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/

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-encrypted-messaging-app-now-supports-text/ https://firstlook.org/theintercept/2015/03/02/signal-iphones... [1] https://whispersystems.org/blog/signal/ https://whispersystems.org/blog/signal/ [2] https://github.com/WhisperSystems/Signal-iOS https://github.com/WhisperSystems/Signal-iOS