[dewey]

I'd suggest WhisperSystem's Signal [0], it's currently available on iOS and Android [1] and open source [2].

[0] https://firstlook.org/theintercept/2015/03/02/signal-iphones...

[1] https://whispersystems.org/blog/signal/

[2] https://github.com/WhisperSystems/Signal-iOS