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Signal is an encrypted communications app for Android and iOS. A desktop version is also available for Linux , Windows , and macOS. It uses the Internet to send one-to-one and group messages, which can include files, voice notes, images and videos, and make one-to-one voice and video calls. Signal uses standard cellular mobile numbers as identifiers, and uses end-to-end encryption to secure all communications to other Signal users.
The applications include mechanisms by which users can independently verify the identity of their messaging correspondents and the integrity of the data channel. Signal is developed by Open Whisper Systems. The clients are published as free and open-source software under the GPLv3 license.
The server code is published under the AGPLv3 license. Signal is the successor of an encrypted voice calling app called RedPhone and an encrypted texting program called TextSecure. The beta versions of RedPhone and TextSecure were first launched in May by Whisper Systems ,  a startup company co-founded by security researcher Moxie Marlinspike and roboticist Stuart Anderson.
In November , Whisper Systems announced that it had been acquired by Twitter. The financial terms of the deal were not disclosed by either company. Marlinspike could help the then-startup improve its security".
Open Whisper Systems' website was launched in January In February , 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.
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. On October 4, , the American Civil Liberties Union ACLU and Open Whisper Systems published a series of documents revealing that OWS had received a subpoena requiring them to provide information associated with two phone numbers for a federal grand jury investigation in the first half of Signal allows users to make voice and video  calls to other Signal users on iOS and Android.
All calls are made over a Wi-Fi or data connection and with the exception of data fees are free of charge, including long distance and international. The apps also support group messaging. All communications to other Signal users are automatically end-to-end encrypted. The keys that are used to encrypt the user's communications are generated and stored at the endpoints i. For authentication, Signal users can compare key fingerprints or scan QR codes out-of-band. The Android version of Signal allows the user to set a passphrase that encrypts the local message database and the user's encryption keys.
Signal also allows users to set timers to messages. Signal excludes users' messages from non-encrypted cloud backups by default. Signal requires that the user provides a phone number for verification,  eliminating the need for user names or passwords and facilitating contact discovery see below.
A workaround is to use a secondary phone number. On iOS, a number can only be registered to one device at a time; on Android, different users on the same device can correspond to different numbers. Signal also requires that the primary device be an Android or iOS based smartphone with an Internet connection. A desktop app that can link with a Signal mobile client is also available.
In July , the Internet Society published a user study that assessed the ability of Signal users to detect and deter man-in-the-middle attacks. 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. As of October [update] , the Signal Protocol has been implemented into WhatsApp , Facebook Messenger , and Google Allo , making it possible for the conversations of "more than a billion people worldwide" to be encrypted.
Signal relies on centralized servers that are maintained by Open Whisper Systems. 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. The servers store registered users' phone numbers, public key material and push tokens which are necessary for setting up calls and transmitting messages. All client-server communications are protected by TLS.
The group messaging mechanism is designed so that the servers do not have access to the membership list, group title, or group icon.
Signal's server architecture was federated between December and February In December , it was announced that the messaging protocol that is used in Signal had successfully been integrated into the Android-based open-source operating system CyanogenMod. According to Open Whisper Systems, the Cyanogen team ran their own Signal messaging server for WhisperPush clients, which federated with Open Whisper Systems' Signal server, so that both clients could exchange messages with each other.
In May , Signal's lead developer Moxie Marlinspike requested that a third-party client called LibreSignal not use the Signal service or the Signal name. The complete source code of the Signal clients for Android, iOS and desktop is available on GitHub under a free software license. It also allows advanced users to compile their own copies of the applications and compare them with the versions that are distributed by Open Whisper Systems.
Signal's servers are also open source. Signal is officially distributed through the Google Play store, Apple's App Store , and the official website. Applications distributed via Google Play are signed by the developer of the application, and the Android operating system checks that updates are signed with the same key , preventing others from distributing updates that the developer themselves did not sign.
Capitol to ensure that lawmakers and staff members have secure communications technology. One of the most widely respected encrypted communication apps, Signal, from Open Whisper Systems, has received significant financial support from the U.
Senate for use by senators and their staff. Following the Democratic National Committee email leak , Vanity Fair reported that Marc Elias , the general counsel for Hillary Clinton 's presidential campaign, had instructed DNC staffers to exclusively use Signal when saying anything "remotely contentious or disparaging" about Republican presidential nominee Donald Trump.
In December , Egypt blocked access to Signal. As of January [update] , Signal is blocked in Iran. Signal is developed by a software group called Open Whisper Systems.
The project receives donations via the Freedom of the Press Foundation ,   which has acted as Open Whisper Systems' fiscal sponsor since December From Wikipedia, the free encyclopedia. GPLv3    with proprietary dependencies on Google libraries  Server: Countries where Signal's domain fronting is enabled by default. Countries where Signal is blocked January Open Whisper Systems and Signal Foundation. Freedom of speech portal Free software portal Cryptography portal Telecommunications portal.
Retrieved 18 January Retrieved 16 January Retrieved 2 April Retrieved 7 April Retrieved 20 March Retrieved 31 October Retrieved 14 January Retrieved 5 November Retrieved 8 November Signal, a messaging app for mobile phones, has earned widespread deserved praise as a free software encrypted voice and text communication client, but currently has a proprietary dependency on Google libraries.
Helping the project remove this dependency and operate the necessary infrastructure is a high priority. Retrieved 21 November Archived from the original on 30 May Retrieved 22 January Retrieved 21 December The Wall Street Journal. Retrieved 10 July The Twitter Developer Blog. Archived from the original on 24 October Archived from the original on 6 January Archived from the original on 31 July Retrieved 14 July Retrieved 24 March Retrieved 27 August Retrieved 29 July Der Standard in German.
Retrieved 1 August Retrieved 19 March Retrieved 4 December Retrieved 5 May Retrieved 26 September The New York Times. The New York Times Company. Retrieved 4 October American Civil Liberties Union.