Implement OpenSSH strict key exchange extension

src/Renci.SshNet/Session.cs

@@ -154,6 +154,17 @@ public class Session : ISession
         /// </summary>
         private bool _isDisconnecting;
 
+        /// <summary>
+        /// Indicates whether it is the init kex.
+        /// </summary>
+        private bool _isInitialKex;
+
+        /// <summary>
+        /// Indicates whether server supports strict key exchange.
+        /// <see href="https://github.com/openssh/openssh-portable/blob/master/PROTOCOL"/> 1.10.
+        /// </summary>
+        private bool _isStrictKex;
+
         private IKeyExchange _keyExchange;
 
         private HashAlgorithm _serverMac;
@@ -281,35 +292,11 @@ public bool IsConnected
         /// </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
-            {
-                _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;
-            }
-        }
+        public Message ClientInitMessage { get; private set; }
 
         /// <summary>
         /// Gets the server version string.
@@ -617,6 +604,8 @@ public void Connect()
                 // Send our key exchange init.
                 // We need to do this before starting the message listener to avoid the case where we receive the server
                 // key exchange init and we continue the key exchange before having sent our own init.
+                _isInitialKex = true;
+                ClientInitMessage = BuildClientInitMessage(includeStrictKexPseudoAlgorithm: true);
                 SendMessage(ClientInitMessage);
 
                 // Mark the message listener threads as started
@@ -741,6 +730,8 @@ public async Task ConnectAsync(CancellationToken cancellationToken)
                 // Send our key exchange init.
                 // We need to do this before starting the message listener to avoid the case where we receive the server
                 // key exchange init and we continue the key exchange before having sent our own init.
+                _isInitialKex = true;
+                ClientInitMessage = BuildClientInitMessage(includeStrictKexPseudoAlgorithm: true);
                 SendMessage(ClientInitMessage);
 
                 // Mark the message listener threads as started
@@ -1107,13 +1098,20 @@ internal void SendMessage(Message message)
                     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++;
+                if (_isStrictKex && message is NewKeysMessage)
+                {
+                    _outboundPacketSequence = 0;
+                }
+                else
+                {
+                    // 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++;
+                }
             }
         }
 
@@ -1344,6 +1342,13 @@ private Message ReceiveMessage(Socket socket)
 
             _inboundPacketSequence++;
 
+            // The below code mirrors from https://github.com/openssh/openssh-portable/commit/1edb00c58f8a6875fad6a497aa2bacf37f9e6cd5
+            // It ensures the integrity of key exchange process.
+            if (_inboundPacketSequence == uint.MaxValue && _isInitialKex)
+            {
+                throw new SshConnectionException("Inbound packet sequence number is about to wrap during initial key exchange.", DisconnectReason.KeyExchangeFailed);
+            }
+
             return LoadMessage(data, messagePayloadOffset, messagePayloadLength);
         }
 
@@ -1455,8 +1460,20 @@ internal void OnKeyExchangeInitReceived(KeyExchangeInitMessage message)
 
             _keyExchangeCompletedWaitHandle.Reset();
 
+            if (_isInitialKex && message.KeyExchangeAlgorithms.Contains("kex-strict-s-v00@openssh.com"))
+            {
+                _isStrictKex = true;
+
+                DiagnosticAbstraction.Log(string.Format("[{0}] Enabling strict key exchange extension.", ToHex(SessionId)));
+
+                if (_inboundPacketSequence != 1)
+                {
+                    throw new SshConnectionException("KEXINIT was not the first packet during strict key exchange.", DisconnectReason.KeyExchangeFailed);
+                }
+            }
+
             // Disable messages that are not key exchange related
-            _sshMessageFactory.DisableNonKeyExchangeMessages();
+            _sshMessageFactory.DisableNonKeyExchangeMessages(_isStrictKex);
 
             _keyExchange = _serviceFactory.CreateKeyExchange(ConnectionInfo.KeyExchangeAlgorithms,
                                                              message.KeyExchangeAlgorithms);
@@ -1533,6 +1550,17 @@ internal void OnNewKeysReceived(NewKeysMessage message)
             // Enable activated messages that are not key exchange related
             _sshMessageFactory.EnableActivatedMessages();
 
+            if (_isInitialKex)
+            {
+                _isInitialKex = false;
+                ClientInitMessage = BuildClientInitMessage(includeStrictKexPseudoAlgorithm: false);
+            }
+
+            if (_isStrictKex)
+            {
+                _inboundPacketSequence = 0;
+            }
+
             NewKeysReceived?.Invoke(this, new MessageEventArgs<NewKeysMessage>(message));
 
             // Signal that key exchange completed
@@ -2067,7 +2095,28 @@ private void Reset()
         private static SshConnectionException CreateConnectionAbortedByServerException()
         {
             return new SshConnectionException("An established connection was aborted by the server.",
-                                              DisconnectReason.ConnectionLost);
+            DisconnectReason.ConnectionLost);
+        }
+
+        private KeyExchangeInitMessage BuildClientInitMessage(bool includeStrictKexPseudoAlgorithm)
+        {
+            return new KeyExchangeInitMessage
+            {
+                KeyExchangeAlgorithms = includeStrictKexPseudoAlgorithm ?
+                                        ConnectionInfo.KeyExchangeAlgorithms.Keys.Concat(["kex-strict-c-v00@openssh.com"]).ToArray() :
+                                        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,
+            };
         }
 
         private bool _disposed;

src/Renci.SshNet/SshMessageFactory.cs

@@ -115,16 +115,41 @@ public Message Create(byte messageNumber)
             return enabledMessageMetadata.Create();
         }
 
-        public void DisableNonKeyExchangeMessages()
+        /// <summary>
+        /// Disables non-KeyExchange messages.
+        /// </summary>
+        /// <param name="strict">
+        /// <see langword="true"/> to indicate the strict key exchange mode; otherwise <see langword="false"/>.
+        /// <para>In strict key exchange mode, only below messages are allowed:</para>
+        /// <list type="bullet">
+        /// <item>SSH_MSG_KEXINIT -> 20</item>
+        /// <item>SSH_MSG_NEWKEYS -> 21</item>
+        /// <item>SSH_MSG_DISCONNECT -> 1</item>
+        /// </list>
+        /// <para>Note:</para>
+        /// <para>  The relevant KEX Reply MSG will be allowed from a sub class of KeyExchange class.</para>
+        /// <para>  For example, it calls <c>Session.RegisterMessage("SSH_MSG_KEX_ECDH_REPLY");</c> if the curve25519-sha256 KEX algorithm is selected per negotiation.</para>
+        /// </param>
+        public void DisableNonKeyExchangeMessages(bool strict)
         {
             for (var i = 0; i < AllMessages.Length; i++)
             {
                 var messageMetadata = AllMessages[i];
 
                 var messageNumber = messageMetadata.Number;
-                if (messageNumber is (> 2 and < 20) or > 30)
+                if (strict)
+                {
+                    if (messageNumber is not 20 and not 21 and not 1)
+                    {
+                        _enabledMessagesByNumber[messageNumber] = null;
+                    }
+                }
+                else
                 {
-                    _enabledMessagesByNumber[messageNumber] = null;
+                    if (messageNumber is (> 2 and < 20) or > 30)
+                    {
+                        _enabledMessagesByNumber[messageNumber] = null;
+                    }
                 }
             }
         }

test/Renci.SshNet.Tests/Classes/SessionTest_ConnectToServerFails.cs

@@ -87,7 +87,7 @@ public void IsConnectedShouldReturnFalse()
         }
 
         [TestMethod]
-        public void SendMessageShouldThrowShhConnectionException()
+        public void SendMessageShouldThrowSshConnectionException()
         {
             try
             {
@@ -189,7 +189,7 @@ public void ISession_MessageListenerCompletedShouldBeSignaled()
         }
 
         [TestMethod]
-        public void ISession_SendMessageShouldThrowShhConnectionException()
+        public void ISession_SendMessageShouldThrowSshConnectionException()
         {
             var session = (ISession)_session;
 

test/Renci.SshNet.Tests/Classes/SessionTest_Connected.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Linq;
 using System.Threading;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using Moq;
@@ -30,6 +31,31 @@ public void ClientVersionIsRenciSshNet()
             Assert.AreEqual("SSH-2.0-Renci.SshNet.SshClient.0.0.1", Session.ClientVersion);
         }
 
+        [TestMethod]
+        public void IncludeStrictKexPseudoAlgorithmInInitKex()
+        {
+            Assert.IsTrue(ServerBytesReceivedRegister.Count > 0);
+
+            var kexInitMessage = new KeyExchangeInitMessage();
+            kexInitMessage.Load(ServerBytesReceivedRegister[0], 4 + 1 + 1, ServerBytesReceivedRegister[0].Length - 4 - 1 - 1);
+            Assert.IsTrue(kexInitMessage.KeyExchangeAlgorithms.Contains("kex-strict-c-v00@openssh.com"));
+        }
+
+        [TestMethod]
+        public void ShouldNotIncludeStrictKexPseudoAlgorithmInSubsequentKex()
+        {
+            ServerBytesReceivedRegister.Clear();
+            Session.SendMessage(Session.ClientInitMessage);
+
+            Thread.Sleep(100);
+
+            Assert.IsTrue(ServerBytesReceivedRegister.Count > 0);
+
+            var kexInitMessage = new KeyExchangeInitMessage();
+            kexInitMessage.Load(ServerBytesReceivedRegister[0], 4 + 1 + 1, ServerBytesReceivedRegister[0].Length - 4 - 1 - 1);
+            Assert.IsFalse(kexInitMessage.KeyExchangeAlgorithms.Contains("kex-strict-c-v00@openssh.com"));
+        }
+
         [TestMethod]
         public void ConnectionInfoShouldReturnConnectionInfoPassedThroughConstructor()
         {

test/Renci.SshNet.Tests/Classes/SessionTest_ConnectedBase.cs

@@ -46,8 +46,7 @@ public abstract class SessionTest_ConnectedBase
         protected Session Session { get; private set; }
         protected Socket ClientSocket { get; private set; }
         protected Socket ServerSocket { get; private set; }
-        internal SshIdentification ServerIdentification { get; set; }
-        protected bool CallSessionConnectWhenArrange { get; set; }
+        protected SshIdentification ServerIdentification { get; private set; }
 
         /// <summary>
         /// Should the "server" wait for the client kexinit before sending its own.
@@ -163,8 +162,6 @@ protected virtual void SetupData()
 
             ClientSocket = new DirectConnector(_socketFactory).Connect(ConnectionInfo);
 
-            CallSessionConnectWhenArrange = true;
-
             void SendKeyExchangeInit()
             {
                 var keyExchangeInitMessage = new KeyExchangeInitMessage
@@ -204,7 +201,7 @@ private void SetupMocks()
             _ = ServiceFactoryMock.Setup(p => p.CreateProtocolVersionExchange())
                                   .Returns(_protocolVersionExchangeMock.Object);
             _ = _protocolVersionExchangeMock.Setup(p => p.Start(Session.ClientVersion, ClientSocket, ConnectionInfo.Timeout))
-                                            .Returns(() => ServerIdentification);
+                                            .Returns(ServerIdentification);
             _ = ServiceFactoryMock.Setup(p => p.CreateKeyExchange(ConnectionInfo.KeyExchangeAlgorithms, new[] { _keyExchangeAlgorithm })).Returns(_keyExchangeMock.Object);
             _ = _keyExchangeMock.Setup(p => p.Name)
                                 .Returns(_keyExchangeAlgorithm);
@@ -252,10 +249,7 @@ protected void Arrange()
             SetupData();
             SetupMocks();
 
-            if (CallSessionConnectWhenArrange)
-            {
-                Session.Connect();
-            }
+            Session.Connect();
         }
 
         protected virtual void ClientAuthentication_Callback()