Fix encoding and decoding of AUTH paket according to RFC. (#2076)

.github/workflows/ReleaseNotes.md

@@ -1,3 +1,4 @@
+* Core: Fixed issue when parsing AUTH packet with 0 length body (#2039).
 * nuget: Changed code signing and nuget certificate (**BREAKING CHANGE**).
 * TopicTemplates: Updated samples, parameter validation (#2022).
 * ManagedClient: Switch SubscribeAsync/UnsubscribeAsync to IEnumerable<string> (#2026).

Source/MQTTnet/Formatter/V5/MqttV5PacketDecoder.cs

@@ -68,12 +68,12 @@ namespace MQTTnet.Formatter.V5
 
         MqttPacket DecodeAuthPacket(ArraySegment<byte> body)
         {
-            ThrowIfBodyIsEmpty(body);
-
             _bufferReader.SetBuffer(body.Array, body.Offset, body.Count);
 
             var packet = new MqttAuthPacket();
 
+            // MQTT spec: The Reason Code and Property Length can be omitted if the Reason Code is 0x00 (Success) and there are no Properties.
+            // In this case the AUTH has a Remaining Length of 0.
             if (_bufferReader.EndOfStream)
             {
                 packet.ReasonCode = MqttAuthenticateReasonCode.Success;

Source/MQTTnet/Formatter/V5/MqttV5PacketEncoder.cs

@@ -13,7 +13,7 @@ namespace MQTTnet.Formatter.V5
     public sealed class MqttV5PacketEncoder
     {
         const int FixedHeaderSize = 1;
-        
+
         readonly MqttBufferWriter _bufferWriter;
         readonly MqttV5PropertiesWriter _propertiesWriter = new MqttV5PropertiesWriter(new MqttBufferWriter(1024, 4096));
 
@@ -64,13 +64,20 @@ namespace MQTTnet.Formatter.V5
 
         byte EncodeAuthPacket(MqttAuthPacket packet)
         {
-            _bufferWriter.WriteByte((byte)packet.ReasonCode);
-
             _propertiesWriter.WriteAuthenticationMethod(packet.AuthenticationMethod);
             _propertiesWriter.WriteAuthenticationData(packet.AuthenticationData);
             _propertiesWriter.WriteReasonString(packet.ReasonString);
             _propertiesWriter.WriteUserProperties(packet.UserProperties);
 
+            // MQTT spec: The Reason Code and Property Length can be omitted if the Reason Code is 0x00 (Success) and there are no Properties.
+            // In this case the AUTH has a Remaining Length of 0.
+            if (packet.ReasonCode == MqttAuthenticateReasonCode.Success && _propertiesWriter.Length == 0)
+            {
+                return MqttBufferWriter.BuildFixedHeader(MqttControlPacketType.Auth);
+            }
+
+            _bufferWriter.WriteByte((byte)packet.ReasonCode);
+
             _propertiesWriter.WriteTo(_bufferWriter);
             _propertiesWriter.Reset();
 
@@ -526,7 +533,7 @@ namespace MQTTnet.Formatter.V5
 
         static void ThrowIfPacketIdentifierIsInvalid(ushort packetIdentifier, MqttPacket packet)
         {
-            // SUBSCRIBE, UNSUBSCRIBE, and PUBLISH(in cases where QoS > 0) Control Packets MUST contain a non-zero 16 - bit Packet Identifier[MQTT - 2.3.1 - 1]. 
+            // SUBSCRIBE, UNSUBSCRIBE, and PUBLISH(in cases where QoS > 0) Control Packets MUST contain a non-zero 16 - bit Packet Identifier[MQTT - 2.3.1 - 1].
 
             if (packetIdentifier == 0)
             {