You might check out Conversations too (https://conversations.im/), it's a federated XMPP (with some really clever extensions) based approach that uses the signal protocol.
https://github.com/WhisperSystems/Signal-Server
It's sure become fashionable to hate on Signal/OWS. The price of actually successfully bringing good encryption mainstream?
Edit: The comment I replied to originally included a question on whether Signals server is even open source.
Sorry about that, I had looked it up just a second after I posted and then removed that line.
> hate on Signal/OWS
I don't mean to hate on them, just their stance on getting away from Google, using F-Droid, federating their service and many other things has been more than a bit of a mess or disappointment. I'm glad they finally made a step in the right direction and I hope this will continue.
As I said in another reply, I can get a completely non-technical user to start using Signal with me in a minute, and have them have an experience that is as accessible as WhatsApp. That's simply not true for any of the other options I'm aware of (and it relates directly to most of the points you raise, with the notable exception of F-Droid. There I also find their reasoning weakest).
And that "no more" is really important. "Just slightly more" won't do. Especially not if it's "slightly more to someone who frequents HN". Because that's likely already prohibitive to most.
Take a person that just barely knows how to operate the play store. I can instruct that person over phone how to start chatting with me securely in a minute or two:
Go to the play store, download signal, open signal, I'm already there in your contact list. Write me a message. Done.
That's an amazing achievement. The much maligned fact that phone numbers are used as identifiers is key to that experience, too, because my phone number already is on that persons phone.
And yeah, I can't verify Whatsapp, but I still trust that Moxie et.al. have checked their implementation. Still that's why I push people to switch to Signal rather than stay on WhatsApp. Doubly so as WhatsApp belongs to Facebook now (I loved their original 1 Dollar a year business model, and if they still had that I would trust them a lot more).
Even with doubts about Facebook, it almost certainly is a massive win that WhatsApp implemented this. It makes WhatsApp immune to being subpoenaed for conversations, and thus they have a clear motive to implement it properly, too.
I wish there would be more devs building modern clients for XMPP instead of building the 100th communication system.