Signal

Signal is an end-to-end encrypted messenger run by a non-profit foundation, and its protocol is the reference implementation that several other apps — including WhatsApp — licensed for their own encryption.

What separates it is how little it stores: it holds effectively no metadata about who talks to whom, which is a stronger position than encryption alone. The app is open source and independently auditable.

The cost is convenience: fewer features, a smaller user base, and the need for the other person to install it too. It suits conversations where confidentiality genuinely matters rather than everyday chat.