Signal
An end-to-end encrypted messaging app.
Signal sends end-to-end encrypted messages and voice/video calls between mobile, desktop, and web clients. The Signal Protocol it pioneered is also used by WhatsApp, Google Messages RCS, and Skype, which means a substantial fraction of the world's day-to-day messaging now travels over a protocol Signal designed.
Moxie Marlinspike and Trevor Perrin developed the protocol; Open Whisper Systems was the original company behind Signal, and the Signal Foundation has run the messaging service as a US-based 501(c)(3) since 2018. Signal's funding model is unusual — donations and a $50 million loan from Brian Acton (WhatsApp co-founder) — which the foundation argues lets it keep user privacy as the singular goal.
Signal is widely used by journalists, activists, lawyers, and people in countries with intrusive surveillance. The desktop clients, mobile apps, and CLI all share the same protocol; the server is open source under AGPL-3.0, though running an independent Signal server is impractical because Signal Messenger LLC operates the centralised user-discovery infrastructure.
License: AGPL-3.0-or-later (server) / GPL-3.0-only (clients)
Category: Communication
Website: https://signal.org/
Install
Mobile: App Store or Google Play Desktop: Debian/Ubuntu: see https://signal.org/download/linux/ Flatpak: flatpak install flathub org.signal.Signal macOS: brew install --cask signal
Authors
- Moxie Marlinspike, Trevor Perrin (creators)
- Signal Foundation