            _exceptionWaitHandle.Set();

            var disconnectReceived = DisconnectReceived;
            if (disconnectReceived != null)
                disconnectReceived(this, new MessageEventArgs<DisconnectMessage>(message));

            var disconnected = Disconnected;
            if (disconnected != null)
                disconnected(this, new EventArgs());

            // disconnect socket, and dispose it
            SocketDisconnectAndDispose();
        }

        /// <summary>
        /// Called when <see cref="IgnoreMessage"/> received.
        /// </summary>
        /// <param name="message"><see cref="IgnoreMessage"/> message.</param>
        internal void OnIgnoreReceived(IgnoreMessage message)
        {
            var handlers = IgnoreReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<IgnoreMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="UnimplementedMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="UnimplementedMessage"/> message.</param>
        internal void OnUnimplementedReceived(UnimplementedMessage message)
        {
            var handlers = UnimplementedReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<UnimplementedMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="DebugMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="DebugMessage"/> message.</param>
        internal void OnDebugReceived(DebugMessage message)
        {
            var handlers = DebugReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<DebugMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ServiceRequestMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ServiceRequestMessage"/> message.</param>
        internal void OnServiceRequestReceived(ServiceRequestMessage message)
        {
            var handlers = ServiceRequestReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ServiceRequestMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ServiceAcceptMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ServiceAcceptMessage"/> message.</param>
        internal void OnServiceAcceptReceived(ServiceAcceptMessage message)
        {
            var handlers = ServiceAcceptReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ServiceAcceptMessage>(message));

            _serviceAccepted.Set();
        }

        internal void OnKeyExchangeDhGroupExchangeGroupReceived(KeyExchangeDhGroupExchangeGroup message)
        {
            var handlers = KeyExchangeDhGroupExchangeGroupReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<KeyExchangeDhGroupExchangeGroup>(message));
        }

        internal void OnKeyExchangeDhGroupExchangeReplyReceived(KeyExchangeDhGroupExchangeReply message)
        {
            var handlers = KeyExchangeDhGroupExchangeReplyReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<KeyExchangeDhGroupExchangeReply>(message));
        }

        /// <summary>
        /// Called when <see cref="KeyExchangeInitMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="KeyExchangeInitMessage"/> message.</param>
        internal void OnKeyExchangeInitReceived(KeyExchangeInitMessage message)
        {
            _keyExchangeInProgress = true;

            _keyExchangeCompletedWaitHandle.Reset();

            // Disable messages that are not key exchange related
            _sshMessageFactory.DisableNonKeyExchangeMessages();

            _keyExchange = _serviceFactory.CreateKeyExchange(ConnectionInfo.KeyExchangeAlgorithms,
                message.KeyExchangeAlgorithms);

            ConnectionInfo.CurrentKeyExchangeAlgorithm = _keyExchange.Name;

            _keyExchange.HostKeyReceived += KeyExchange_HostKeyReceived;

            //  Start the algorithm implementation
            _keyExchange.Start(this, message);

            var keyExchangeInitReceived = KeyExchangeInitReceived;
            if (keyExchangeInitReceived != null)
                keyExchangeInitReceived(this, new MessageEventArgs<KeyExchangeInitMessage>(message));
        }

        internal void OnKeyExchangeDhReplyMessageReceived(KeyExchangeDhReplyMessage message)
        {
            var handlers = KeyExchangeDhReplyMessageReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<KeyExchangeDhReplyMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="NewKeysMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="NewKeysMessage"/> message.</param>
        internal void OnNewKeysReceived(NewKeysMessage message)
        {
            //  Update sessionId
            if (SessionId == null)
            {
                SessionId = _keyExchange.ExchangeHash;
            }

            //  Dispose of old ciphers and hash algorithms
            if (_serverMac != null)
            {
                _serverMac.Dispose();
                _serverMac = null;
            }

            if (_clientMac != null)
            {
                _clientMac.Dispose();
                _clientMac = null;
            }

            //  Update negotiated algorithms
            _serverCipher = _keyExchange.CreateServerCipher();
            _clientCipher = _keyExchange.CreateClientCipher();
            _serverMac = _keyExchange.CreateServerHash();
            _clientMac = _keyExchange.CreateClientHash();
            _clientCompression = _keyExchange.CreateCompressor();
            _serverDecompression = _keyExchange.CreateDecompressor();

            //  Dispose of old KeyExchange object as it is no longer needed.
            if (_keyExchange != null)
            {
                _keyExchange.HostKeyReceived -= KeyExchange_HostKeyReceived;
                _keyExchange.Dispose();
                _keyExchange = null;
            }

            // Enable activated messages that are not key exchange related
            _sshMessageFactory.EnableActivatedMessages();

            var newKeysReceived = NewKeysReceived;
            if (newKeysReceived != null)
                newKeysReceived(this, new MessageEventArgs<NewKeysMessage>(message));

            //  Signal that key exchange completed
            _keyExchangeCompletedWaitHandle.Set();

            _keyExchangeInProgress = false;
        }

        /// <summary>
        /// Called when client is disconnecting from the server.
        /// </summary>
        void ISession.OnDisconnecting()
        {
            _isDisconnecting = true;
        }

        /// <summary>
        /// Called when <see cref="RequestMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="RequestMessage"/> message.</param>
        internal void OnUserAuthenticationRequestReceived(RequestMessage message)
        {
            var handlers = UserAuthenticationRequestReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<RequestMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="FailureMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="FailureMessage"/> message.</param>
        internal void OnUserAuthenticationFailureReceived(FailureMessage message)
        {
            var handlers = UserAuthenticationFailureReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<FailureMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="SuccessMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="SuccessMessage"/> message.</param>
        internal void OnUserAuthenticationSuccessReceived(SuccessMessage message)
        {
            var handlers = UserAuthenticationSuccessReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<SuccessMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="BannerMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="BannerMessage"/> message.</param>
        internal void OnUserAuthenticationBannerReceived(BannerMessage message)
        {
            var handlers = UserAuthenticationBannerReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<BannerMessage>(message));
        }


        /// <summary>
        /// Called when <see cref="InformationRequestMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="InformationRequestMessage"/> message.</param>
        internal void OnUserAuthenticationInformationRequestReceived(InformationRequestMessage message)
        {
            var handlers = UserAuthenticationInformationRequestReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<InformationRequestMessage>(message));
        }

        internal void OnUserAuthenticationPasswordChangeRequiredReceived(PasswordChangeRequiredMessage message)
        {
            var handlers = UserAuthenticationPasswordChangeRequiredReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<PasswordChangeRequiredMessage>(message));
        }

        internal void OnUserAuthenticationPublicKeyReceived(PublicKeyMessage message)
        {
            var handlers = UserAuthenticationPublicKeyReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<PublicKeyMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="GlobalRequestMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="GlobalRequestMessage"/> message.</param>
        internal void OnGlobalRequestReceived(GlobalRequestMessage message)
        {
            var handlers = GlobalRequestReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<GlobalRequestMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="RequestSuccessMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="RequestSuccessMessage"/> message.</param>
        internal void OnRequestSuccessReceived(RequestSuccessMessage message)
        {
            var handlers = RequestSuccessReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<RequestSuccessMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="RequestFailureMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="RequestFailureMessage"/> message.</param>
        internal void OnRequestFailureReceived(RequestFailureMessage message)
        {
            var handlers = RequestFailureReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<RequestFailureMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ChannelOpenMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ChannelOpenMessage"/> message.</param>
        internal void OnChannelOpenReceived(ChannelOpenMessage message)
        {
            var handlers = ChannelOpenReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ChannelOpenMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ChannelOpenConfirmationMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ChannelOpenConfirmationMessage"/> message.</param>
        internal void OnChannelOpenConfirmationReceived(ChannelOpenConfirmationMessage message)
        {
            var handlers = ChannelOpenConfirmationReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ChannelOpenConfirmationMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ChannelOpenFailureMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ChannelOpenFailureMessage"/> message.</param>
        internal void OnChannelOpenFailureReceived(ChannelOpenFailureMessage message)
        {
            var handlers = ChannelOpenFailureReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ChannelOpenFailureMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ChannelWindowAdjustMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ChannelWindowAdjustMessage"/> message.</param>
        internal void OnChannelWindowAdjustReceived(ChannelWindowAdjustMessage message)
        {
            var handlers = ChannelWindowAdjustReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ChannelWindowAdjustMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ChannelDataMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ChannelDataMessage"/> message.</param>
        internal void OnChannelDataReceived(ChannelDataMessage message)
        {
            var handlers = ChannelDataReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ChannelDataMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ChannelExtendedDataMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ChannelExtendedDataMessage"/> message.</param>
        internal void OnChannelExtendedDataReceived(ChannelExtendedDataMessage message)
        {
            var handlers = ChannelExtendedDataReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ChannelExtendedDataMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ChannelCloseMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ChannelCloseMessage"/> message.</param>
        internal void OnChannelEofReceived(ChannelEofMessage message)
        {
            var handlers = ChannelEofReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ChannelEofMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ChannelCloseMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ChannelCloseMessage"/> message.</param>
        internal void OnChannelCloseReceived(ChannelCloseMessage message)
        {
            var handlers = ChannelCloseReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ChannelCloseMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ChannelRequestMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ChannelRequestMessage"/> message.</param>
        internal void OnChannelRequestReceived(ChannelRequestMessage message)
        {
            var handlers = ChannelRequestReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ChannelRequestMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ChannelSuccessMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ChannelSuccessMessage"/> message.</param>
        internal void OnChannelSuccessReceived(ChannelSuccessMessage message)
        {
            var handlers = ChannelSuccessReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ChannelSuccessMessage>(message));
        }

        /// <summary>
        /// Called when <see cref="ChannelFailureMessage"/> message received.
        /// </summary>
        /// <param name="message"><see cref="ChannelFailureMessage"/> message.</param>
        internal void OnChannelFailureReceived(ChannelFailureMessage message)
        {
            var handlers = ChannelFailureReceived;
            if (handlers != null)
                handlers(this, new MessageEventArgs<ChannelFailureMessage>(message));
        }

#endregion

        private void KeyExchange_HostKeyReceived(object sender, HostKeyEventArgs e)
        {
            var handlers = HostKeyReceived;
            if (handlers != null)
                handlers(this, e);
        }

#region Message loading functions

        /// <summary>
        /// Registers SSH message with the session.
        /// </summary>
        /// <param name="messageName">The name of the message to register with the session.</param>
        public void RegisterMessage(string messageName)
        {
            _sshMessageFactory.EnableAndActivateMessage(messageName);
        }

        /// <summary>
        /// Unregister SSH message from the session.
        /// </summary>
        /// <param name="messageName">The name of the message to unregister with the session.</param>
        public void UnRegisterMessage(string messageName)
        {
            _sshMessageFactory.DisableAndDeactivateMessage(messageName);
        }

        /// <summary>
        /// Loads a message from a given buffer.
        /// </summary>
        /// <param name="data">An array of bytes from which to construct the message.</param>
        /// <param name="offset">The zero-based byte offset in <paramref name="data"/> at which to begin reading.</param>
        /// <param name="count">The number of bytes to load.</param>
        /// <returns>
        /// A message constructed from <paramref name="data"/>.
        /// </returns>
        /// <exception cref="SshException">The type of the message is not supported.</exception>
        private Message LoadMessage(byte[] data, int offset, int count)
        {
            var messageType = data[offset];

            var message = _sshMessageFactory.Create(messageType);
            message.Load(data, offset + 1, count - 1);

            DiagnosticAbstraction.Log(string.Format("[{0}] Received message '{1}' from server: '{2}'.", ToHex(SessionId), message.GetType().Name, message));

            return message;
        }

        private static string ToHex(byte[] bytes, int offset)
        {
            var byteCount = bytes.Length - offset;

            var builder = new StringBuilder(bytes.Length * 2);

            for (var i = offset; i < byteCount; i++)
            {
                var b = bytes[i];
                builder.Append(b.ToString("X2"));
            }

            return builder.ToString();
        }

        internal static string ToHex(byte[] bytes)
        {
            if (bytes == null)
                return null;

            return ToHex(bytes, 0);
        }

#endregion

        /// <summary>
        /// Establishes a socket connection to the specified host and port.
        /// </summary>
        /// <param name="host">The host name of the server to connect to.</param>
        /// <param name="port">The port to connect to.</param>
        /// <exception cref="SshOperationTimeoutException">The connection failed to establish within the configured <see cref="Renci.SshNet.ConnectionInfo.Timeout"/>.</exception>
        /// <exception cref="SocketException">An error occurred trying to establish the connection.</exception>
        private void SocketConnect(string host, int port)
        {
            var ipAddress = DnsAbstraction.GetHostAddresses(host)[0];
            var ep = new IPEndPoint(ipAddress, port);

            DiagnosticAbstraction.Log(string.Format("Initiating connection to '{0}:{1}'.", host, port));

            _socket = SocketAbstraction.Connect(ep, ConnectionInfo.Timeout);

            const int socketBufferSize = 2 * MaximumSshPacketSize;
            _socket.SendBufferSize = socketBufferSize;
            _socket.ReceiveBufferSize = socketBufferSize;
        }

        /// <summary>
        /// Performs a blocking read on the socket until <paramref name="length"/> bytes are received.
        /// </summary>
        /// <param name="buffer">An array of type <see cref="byte"/> that is the storage location for the received data.</param>
        /// <param name="offset">The position in <paramref name="buffer"/> parameter to store the received data.</param>
        /// <param name="length">The number of bytes to read.</param>
        /// <returns>
        /// The number of bytes read.
        /// </returns>
        /// <exception cref="SshConnectionException">The socket is closed.</exception>
        /// <exception cref="SshOperationTimeoutException">The read has timed-out.</exception>
        /// <exception cref="SocketException">The read failed.</exception>
        private int SocketRead(byte[] buffer, int offset, int length)
        {
            var bytesRead = SocketAbstraction.Read(_socket, buffer, offset, length, InfiniteTimeSpan);
            if (bytesRead == 0)
            {
                // when we're in the disconnecting state (either triggered by client or server), then the
                // SshConnectionException will interrupt the message listener loop (if not already interrupted)
                // and the exception itself will be ignored (in RaiseError)
                throw new SshConnectionException("An established connection was aborted by the server.",
                    DisconnectReason.ConnectionLost);
            }
            return bytesRead;
        }

#if FEATURE_SOCKET_POLL
        /// <summary>
        /// Gets a value indicating whether the socket is connected.
        /// </summary>
        /// <returns>
        /// <c>true</c> if the socket is connected; otherwise, <c>false</c>.
        /// </returns>
        /// <remarks>
        /// <para>
        /// As a first check we verify whether <see cref="Socket.Connected"/> is
        /// <c>true</c>. However, this only returns the state of the socket as of
        /// the last I/O operation.
        /// </para>
        /// <para>
        /// Therefore we use the combination of <see cref="Socket.Poll(int, SelectMode)"/> with mode <see cref="SelectMode.SelectRead"/>
        /// and <see cref="Socket.Available"/> to verify if the socket is still connected.
        /// </para>
        /// <para>
        /// The MSDN doc mention the following on the return value of <see cref="Socket.Poll(int, SelectMode)"/>
        /// with mode <see cref="SelectMode.SelectRead"/>:
        /// <list type="bullet">
        ///     <item>
        ///         <description><c>true</c> if data is available for reading;</description>
        ///     </item>
        ///     <item>
        ///         <description><c>true</c> if the connection has been closed, reset, or terminated; otherwise, returns <c>false</c>.</description>
        ///     </item>
        /// </list>
        /// </para>
        /// <para>
        /// <c>Conclusion:</c> when the return value is <c>true</c> - but no data is available for reading - then
        /// the socket is no longer connected.
        /// </para>
        /// <para>
        /// When a <see cref="Socket"/> is used from multiple threads, there's a race condition
        /// between the invocation of <see cref="Socket.Poll(int, SelectMode)"/> and the moment
        /// when the value of <see cref="Socket.Available"/> is obtained. To workaround this issue
        /// we synchronize reads from the <see cref="Socket"/>.
        /// </para>
        /// </remarks>
#else
        /// <summary>
        /// Gets a value indicating whether the socket is connected.
        /// </summary>
        /// <returns>
        /// <c>true</c> if the socket is connected; otherwise, <c>false</c>.
        /// </returns>
        /// <remarks>
        /// We verify whether <see cref="Socket.Connected"/> is <c>true</c>. However, this only returns the state
        /// of the socket as of the last I/O operation.
        /// </remarks>
#endif
        private bool IsSocketConnected()
        {
            lock (_socketDisposeLock)
            {
#if FEATURE_SOCKET_POLL
                if (!_socket.IsConnected())
                {
                    return false;
                }

                lock (_socketReadLock)
                {
                    var connectionClosedOrDataAvailable = _socket.Poll(0, SelectMode.SelectRead);
                    return !(connectionClosedOrDataAvailable && _socket.Available == 0);
                }
#else
                return _socket.IsConnected();
#endif // FEATURE_SOCKET_POLL
            }
        }

        /// <summary>
        /// Performs a blocking read on the socket until <paramref name="length"/> bytes are received.
        /// </summary>
        /// <param name="buffer">An array of type <see cref="byte"/> that is the storage location for the received data.</param>
        /// <param name="offset">The position in <paramref name="buffer"/> parameter to store the received data.</param>
        /// <param name="length">The number of bytes to read.</param>
        /// <returns>
        /// The number of bytes read.
        /// </returns>
        /// <exception cref="SshOperationTimeoutException">The read has timed-out.</exception>
        /// <exception cref="SocketException">The read failed.</exception>
        private int TrySocketRead(byte[] buffer, int offset, int length)
        {
            return SocketAbstraction.Read(_socket, buffer, offset, length, InfiniteTimeSpan);
        }

        /// <summary>
        /// Performs a blocking read on the socket until a line is read.
        /// </summary>
        /// <param name="timeout">A <see cref="TimeSpan"/> that represents the time to wait until a line is read.</param>
        /// <exception cref="SshOperationTimeoutException">The read has timed-out.</exception>
        /// <exception cref="SocketException">An error occurred when trying to access the socket.</exception>
        /// <returns>
        /// The line read from the socket, or <c>null</c> when the remote server has shutdown and all data has been received.
        /// </returns>
        private string SocketReadLine(TimeSpan timeout)
        {
            var encoding = SshData.Ascii;
            var buffer = new List<byte>();
            var data = new byte[1];

            // read data one byte at a time to find end of line and leave any unhandled information in the buffer
            // to be processed by subsequent invocations
            do
            {
                var bytesRead = SocketAbstraction.Read(_socket, data, 0, data.Length, timeout);
                if (bytesRead == 0)
                    // the remote server shut down the socket
                    break;

                buffer.Add(data[0]);
            }
            while (!(buffer.Count > 0 && (buffer[buffer.Count - 1] == LineFeed || buffer[buffer.Count - 1] == Null)));

            if (buffer.Count == 0)
                return null;
            if (buffer.Count == 1 && buffer[buffer.Count - 1] == 0x00)
                // return an empty version string if the buffer consists of only a 0x00 character
                return string.Empty;
            if (buffer.Count > 1 && buffer[buffer.Count - 2] == CarriageReturn)
                // strip trailing CRLF
                return encoding.GetString(buffer.ToArray(), 0, buffer.Count - 2);
            if (buffer.Count > 1 && buffer[buffer.Count - 1] == LineFeed)
                // strip trailing LF
                return encoding.GetString(buffer.ToArray(), 0, buffer.Count - 1);
            return encoding.GetString(buffer.ToArray(), 0, buffer.Count);
        }

        /// <summary>
        /// Shuts down and disposes the socket.
        /// </summary>
        private void SocketDisconnectAndDispose()
        {
            if (_socket != null)
            {
                lock (_socketDisposeLock)
                {
                    if (_socket != null)
                    {
                        if (_socket.Connected)
                        {
                            try
                            {
                                DiagnosticAbstraction.Log(string.Format("[{0}] Shutting down socket.", ToHex(SessionId)));

                                // interrupt any pending reads; should be done outside of socket read lock as we
                                // actually want shutdown the socket to make sure blocking reads are interrupted
                                //
                                // this may result in a SocketException (eg. An existing connection was forcibly
                                // closed by the remote host) which we'll log and ignore as it means the socket
                                // was already shut down
                                _socket.Shutdown(SocketShutdown.Send);
                            }
                            catch (SocketException ex)
                            {
                                // TODO: log as warning
                                DiagnosticAbstraction.Log("Failure shutting down socket: " + ex);
                            }
                        }

                        DiagnosticAbstraction.Log(string.Format("[{0}] Disposing socket.", ToHex(SessionId)));
                        _socket.Dispose();
                        DiagnosticAbstraction.Log(string.Format("[{0}] Disposed socket.", ToHex(SessionId)));
                        _socket = null;
                    }
                }
            }
        }

        /// <summary>
        /// Listens for incoming message from the server and handles them. This method run as a task on separate thread.
        /// </summary>
        private void MessageListener()
        {
#if FEATURE_SOCKET_SELECT
            var readSockets = new List<Socket> { _socket };
#endif // FEATURE_SOCKET_SELECT

            try
            {
                // remain in message loop until socket is shut down or until we're disconnecting
                while (_socket.IsConnected())
                {
#if FEATURE_SOCKET_SELECT
                    // if the socket is already disposed when Select is invoked, then a SocketException
                    // stating "An operation was attempted on something that is not a socket" is thrown;
                    // we attempt to avoid this exception by having an IsConnected() that can break the
                    // message loop
                    //
                    // note that there's no guarantee that the socket will not be disposed between the
                    // IsConnected() check and the Select invocation; we can't take a "dispose" lock
                    // that includes the Select invocation as we want Dispose() to be able to interrupt
                    // the Select

                    // perform a blocking select to determine whether there's is data available to be
                    // read; we do not use a blocking read to allow us to use Socket.Poll to determine
                    // if the connection is still available (in IsSocketConnected)

                    Socket.Select(readSockets, null, null, -1);

                    // the Select invocation will be interrupted in one of the following conditions:
                    // * data is available to be read
                    //   => the socket will not be removed from "readSockets"
                    // * the socket connection is closed during the Select invocation
                    //   => the socket will be removed from "readSockets"
                    // * the socket is disposed during the Select invocation
                    //   => the socket will not be removed from "readSocket"
                    // 
                    // since we handle the second and third condition the same way and Socket.Connected
                    // allows us to check for both conditions, we use that instead of both checking for
                    // the removal from "readSockets" and the Connection check
                    if (!_socket.IsConnected())
                    {
                        // connection with SSH server was closed or socket was disposed;
                        // break out of the message loop
                        break;
                    }
#elif FEATURE_SOCKET_POLL
                    // when Socket.Select(IList, IList, IList, Int32) is not available or is buggy, we use
                    // Socket.Poll(Int, SelectMode) to block until either data is available or the socket
                    // is closed
                    _socket.Poll(-1, SelectMode.SelectRead);

                    if (!_socket.IsConnected())
                    {
                        // connection with SSH server was closed or socket was disposed;
                        // break out of the message loop
                        break;
                    }
#endif // FEATURE_SOCKET_SELECT

                    var message = ReceiveMessage();
                    if (message == null)
                    {
                        // connection with SSH server was closed;
                        // break out of the message loop
                        break;
                    }

                    // process message
                    message.Process(this);
                }

                // connection with SSH server was closed or socket was disposed
                RaiseError(CreateConnectionAbortedByServerException());
            }
            catch (SocketException ex)
            {
                RaiseError(new SshConnectionException(ex.Message, DisconnectReason.ConnectionLost, ex));
            }
            catch (Exception exp)
            {
                RaiseError(exp);
            }
            finally
            {
                // signal that the message listener thread has stopped
                _messageListenerCompleted.Set();
            }
        }

        private byte SocketReadByte()
        {
            var buffer = new byte[1];
            SocketRead(buffer, 0, 1);
            return buffer[0];
        }

        private void SocketWriteByte(byte data)
        {
            SocketAbstraction.Send(_socket, new[] {data});
        }

        private void ConnectSocks4()
        {
            //  Send socks version number
            SocketWriteByte(0x04);

            //  Send command code
            SocketWriteByte(0x01);

            //  Send port
            SocketWriteByte((byte)(ConnectionInfo.Port / 0xFF));
            SocketWriteByte((byte)(ConnectionInfo.Port % 0xFF));

            //  Send IP
            var ipAddress = DnsAbstraction.GetHostAddresses(ConnectionInfo.Host)[0];
            SocketAbstraction.Send(_socket, ipAddress.GetAddressBytes());

            //  Send username
            var username = SshData.Ascii.GetBytes(ConnectionInfo.ProxyUsername);
            SocketAbstraction.Send(_socket, username);
            SocketWriteByte(0x00);

            //  Read 0
            if (SocketReadByte() != 0)
            {
                throw new ProxyException("SOCKS4: Null is expected.");
            }

            //  Read response code
            var code = SocketReadByte();

            switch (code)
            {
                case 0x5a:
                    break;
                case 0x5b:
                    throw new ProxyException("SOCKS4: Connection rejected.");
                case 0x5c:
                    throw new ProxyException("SOCKS4: Client is not running identd or not reachable from the server.");
                case 0x5d:
                    throw new ProxyException("SOCKS4: Client's identd could not confirm the user ID string in the request.");
                default:
                    throw new ProxyException("SOCKS4: Not valid response.");
            }

            var dummyBuffer = new byte[6]; // field 3 (2 bytes) and field 4 (4) should be ignored
            SocketRead(dummyBuffer, 0, 6);
        }

        private void ConnectSocks5()
        {
            //  Send socks version number
            SocketWriteByte(0x05);

            //  Send number of supported authentication methods
            SocketWriteByte(0x02);

            //  Send supported authentication methods
            SocketWriteByte(0x00); //  No authentication
            SocketWriteByte(0x02); //  Username/Password

            var socksVersion = SocketReadByte();
            if (socksVersion != 0x05)
                throw new ProxyException(string.Format("SOCKS Version '{0}' is not supported.", socksVersion));

            var authenticationMethod = SocketReadByte();
            switch (authenticationMethod)
            {
                case 0x00:
                    break;
                case 0x02:

                    //  Send version
                    SocketWriteByte(0x01);

                    var username = SshData.Ascii.GetBytes(ConnectionInfo.ProxyUsername);

                    if (username.Length > byte.MaxValue)
                        throw new ProxyException("Proxy username is too long.");

                    //  Send username length
                    SocketWriteByte((byte)username.Length);

                    //  Send username
                    SocketAbstraction.Send(_socket, username);

                    var password = SshData.Ascii.GetBytes(ConnectionInfo.ProxyPassword);

                    if (password.Length > byte.MaxValue)
                        throw new ProxyException("Proxy password is too long.");

                    //  Send username length
                    SocketWriteByte((byte)password.Length);

                    //  Send username
                    SocketAbstraction.Send(_socket, password);

                    var serverVersion = SocketReadByte();

                    if (serverVersion != 1)
                        throw new ProxyException("SOCKS5: Server authentication version is not valid.");

                    var statusCode = SocketReadByte();
                    if (statusCode != 0)
                        throw new ProxyException("SOCKS5: Username/Password authentication failed.");

                    break;
                case 0xFF:
                    throw new ProxyException("SOCKS5: No acceptable authentication methods were offered.");
            }

            //  Send socks version number
            SocketWriteByte(0x05);

            //  Send command code
            SocketWriteByte(0x01); //  establish a TCP/IP stream connection

            //  Send reserved, must be 0x00
            SocketWriteByte(0x00);

            var ip = DnsAbstraction.GetHostAddresses(ConnectionInfo.Host)[0];

            //  Send address type and address
            if (ip.AddressFamily == AddressFamily.InterNetwork)
            {
                SocketWriteByte(0x01);
                var address = ip.GetAddressBytes();
                SocketAbstraction.Send(_socket, address);
            }
            else if (ip.AddressFamily == AddressFamily.InterNetworkV6)
            {
                SocketWriteByte(0x04);
                var address = ip.GetAddressBytes();
                SocketAbstraction.Send(_socket, address);
            }
            else
            {
                throw new ProxyException(string.Format("SOCKS5: IP address '{0}' is not supported.", ip));
            }

            //  Send port
            SocketWriteByte((byte)(ConnectionInfo.Port / 0xFF));
            SocketWriteByte((byte)(ConnectionInfo.Port % 0xFF));

            //  Read Server SOCKS5 version
            if (SocketReadByte() != 5)
            {
                throw new ProxyException("SOCKS5: Version 5 is expected.");
            }

            //  Read response code
            var status = SocketReadByte();

            switch (status)
            {
                case 0x00:
                    break;
                case 0x01:
                    throw new ProxyException("SOCKS5: General failure.");
                case 0x02:
                    throw new ProxyException("SOCKS5: Connection not allowed by ruleset.");
                case 0x03:
                    throw new ProxyException("SOCKS5: Network unreachable.");
                case 0x04:
                    throw new ProxyException("SOCKS5: Host unreachable.");
                case 0x05:
                    throw new ProxyException("SOCKS5: Connection refused by destination host.");
                case 0x06:
                    throw new ProxyException("SOCKS5: TTL expired.");
                case 0x07:
                    throw new ProxyException("SOCKS5: Command not supported or protocol error.");
                case 0x08:
                    throw new ProxyException("SOCKS5: Address type not supported.");
                default:
                    throw new ProxyException("SOCKS5: Not valid response.");
            }

            //  Read 0
            if (SocketReadByte() != 0)
            {
                throw new ProxyException("SOCKS5: 0 byte is expected.");
            }

            var addressType = SocketReadByte();
            var responseIp = new byte[16];

            switch (addressType)
            {
                case 0x01:
                    SocketRead(responseIp, 0, 4);
                    break;
                case 0x04:
                    SocketRead(responseIp, 0, 16);
                    break;
                default:
                    throw new ProxyException(string.Format("Address type '{0}' is not supported.", addressType));
            }

            var port = new byte[2];

            //  Read 2 bytes to be ignored
            SocketRead(port, 0, 2);
        }

        private void ConnectHttp()
        {
            var httpResponseRe = new Regex(@"HTTP/(?<version>\d[.]\d) (?<statusCode>\d{3}) (?<reasonPhrase>.+)$");
            var httpHeaderRe = new Regex(@"(?<fieldName>[^\[\]()<>@,;:\""/?={} \t]+):(?<fieldValue>.+)?");

            SocketAbstraction.Send(_socket, SshData.Ascii.GetBytes(string.Format("CONNECT {0}:{1} HTTP/1.0\r\n", ConnectionInfo.Host, ConnectionInfo.Port)));

            //  Sent proxy authorization is specified
            if (!string.IsNullOrEmpty(ConnectionInfo.ProxyUsername))
            {
                var authorization = string.Format("Proxy-Authorization: Basic {0}\r\n",
                                                  Convert.ToBase64String(SshData.Ascii.GetBytes(string.Format("{0}:{1}", ConnectionInfo.ProxyUsername, ConnectionInfo.ProxyPassword)))
                                                  );
                SocketAbstraction.Send(_socket, SshData.Ascii.GetBytes(authorization));
            }

            SocketAbstraction.Send(_socket, SshData.Ascii.GetBytes("\r\n"));

            HttpStatusCode? statusCode = null;
            var contentLength = 0;

            while (true)
            {
                var response = SocketReadLine(ConnectionInfo.Timeout);
                if (response == null)
                    // server shut down socket
                    break;

                if (statusCode == null)
                {
                    var statusMatch = httpResponseRe.Match(response);
                    if (statusMatch.Success)
                    {
                        var httpStatusCode = statusMatch.Result("${statusCode}");
                        statusCode = (HttpStatusCode) int.Parse(httpStatusCode);
                        if (statusCode != HttpStatusCode.OK)
                        {
                            var reasonPhrase = statusMatch.Result("${reasonPhrase}");
                            throw new ProxyException(string.Format("HTTP: Status code {0}, \"{1}\"", httpStatusCode,
                                reasonPhrase));
                        }
                    }

                    continue;
                }

                // continue on parsing message headers coming from the server
                var headerMatch = httpHeaderRe.Match(response);
                if (headerMatch.Success)
                {
                    var fieldName = headerMatch.Result("${fieldName}");
                    if (fieldName.Equals("Content-Length", StringComparison.OrdinalIgnoreCase))
                    {
                        contentLength = int.Parse(headerMatch.Result("${fieldValue}"));
                    }
                    continue;
                }

                // check if we've reached the CRLF which separates request line and headers from the message body
                if (response.Length == 0)
                {
                    //  read response body if specified
                    if (contentLength > 0)
                    {
                        var contentBody = new byte[contentLength];
                        SocketRead(contentBody, 0, contentLength);
                    }
                    break;
                }
            }

            if (statusCode == null)
                throw new ProxyException("HTTP response does not contain status line.");
        }

        /// <summary>
        /// Raises the <see cref="ErrorOccured"/> event.
        /// </summary>
        /// <param name="exp">The <see cref="Exception"/>.</param>
        private void RaiseError(Exception exp)
        {
            var connectionException = exp as SshConnectionException;

            DiagnosticAbstraction.Log(string.Format("[{0}] Raised exception: {1}", ToHex(SessionId), exp));

            if (_isDisconnecting)
            {
                //  a connection exception which is raised while isDisconnecting is normal and
                //  should be ignored
                if (connectionException != null)
                    return;

                // any timeout while disconnecting can be caused by loss of connectivity
                // altogether and should be ignored
                var socketException = exp as SocketException;
                if (socketException != null && socketException.SocketErrorCode == SocketError.TimedOut)
                    return;
            }

            // "save" exception and set exception wait handle to ensure any waits are interrupted
            _exception = exp;
            _exceptionWaitHandle.Set();

            var errorOccured = ErrorOccured;
            if (errorOccured != null)
                errorOccured(this, new ExceptionEventArgs(exp));

            if (connectionException != null)
            {
                DiagnosticAbstraction.Log(string.Format("[{0}] Disconnecting after exception: {1}", ToHex(SessionId), exp));
                Disconnect(connectionException.DisconnectReason, exp.ToString());
            }
        }

        /// <summary>
        /// Resets connection-specific information to ensure state of a previous connection
        /// does not affect new connections.
        /// </summary>
        private void Reset()
        {
            if (_exceptionWaitHandle != null)
                _exceptionWaitHandle.Reset();
            if (_keyExchangeCompletedWaitHandle != null)
                _keyExchangeCompletedWaitHandle.Reset();
            if (_messageListenerCompleted != null)
                _messageListenerCompleted.Set();

            SessionId = null;
            _isDisconnectMessageSent = false;
            _isDisconnecting = false;
            _isAuthenticated = false;
            _exception = null;
            _keyExchangeInProgress = false;
        }

        private static SshConnectionException CreateConnectionAbortedByServerException()
        {
            return new SshConnectionException("An established connection was aborted by the server.",
                DisconnectReason.ConnectionLost);
        }

#region IDisposable implementation

        private bool _disposed;

        /// <summary>
        /// Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
        /// </summary>
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        /// <summary>
        /// Releases unmanaged and - optionally - managed resources
        /// </summary>
        /// <param name="disposing"><c>true</c> to release both managed and unmanaged resources; <c>false</c> to release only unmanaged resources.</param>
        protected virtual void Dispose(bool disposing)
        {
            if (_disposed)
                return;

            if (disposing)
            {
                DiagnosticAbstraction.Log(string.Format("[{0}] Disposing session.", ToHex(SessionId)));

                Disconnect();

                var serviceAccepted = _serviceAccepted;
                if (serviceAccepted != null)
                {
                    serviceAccepted.Dispose();
                    _serviceAccepted = null;
                }

                var exceptionWaitHandle = _exceptionWaitHandle;
                if (exceptionWaitHandle != null)
                {
                    exceptionWaitHandle.Dispose();
                    _exceptionWaitHandle = null;
                }

                var keyExchangeCompletedWaitHandle = _keyExchangeCompletedWaitHandle;
                if (keyExchangeCompletedWaitHandle != null)
                {
                    keyExchangeCompletedWaitHandle.Dispose();
                    _keyExchangeCompletedWaitHandle = null;
                }

                var serverMac = _serverMac;
                if (serverMac != null)
                {
                    serverMac.Dispose();
                    _serverMac = null;
                }

                var clientMac = _clientMac;
                if (clientMac != null)
                {
                    clientMac.Dispose();
                    _clientMac = null;
                }

                var keyExchange = _keyExchange;
                if (keyExchange != null)
                {
                    keyExchange.HostKeyReceived -= KeyExchange_HostKeyReceived;
                    keyExchange.Dispose();
                    _keyExchange = null;
                }

                var messageListenerCompleted = _messageListenerCompleted;
                if (messageListenerCompleted != null)
                {
                    messageListenerCompleted.Dispose();
                    _messageListenerCompleted = null;
                }

                _disposed = true;
            }
        }

        /// <summary>
        /// Releases unmanaged resources and performs other cleanup operations before the
        /// <see cref="Session"/> is reclaimed by garbage collection.
        /// </summary>
        ~Session()
        {
            Dispose(false);
        }

#endregion IDisposable implementation

#region ISession implementation

        /// <summary>
        /// Gets or sets the connection info.
        /// </summary>
        /// <value>The connection info.</value>
        IConnectionInfo ISession.ConnectionInfo
        {
            get { return ConnectionInfo; }
        }

        WaitHandle ISession.MessageListenerCompleted
        {
            get { return _messageListenerCompleted; }
        }

        /// <summary>
        /// Create a new SSH session channel.
        /// </summary>
        /// <returns>
        /// A new SSH session channel.
        /// </returns>
        IChannelSession ISession.CreateChannelSession()
        {
            return new ChannelSession(this, NextChannelNumber, InitialLocalWindowSize, LocalChannelDataPacketSize);
        }

        /// <summary>
        /// Create a new channel for a locally forwarded TCP/IP port.
        /// </summary>
        /// <returns>
        /// A new channel for a locally forwarded TCP/IP port.
        /// </returns>
        IChannelDirectTcpip ISession.CreateChannelDirectTcpip()
        {
            return new ChannelDirectTcpip(this, NextChannelNumber, InitialLocalWindowSize, LocalChannelDataPacketSize);
        }

        /// <summary>
        /// Creates a "forwarded-tcpip" SSH channel.
        /// </summary>
        /// <returns>
        /// A new "forwarded-tcpip" SSH channel.
        /// </returns>
        IChannelForwardedTcpip ISession.CreateChannelForwardedTcpip(uint remoteChannelNumber, uint remoteWindowSize,
            uint remoteChannelDataPacketSize)
        {
            return new ChannelForwardedTcpip(this,
                                             NextChannelNumber,
                                             InitialLocalWindowSize,
                                             LocalChannelDataPacketSize,
                                             remoteChannelNumber,
                                             remoteWindowSize,
                                             remoteChannelDataPacketSize);
        }

        /// <summary>
        /// Sends a message to the server.
        /// </summary>
        /// <param name="message">The message to send.</param>
        /// <exception cref="SshConnectionException">The client is not connected.</exception>
        /// <exception cref="SshOperationTimeoutException">The operation timed out.</exception>
        /// <exception cref="InvalidOperationException">The size of the packet exceeds the maximum size defined by the protocol.</exception>
        void ISession.SendMessage(Message message)
        {
            SendMessage(message);
        }

        /// <summary>
        /// Sends a message to the server.
        /// </summary>
        /// <param name="message">The message to send.</param>
        /// <returns>
        /// <c>true</c> if the message was sent to the server; otherwise, <c>false</c>.
        /// </returns>
        /// <exception cref="InvalidOperationException">The size of the packet exceeds the maximum size defined by the protocol.</exception>
        /// <remarks>
        /// This methods returns <c>false</c> when the attempt to send the message results in a
        /// <see cref="SocketException"/> or a <see cref="SshException"/>.
        /// </remarks>
        bool ISession.TrySendMessage(Message message)
        {
            return TrySendMessage(message);
        }

#endregion ISession implementation
    }
}