I saw Elon Musk tweet about this awhile back. Looks like Snowden uses it as well.
signal.org
Signal allows users to send texts, videos, audio or picture messages with end-to-end encryption, just as they would via a normal text message. "Signal’s advanced privacy-preserving technology is always enabled, so you can focus on sharing the moments that matter with the people who matter to you," the app writes in its description.

Signal Messenger: Speak Freely
Say "hello" to a different messaging experience. An unexpected focus on privacy, combined with all of the features you expect.

