Signal (software)

Signal is an open-source, encrypted messaging service for instant messaging, voice calls, and video calls. The instant messaging function includes sending text, voice notes, images, videos, and other files. Communication may be one-to-one between users or may involve group messaging.

The application uses a centralized computing architecture and is cross-platform software. It is developed by the nonprofit Signal Foundation and its subsidiary Signal Messenger LLC. Signal's software is free and open-source. Its mobile clients, desktop client, and server are all published under the AGPL-3.0-only license. The official Android app generally uses the proprietary Google Play Services, although it is designed to be able to work without them. Signal is available on Aptoide. Signal is also distributed for iOS and desktop programs for Windows, macOS, and Linux. Registration for desktop use requires an iOS or Android device.

Signal uses mobile telephone numbers to register and manage user accounts, though configurable usernames were added in March 2024 to allow users to hide their phone numbers from other users. After removing support for SMS on Android in 2023, the app now secures all communications with end-to-end encryption. The client software includes mechanisms by which users can independently verify their contacts' identities and the data channel's integrity.

The nonprofit Signal Foundation was launched in February 2018 with initial funding of $50 million from WhatsApp co-founder Brian Acton. As of January 2025^[update], the platform had approximately 70 million monthly active users. As of January 2025^[update], it had been downloaded more than 220 million times.

Signal is the successor of the RedPhone encrypted voice calling app and the TextSecure encrypted texting program. The beta versions of RedPhone and TextSecure were first launched in May 2010 by Whisper Systems, a startup company co-founded by security researcher Moxie Marlinspike and roboticist Stuart Anderson. Whisper Systems also produced a firewall and tools for encrypting other forms of data. All of these were proprietary enterprise mobile security software and were only available for Android.

In November 2011, Whisper Systems announced that it had been acquired by Twitter. Neither company disclosed the financial terms of the deal. The acquisition was done "primarily so that Mr. Marlinspike could help the then-startup improve its security". Shortly after the acquisition, Whisper Systems' RedPhone service was made unavailable. Some criticized the removal, arguing that the software was "specifically targeted [to help] people under repressive regimes" and that it left people like the Egyptians in "a dangerous position" during the events of the Egyptian revolution of 2011.

Twitter released TextSecure as free and open-source software under the GPLv3 license in December 2011. RedPhone was also released under the same license in July 2012. Marlinspike later left Twitter and founded Open Whisper Systems as a collaborative Open Source project for the continued development of TextSecure and RedPhone.

Open Whisper Systems' website was launched in January 2013.