Signal (software)

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Signal is a free, open-source messaging app that focuses on privacy. It lets you send text messages, make voice and video calls, and have group chats, all with end-to-end encryption by default. The app is developed by the Signal Foundation and its subsidiary Signal Messenger LLC, and it runs on Android, iOS, Windows, macOS, and Linux. The software is released under the AGPL-3.0-only license and is funded mainly through donations.

Key ideas you should know:
- Privacy first: Signal uses the Signal Protocol to keep messages and calls private. It combines strong cryptography with features like forward secrecy and contact verification (via fingerprints or QR codes) to confirm who you’re talking to.
- Phone-number-based sign-up with options: You sign up with a phone number (which can be a VoIP number or landline). In 2024, Signal added public usernames so you can message someone without sharing your phone number.
- What’s encrypted: One-to-one chats, group chats, voice calls, and video calls are end-to-end encrypted. Up until 2023 Android users could choose to send SMS/MMS in addition to Signal messages, but that option was removed in 2023.
- Security features: You can enable screen lock, set message timers to auto-delete, and blur faces in photos. Message backups can be kept encrypted, and Signal emphasizes that its servers don’t store who called whom.
- Open-source and audits: The Signal client apps (Android, iOS, desktop) are open source. The server software is mostly open, though some anti-spam components are closed source. The project has undergone independent security reviews and endorsements from privacy advocates.

Brief history:
- 2010–2011: TextSecure and RedPhone emerge from Whisper Systems and later get acquired by Twitter.
- 2013–2014: Open Whisper Systems continues development; Signal protocol evolves to support encrypted group chats.
- 2014: Signal launches as a combined app for encrypted calling and messaging.
- 2015–2016: TextSecure and RedPhone are merged into Signal; Signal Desktop debuts as a Chrome app and later becomes a standalone Electron app.
- 2018: The Signal Foundation is founded with $50 million from Brian Acton to support ongoing privacy-focused development.
- 2019–2020s: Signal adds features like iPad support, view-once media, stickers, and more. It gains prominence during protests and political events for its strong privacy.
- 2021–2025: Signal sees waves of new users and occasional outages during traffic surges; leadership changes occur; the platform expands to more regions and devices.

Notable points:
- Signal’s influence is broad: many journalists, NGOs, and even some government groups use it for private communication.
- It has faced censorship and blocking in several countries at different times, and it has adapted with changes in hosting and delivery methods.
- The project is funded by donations and is run by a nonprofit foundation, with the goal of keeping private communication accessible and widespread.

In short, Signal is a privacy-focused, open-source messaging platform that provides secure, encrypted communications across devices and platforms, backed by a nonprofit foundation and a strong commitment to user control and transparency.