The premise of the article --- and it's a good premise --- is that the secure chat applications you should consider are those that implement Signal Protocol. Matrix does not.

correct; Matrix doesn't implement Signal Protocol. We use the Olm ratchet as a 1:1 ratchet which is an independent implementation of the Double Ratchet (aka the Ratchet formally known as Axolotl), and have added our own group ratchet called Megolm to handle the semantics required for Matrix.

In terms of how it compares to Signal: we are still in alpha (although Olm now ships on the develop branch of matrix-js-sdk and dependencies), and so obviously Signal is infinitely more mature. However, we are getting the core ratchet publicly audited during July before we push all the E2E formally live, which should give a better idea of how reputable to consider it.

do you have any resources for building a secure chat application, or maybe more accurately, implementing secure chat as a feature of an application?

That's not an undertaking that should be taken lightly. At minimum, you should have a team wherein one of the people making the final design decisions is a cryptography expert.

And I don't mean "I learned how to do textbook RSA in college", I mean more of, "Can tease a previously undiscovered cache-timing side-channel out of a crypto library". (For example, the recent libgcrypt advisories.)

If the words "padding oracle" or acronyms like AEAD sound strange and foreign, that person is not qualified to fill that role.

I would wager most developers lack the background to make a messaging service that is actually secure. (To anyone reading this: Please don't let this fact magnify any sense of impostor syndrome you may have. You're far from alone. Even the experts won't embark on this endeavor without peer review.)

to be clear, I was hoping to build it casually on top of a protocol or library. I would not be planning on rolling my own crypto or anything like that. I want a feature of my application to be encrypted chat which given the large availability or apps & libraries, I was hoping could be more of an integration than a build out.

In that case, the Signal protocol is open source.

https://github.com/WhisperSystems/libsignal-protocol-c

https://github.com/WhisperSystems/libsignal-protocol-java

More generally (i.e. not for messaging apps), libsodium is great for application-layer cryptography:

https://download.libsodium.org/doc/bindings_for_other_langua...

thanks, I did a cursory look a little while ago and have been thinking of signal. Thanks for the resources. Will check out.

Would also love to hear a point-by-point comparison of the two..

It looks like it uses Olm, which aspired to be an early implementation of the Signal protocol (when it was called Axolotl):

https://matrix.org/git/olm/about/

I've not audited their implementation, but at least that one design choice is sane.

IIRC there were some design changes between the early Axolotl ratchet and the Signal Protocol. If you're interested in using Olm, I would make "find out what the differences are and why" my first priority.

Ah ok I'm not necessarily so much interested in using it, but have been looking into the details of the signal ratchet recently, and was wondering what the specific changes / rationale..

If I had the time, I'd do a thorough analysis of the two and publish my findings.