The DHT system for identities is cool, but the thing that gets me is that they don't have support for SRTP with ZRTP, only SRTP with SDES. There's no perfect forward secrecy, and a bunch of other features that ZRTP has.
A more comparable thing to Signal is Matrix [1] and it's client Riot [2]. Theoretically, it has perfect forward secrecy. It's not as distributed as Ring is, but there is no centralized server as in Signal either.
Riot is also in F-Droid. So while not a GNU member, I find it sufficiently open and free.
A totally distributed approach with forward secrecy is Tox [3]. It needs a good code audit, though. And as in Ring, P2P causes data consumption issues in mobile.
the ID service in Matrix is optional, and should be called an 'ID mapping service', really. All it does is to map 3rd party IDs such as email addresses to decentralised matrix IDs. Currently it is a logically centralised service, and this is a major shortcoming: in future we intend to decentralise it as much as possible. This is Hard however, and we're kinda hoping that a decentralised equivalent to Keybase will come along and do the work for us :) Until then, you don't have to use the existing centralised thing if you don't like it :)
A more comparable thing to Signal is Matrix [1] and it's client Riot [2]. Theoretically, it has perfect forward secrecy. It's not as distributed as Ring is, but there is no centralized server as in Signal either.
Riot is also in F-Droid. So while not a GNU member, I find it sufficiently open and free.
A totally distributed approach with forward secrecy is Tox [3]. It needs a good code audit, though. And as in Ring, P2P causes data consumption issues in mobile.
[1] https://matrix.org/
[2] https://riot.im/
[3] https://tox.chat/