Signal– Secure Instant Messaging & Voice State-of-the-art end-to-end encryption (powered by the open source Signal Protocol) keeps your conversations secure. Signal is in the process of releasing their next version that will support non-phone number based accounts.
Install Signal
https://signal.org/download/
Signal Article
https://medium.com/@mshelton/locking-down-signal-d71678f653d3
https://www.wired.com/story/signal-tips-private-messaging-encryption