Expose certificate selection in MQTTnet options

Source/MQTTnet/Client/Options/MqttClientCertificateSelectionEventArgs.cs

@@ -0,0 +1,36 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Security.Cryptography.X509Certificates;
+
+namespace MQTTnet.Client
+{
+    public sealed class MqttClientCertificateSelectionEventArgs : EventArgs
+    {
+        public MqttClientCertificateSelectionEventArgs(
+            string targetHost,
+            X509CertificateCollection localCertificates,
+            X509Certificate remoteCertificate,
+            string[] acceptableIssuers,
+            MqttClientTcpOptions tcpOptions)
+        {
+            TargetHost = targetHost;
+            LocalCertificates = localCertificates;
+            RemoveCertificate = remoteCertificate;
+            AcceptableIssuers = acceptableIssuers;
+            TcpOptions = tcpOptions ?? throw new ArgumentNullException(nameof(tcpOptions));
+        }
+
+        public string[] AcceptableIssuers { get; }
+
+        public X509CertificateCollection LocalCertificates { get; }
+
+        public X509Certificate RemoveCertificate { get; }
+
+        public string TargetHost { get; }
+
+        public MqttClientTcpOptions TcpOptions { get; }
+    }
+}

Source/MQTTnet/Client/Options/MqttClientCertificateValidationEventArgs.cs

@@ -10,11 +10,20 @@ namespace MQTTnet.Client
 {
     public sealed class MqttClientCertificateValidationEventArgs : EventArgs
     {
-        public X509Certificate Certificate { get; set; }
+        public MqttClientCertificateValidationEventArgs(X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors, IMqttClientChannelOptions clientOptions)
+        {
+            Certificate = certificate;
+            Chain = chain;
+            ClientOptions = clientOptions;
+            SslPolicyErrors = sslPolicyErrors;
+        }
 
-        public X509Chain Chain { get; set; }
+        public X509Certificate Certificate { get; }
+
+        public X509Chain Chain { get; }
+
+        public IMqttClientChannelOptions ClientOptions { get; }
 
-        public IMqttClientChannelOptions ClientOptions { get; set; }
 #if NET452 || NET461 || NET48
         /// <summary>
         ///     Can be a host string name or an object derived from WebRequest.
@@ -22,6 +31,6 @@ namespace MQTTnet.Client
         public object Sender { get; set; }
 #endif
 
-        public SslPolicyErrors SslPolicyErrors { get; set; }
+        public SslPolicyErrors SslPolicyErrors { get; }
     }
 }

Source/MQTTnet/Client/Options/MqttClientTlsOptions.cs

@@ -14,6 +14,8 @@ namespace MQTTnet.Client
     {
         public Func<MqttClientCertificateValidationEventArgs, bool> CertificateValidationHandler { get; set; }
 
+        public Func<MqttClientCertificateSelectionEventArgs, X509Certificate> CertificateSelectionHandler { get; set; }
+
         public bool UseTls { get; set; }
 
         public bool IgnoreCertificateRevocationErrors { get; set; }

Source/MQTTnet/Client/Options/MqttClientTlsOptionsBuilder.cs

@@ -49,6 +49,17 @@ namespace MQTTnet.Client
             return this;
         }
 
+        public MqttClientTlsOptionsBuilder WithCertificateSelectionHandler(Func<MqttClientCertificateSelectionEventArgs, X509Certificate> certificateSelectionHandler)
+        {
+            if (certificateSelectionHandler == null)
+            {
+                throw new ArgumentNullException(nameof(certificateSelectionHandler));
+            }
+
+            _tlsOptions.CertificateSelectionHandler = certificateSelectionHandler;
+            return this;
+        }
+
         public MqttClientTlsOptionsBuilder WithClientCertificates(IEnumerable<X509Certificate2> certificates)
         {
             if (certificates == null)

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -106,10 +106,29 @@ namespace MQTTnet.Implementations
                     {
                         if (_tcpOptions.RemoteEndpoint is DnsEndPoint dns)
                         {
-                            targetHost = dns.Host;    
+                            targetHost = dns.Host;
                         }
                     }
-                    var sslStream = new SslStream(networkStream, false, InternalUserCertificateValidationCallback);
+
+                    SslStream sslStream;
+                    if (_tcpOptions.TlsOptions.CertificateSelectionHandler != null)
+                    {
+                        sslStream = new SslStream(
+                            networkStream,
+                            false,
+                            InternalUserCertificateValidationCallback,
+                            InternalUserCertificateSelectionCallback);
+                    }
+                    else
+                    {
+                        // Use a different constructor depending on the options for MQTTnet so that we do not have
+                        // to copy the exact same behavior of the selection handler.
+                        sslStream = new SslStream(
+                            networkStream,
+                            false,
+                            InternalUserCertificateValidationCallback);
+                    }
+
                     try
                     {
 #if NETCOREAPP3_1_OR_GREATER
@@ -134,9 +153,7 @@ namespace MQTTnet.Implementations
                             {
                                 TrustMode = X509ChainTrustMode.CustomRootTrust,
                                 VerificationFlags = X509VerificationFlags.IgnoreEndRevocationUnknown,
-                                RevocationMode = _tcpOptions.TlsOptions.IgnoreCertificateRevocationErrors
-                                    ? X509RevocationMode.NoCheck
-                                    : _tcpOptions.TlsOptions.RevocationMode
+                                RevocationMode = _tcpOptions.TlsOptions.IgnoreCertificateRevocationErrors ? X509RevocationMode.NoCheck : _tcpOptions.TlsOptions.RevocationMode
                             };
 
                             sslOptions.CertificateChainPolicy.CustomTrustStore.AddRange(_tcpOptions.TlsOptions.TrustChain);
@@ -292,19 +309,34 @@ namespace MQTTnet.Implementations
             }
         }
 
+        X509Certificate InternalUserCertificateSelectionCallback(
+            object sender,
+            string targetHost,
+            X509CertificateCollection localCertificates,
+            X509Certificate remoteCertificate,
+            string[] acceptableIssuers)
+        {
+            var certificateSelectionHandler = _tcpOptions?.TlsOptions?.CertificateSelectionHandler;
+            if (certificateSelectionHandler != null)
+            {
+                var eventArgs = new MqttClientCertificateSelectionEventArgs(targetHost, localCertificates, remoteCertificate, acceptableIssuers, _tcpOptions);
+                return certificateSelectionHandler(eventArgs);
+            }
+
+            if (localCertificates?.Count > 0)
+            {
+                return localCertificates[0];
+            }
+
+            return null;
+        }
+
         bool InternalUserCertificateValidationCallback(object sender, X509Certificate x509Certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
         {
             var certificateValidationHandler = _tcpOptions?.TlsOptions?.CertificateValidationHandler;
             if (certificateValidationHandler != null)
             {
-                var eventArgs = new MqttClientCertificateValidationEventArgs
-                {
-                    Certificate = x509Certificate,
-                    Chain = chain,
-                    SslPolicyErrors = sslPolicyErrors,
-                    ClientOptions = _tcpOptions
-                };
-
+                var eventArgs = new MqttClientCertificateValidationEventArgs(x509Certificate, chain, sslPolicyErrors, _tcpOptions);
                 return certificateValidationHandler(eventArgs);
             }
 

Source/MQTTnet/Implementations/MqttWebSocketChannel.cs

@@ -109,7 +109,7 @@ namespace MQTTnet.Implementations
             // A single WebSocket data frame can contain multiple or partial MQTT Control Packets. The receiver MUST NOT assume that MQTT Control Packets are aligned on WebSocket frame boundaries [MQTT-6.0.0-2].
             await _webSocket.SendAsync(buffer, WebSocketMessageType.Binary, isEndOfPacket, cancellationToken).ConfigureAwait(false);
 #else
-            // The lock is required because the client will throw an exception if _SendAsync_ is 
+            // The lock is required because the client will throw an exception if _SendAsync_ is
             // called from multiple threads at the same time. But this issue only happens with several
             // framework versions.
             if (_sendLock == null)
@@ -156,7 +156,7 @@ namespace MQTTnet.Implementations
 #else
             var proxyUri = new Uri(_options.ProxyOptions.Address);
             WebProxy webProxy;
-            
+
             if (!string.IsNullOrEmpty(_options.ProxyOptions.Username) && !string.IsNullOrEmpty(_options.ProxyOptions.Password))
             {
                 var credentials = new NetworkCredential(_options.ProxyOptions.Username, _options.ProxyOptions.Password, _options.ProxyOptions.Domain);
@@ -164,9 +164,9 @@ namespace MQTTnet.Implementations
             }
             else
             {
-                webProxy = new WebProxy(proxyUri, _options.ProxyOptions.BypassOnLocal, _options.ProxyOptions.BypassList);    
+                webProxy = new WebProxy(proxyUri, _options.ProxyOptions.BypassOnLocal, _options.ProxyOptions.BypassList);
             }
-            
+
             if (_options.ProxyOptions.UseDefaultCredentials)
             {
                 // Only update the property if required because setting it to false will alter
@@ -234,7 +234,7 @@ namespace MQTTnet.Implementations
             {
                 clientWebSocket.Options.Credentials = _options.Credentials;
             }
-            
+
             var certificateValidationHandler = _options.TlsOptions?.CertificateValidationHandler;
             if (certificateValidationHandler != null)
             {
@@ -245,7 +245,7 @@ namespace MQTTnet.Implementations
 #elif WINDOWS_UWP
                 throw new NotSupportedException("Remote certificate validation callback is not supported when using 'uap10.0'.");
 #elif NET452 || NET461 || NET48
-                ServicePointManager.ServerCertificateValidationCallback += (sender, certificate, chain, sslPolicyErrors) => 
+                ServicePointManager.ServerCertificateValidationCallback += (sender, certificate, chain, sslPolicyErrors) =>
                 {
                     var context = new MqttClientCertificateValidationEventArgs
                     {
@@ -262,17 +262,16 @@ namespace MQTTnet.Implementations
                 clientWebSocket.Options.RemoteCertificateValidationCallback = (sender, certificate, chain, sslPolicyErrors) =>
                 {
                     // TODO: Find a way to add client options to same callback. Problem is that they have a different type.
-                    var context = new MqttClientCertificateValidationEventArgs
-                    {
-                        Certificate = certificate,
-                        Chain = chain,
-                        SslPolicyErrors = sslPolicyErrors,
-                        ClientOptions = _options
-                    };
-
+                    var context = new MqttClientCertificateValidationEventArgs(certificate, chain, sslPolicyErrors, _options);
                     return certificateValidationHandler(context);
                 };
 #endif
+
+                var certificateSelectionHandler = _options.TlsOptions?.CertificateSelectionHandler;
+                if (certificateSelectionHandler != null)
+                {
+                    throw new NotSupportedException("Remote certificate selection callback is not supported for WebSocket connections.");
+                }
             }
         }
     }