Signal recently released a standalone app for Debian based Linux (meaning Tails and others), Windows and Mac.
For those unaware: Signal is a free, open source, end to end encryption application that is one of the more secure methods of A to B communication.
Open Whisper Systems (organization that makes Signal) itself cannot read your messages simply because you control the encryption keys (that is the keys are stored locally on the device you are using) when using Signal.
Go here: https://signal.org/download/
Edit: I forgot Signal requires a phone number to sign up. More secure alternatives include Ricochet, Bitmessage, Wickr, and Jabber
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