Consider Signal
Created by: Schweineschwarte
What's with the open source programm „TextSecure“? It's an alternative for WhatsApp and it's opensource (GPLv3). It encrypt messages as standard and features forward secrecy and deniability guarantees.
Furthermore, it gives an audit about Textsecure from the Ruhr University Bochum with the result: "Furthermore, we formally prove that—if our mitigation is applied—TEXTSECURE’s push messaging can indeed achieve the goals of authenticity and confidentiality."
https://en.wikipedia.org/wiki/TextSecure https://github.com/WhisperSystems/TextSecure https://www.whispersystems.org/blog/advanced-ratcheting/ https://eprint.iacr.org/2014/904.pdf
Edited by Yegor Timoshenko