Signal (software)

Signal is an encrypted messaging service for instant messaging, voice, and video calls. The instant messaging function includes sending text, voice notes, , videos, and . Communication may be one-to-one between users, or for group messaging. The application uses a centralized computing architecture, and is cross-platform software. It is developed by the non-profit 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 work without them. 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 as an identifier for users. It secures all communications with end-to-end encryption. The client software includes mechanisms by which users can independently verify the identity of their contacts and the integrity of the data channel. The non-profit Signal Foundation was launched in February 2018 with initial funding of $50 million from Brian Acton. , the platform had approximately 40 million monthly active users. , it was downloaded more than 105 million times. Until the feature was removed in 2023, the Android version was also optionally capable of functioning as an SMS app. 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.