[mrbiber]

I'm surprised to see that matrix.org / riot.im is not mentioned as an alternative to Signal. They have a federated, open protocol, usable open-source apps for web/Android/iOS, registration without a phone number (even email is optional), do not require Google Play, and support double-ratchet encryption (in beta, it's not activated by default yet, but it will be activated by default in private rooms in the future).

[vertex-four]

Because, frankly, the UX around encryption in Matrix is dire, as well as being beta. I've never seen someone actually use the same keys for two conversations over as many days, due to switching devices, logging out of the web interface, etc.