Messaging apps that perform [https://en.wikipedia.org/wiki/End-to-end_encryption end-to-end encryption] and publish their methods and source code in the public domain are considered here as private messaging tools. You can see a review of multiple apps on the [https://www.eff.org/secure-messaging-scorecard EFF's secure messaging scorecard].
*Signal for [https://ssd.eff.org/en/module/how-use-signal-android Android ] and [https://ssd.eff.org/en/module/how-use-signal-ios iPhone ] from [https://whispersystems.org/ WhisperSystem]
*[https://telegram.org Telegram] for all smartphone and desktop platforms
*[https://www.surespot.me SureSpot] for Android and iPhone