added support for Tls1.3 protocol

Source/MQTTnet.Extensions.Wrappers.WebSocket4Net/WebSocket4NetMqttClientAdapterFactory.cs

@@ -52,7 +52,11 @@ namespace MQTTnet.TestApp.NetCore
                 if (channelOptions.TlsOptions.UseTls)
                 {
                     uri = "wss://" + channelOptions.Uri;
+#if NETCOREAPP3_0 || NET5_0
+                    sslProtocols = SslProtocols.Tls13;
+#else
                     sslProtocols = SslProtocols.Tls12;
+#endif
                 }
 
                 var subProtocol = channelOptions.SubProtocols.FirstOrDefault() ?? string.Empty;

Source/MQTTnet.Server/Mqtt/MqttServerService.cs

@@ -212,9 +212,15 @@ namespace MQTTnet.Server.Mqtt
             // Configure encrypted connections
             if (_settings.EncryptedTcpEndPoint.Enabled)
             {
+#if NETCOREAPP3_0 || NET5_0
+                options
+                    .WithEncryptedEndpoint()
+                    .WithEncryptionSslProtocol(SslProtocols.Tls13);
+#else
                 options
                     .WithEncryptedEndpoint()
                     .WithEncryptionSslProtocol(SslProtocols.Tls12);
+#endif
 
                 if (!string.IsNullOrEmpty(_settings.EncryptedTcpEndPoint?.Certificate?.Path))
                 {

Source/MQTTnet/Client/Options/MqttClientOptionsBuilderTlsParameters.cs

@@ -19,7 +19,11 @@ namespace MQTTnet.Client.Options
 
         public Func<MqttClientCertificateValidationCallbackContext, bool> CertificateValidationHandler { get; set; }
 
+#if NETCOREAPP3_0 || NET5_0
+        public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls13;
+#else
         public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12;
+#endif
 
 #if WINDOWS_UWP
         public IEnumerable<IEnumerable<byte>> Certificates { get; set; }

Source/MQTTnet/Client/Options/MqttClientTlsOptions.cs

@@ -26,7 +26,11 @@ namespace MQTTnet.Client.Options
         public List<SslApplicationProtocol> ApplicationProtocols { get; set; }
 #endif
 
+#if NETCOREAPP3_0 || NET5_0
+        public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls13;
+#else
         public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12;
+#endif
 
         [Obsolete("This property will be removed soon. Use CertificateValidationHandler instead.")]
         public Func<X509Certificate, X509Chain, SslPolicyErrors, IMqttClientOptions, bool> CertificateValidationCallback { get; set; }

Source/MQTTnet/Implementations/MqttTcpChannel.Uwp.cs

@@ -39,7 +39,11 @@ namespace MQTTnet.Implementations
 
             CreateStreams();
 
+#if NETCOREAPP3_0 || NET5_0
+            IsSecureConnection = socket.Information.ProtectionLevel >= SocketProtectionLevel.Tls13;
+#else
             IsSecureConnection = socket.Information.ProtectionLevel >= SocketProtectionLevel.Tls12;
+#endif
             ClientCertificate = clientCertificate;
 
             Endpoint = _socket.Information.RemoteAddress + ":" + _socket.Information.RemotePort;
@@ -75,11 +79,23 @@ namespace MQTTnet.Implementations
                     _socket.Control.IgnorableServerCertificateErrors.Add(ignorableChainValidationResult);
                 }
 
+#if NETCOREAPP3_0 || NET5_0
+                var socketProtectionLevel = SocketProtectionLevel.Tls13;
+                if (_options.TlsOptions.SslProtocol == SslProtocols.Tls12)
+                {
+                    socketProtectionLevel = SocketProtectionLevel.Tls12;
+                }
+                else if (_options.TlsOptions.SslProtocol == SslProtocols.Tls11)
+                {
+                    socketProtectionLevel = SocketProtectionLevel.Tls11;
+                }
+#else                
                 var socketProtectionLevel = SocketProtectionLevel.Tls12;
                 if (_options.TlsOptions.SslProtocol == SslProtocols.Tls11)
                 {
                     socketProtectionLevel = SocketProtectionLevel.Tls11;
                 }
+#endif
                 else if (_options.TlsOptions.SslProtocol == SslProtocols.Tls)
                 {
                     socketProtectionLevel = SocketProtectionLevel.Tls10;