Every Byte of a TLS Connection Explained and Reproduced

However, Wireshark can mislead you if you don't understand what you're looking at.

It's a bit like having a low-level debugger. If you're happy with low-level C, and you're looking at a debugger and it says variable 'k' which you know is a uint8_t which loops from 0 to 5, currently has the value 65 you should say to yourself. "Hmm, I bet that the compiler used the same place to store variable 'c' that's a single byte from a text string, so this is just the capital letter A and the compiler has realised it doesn't need to store the value of k even if it's technically in scope..." rather than "OMG my loop variable somehow massively exceeded its expected range, maybe cosmic rays have damaged the RAM".

With TLS for example if you give it a whole TLS 1.2 sesssion, Wireshark will say oh, this is TLS 1.2. Fine. But if you show it only a TLS 1.2 connection that failed, Wireshark will say "Oh, this is TLS 1.0". Why? Well, the low-level protocol has been bodged over the years because of crappy middleboxes, so Wireshark doesn't actually know for sure, and rather than say "I don't know yet, I need to see more of the connection" it says TLS 1.0

This can be a problem because you'll get amateurs saying "Our system can't talk to your server because you only do TLS 1.0" and you say "No. You are wrong" and they say "Look, here's a Wireshark trace" and sure enough Wireshark is telling them it's TLS 1.0 because their system has disconnected early (e.g. because they disabled all the crypto algorithms you allow), and so Wireshark wasn't sure and labels it TLS 1.0 rather than TLS 1.2

This is going to happen again with TLS 1.3. TLS 1.3 deliberately says "Hi I'm TLS 1.2" (middleboxes again) and so that's what Wireshark will report (until you get a newer version that knows to look inside the supported_versions extension field for the version) and so you can bet that amateurs are going to say "Your service only does TLS 1.2" when actually their connection failed for some reason and they don't understand how to read the Wireshark trace.

very fine work. the data flow and annotation are clear and concise. good job

Can you do this for DTLS, too? :=-)

Would you be interested in some articles about his? A lot of our infrastructure bases trust around self-signed CA chains and mutual TLS authentication in different configurations, mostly involving elastic stacks, vault, nomad, consul - and we're migrating a lot of the cert handling to be fully automated. We're also extending this to a number of our java applications as well.

I might be able to shed some light on there, as this topic isn't actually that hard. It just requires some very careful thinking.

This is the third request I've gotten for DTLS. Where are you weirdos coming from? :)