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Signal (software)


Signal is a cross-platform encrypted messaging service developed by the Signal Foundation and Signal Messenger LLC. It uses the Internet to send one-to-one and group messages, which can include files, voice notes, images and videos. Its mobile apps can also make one-to-one voice and video calls, and the Android version can optionally function 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 February 2014, Open Whisper Systems introduced the second version of their TextSecure Protocol (now Signal Protocol), which added end-to-end encrypted group chat and instant messaging capabilities to TextSecure. Toward the end of July 2014, they announced plans to unify the RedPhone and TextSecure applications as Signal. This announcement coincided with the initial release of Signal as a RedPhone counterpart for iOS. The developers said that their next steps would be to provide TextSecure instant messaging capabilities for iOS, unify the RedPhone and TextSecure applications on Android, and launch a web client. Signal was the first iOS app to enable easy, strongly encrypted voice calls for free. TextSecure compatibility was added to the iOS application in March 2015.

TextSecure allowed the user to set a passphrase that encrypted the local message database and the user's encryption keys. This did not encrypt the user's contact database or message timestamps. The Signal applications on Android and iOS can be locked with the phone's pin, passphrase, or biometric authentication. The user can define a "screen lock timeout" interval, providing an additional protection mechanism in case the phone is lost or stolen.

The Signal Protocol also supports end-to-end encrypted group chats. The group chat protocol is a combination of a pairwise double ratchet and multicast encryption. In addition to the properties provided by the one-to-one protocol, the group chat protocol provides speaker consistency, out-of-order resilience, dropped message resilience, computational equality, trust equality, subgroup messaging, as well as contractible and expandable membership.

Signal relies on centralized servers that are maintained by Signal Messenger. In addition to routing Signal's messages, the servers also facilitate the discovery of contacts who are also registered Signal users and the automatic exchange of users' public keys. By default, Signal's voice and video calls are peer-to-peer. If the caller is not in the receiver's address book, the call is routed through a server in order to hide the users' IP addresses.