View source for Software:Signal

{{short description|Privacy-focused encrypted messaging app}}
{{Infobox software
| name = Signal
| logo = Signal-Logo.svg
| logo size = 150px
| logo alt = Logo of Signal (2020–present)
| screenshot = 
| screenshot alt = Screenshot of Signal
| caption = Screenshot of Signal version 5.1 on iOS (January 2021)
| collapsible = no
| developer = {{ Plainlist|
* Signal Technology Foundation,
* Signal Messenger LLC and contributors 
}}
| released = {{Start date and years ago|df=yes|2014|07|29}}<ref name="Greenberg-2014-07-29" /><ref name="Marlinspike-2014-07-29">{{cite web |url=https://whispersystems.org/blog/signal/ |title=Free, Worldwide, Encrypted Phone Calls for iPhone |publisher=Open Whisper Systems |first=Moxie |last=Marlinspike |date=29 July 2014 |access-date=16 January 2017 |archive-date=31 August 2017 |archive-url=https://web.archive.org/web/20170831113358/https://whispersystems.org/blog/signal/ |url-status=live}}</ref>
| ver layout = stacked
| latest release version = {{Multiple releases
  | branch1 = Android
  | version1 = {{wikidata|property|preferred|references|edit|Q19718090|P348|P400=Q94|P548=Q2804309}}
  | date1 = {{Start date and age|{{wikidata|qualifier|preferred|single|Q19718090|P348|P400=Q94|P548=Q2804309|P577}}}}
  | branch2 = iOS
  | version2 = {{wikidata|property|preferred|references|edit|Q19718090|P348|P400=Q48493|P548=Q2804309}}
  | date2 = {{Start date and age|{{wikidata|qualifier|preferred|single|Q19718090|P348|P400=Q48493|P548=Q2804309|P577}}}}
  | branch3 = Desktop
  | version3 = {{wikidata|property|preferred|references|edit|Q19718090|P348|P400=Q16338|P548=Q2804309}}
  | date3 = {{Start date and age|{{wikidata|qualifier|preferred|single|Q19718090|P348|P400=Q16338|P548=Q2804309|P577}}}}
}}
| latest preview version = {{Multiple releases
  | branch1 = Android ([[Software release life cycle#Beta|Beta]])
  | version1 = {{wikidata|property|references|edit|Q19718090|P348|P400=Q94|P548=Q3295609}}
  | date1 = {{Start date and age|{{wikidata|qualifier|single|Q19718090|P348|P400=Q94|P548=Q3295609|P577}}}}
  | branch2 = iOS (Beta)
  | version2 = {{wikidata|property|references|edit|Q19718090|P348|P400=Q48493|P548=Q3295609}}
  | date2 = {{Start date and age|{{wikidata|qualifier|single|Q19718090|P348|P400=Q48493|P548=Q3295609|P577}}}}
  | branch3 = Desktop (Beta)
  | version3 = {{wikidata|property|references|edit|Q19718090|P348|P400=Q16338|P548=Q3295609}}
  | date3 = {{Start date and age|{{wikidata|qualifier|single|Q19718090|P348|P400=Q16338|P548=Q3295609|P577}}}}
}}
| operating system = {{Plainlist|
* Android 4.4 or later
* [[Software:IOS 12|iOS 12.2]] or later
* [[Software:Windows 7|Windows 7]] or later (64-bit)<ref name="standalone-signal-desktop">{{cite web |last1=Nonnenberg |first1=Scott |title=Standalone Signal Desktop |url=https://signal.org/blog/standalone-signal-desktop/ |publisher=Open Whisper Systems |access-date=31 October 2017 |date=31 October 2017 |archive-date=15 February 2020 |archive-url=https://web.archive.org/web/20200215121725/https://signal.org/blog/standalone-signal-desktop/ |url-status=live}}</ref>
* macOS 10.10 or later<ref name="Installing-Signal">{{cite web |url=https://support.signal.org/hc/en-us/articles/360008216551#install_desktop |title=Installing Signal - Signal Support |access-date=20 March 2019 |archive-date=23 February 2020 |archive-url=https://web.archive.org/web/20200223121428/https://support.signal.org/hc/en-us/articles/360008216551#install_desktop |url-status=live}}</ref>
* [[Software:Linux distribution|Linux distribution]]s supporting [[Software:APT (Debian)|APT]]<ref name="standalone-signal-desktop"/>
* [[Software:FreeBSD|FreeBSD]]
  }}
| platform = 
| size = <!-- language varies based on the platform
| language count = 51
| language footnote = <ref>{{cite web |title=Signal Messenger localization |url=https://www.transifex.com/signalapp/public/ |website=Transifex |access-date=5 November 2018}}</ref>-->
| programming language = 
| genre = Encrypted voice calling, video calling and [[Social:Instant messaging|instant messaging]]
| license = [[Software:GNU Affero General Public License|AGPL-3.0-only]]<ref name="signal-android-github" /><ref name="signal-ios-github" /><ref name="signal-desktop-github"/><ref name="Signal-Server" />{{efn|name="mobile license"}}{{efn|name=SpamDetection}}
}}

'''Signal''' is an [[Encryption|encrypted]] [[Social:Message|messaging]] service for [[Social:Instant messaging|instant messaging]], voice, and [[Videotelephony|video calls]].<ref name="Mott-2017-03-14"/><ref name="desktop-v1.35.1">{{cite web |last1=Perez |first1=Josh |title=Release v1.35.1 |url=https://github.com/signalapp/Signal-Desktop/releases/tag/v1.35.1 |website=github.com |publisher=Signal |access-date=3 September 2020 |date=2 September 2020 |archive-date=12 November 2020 |archive-url=https://web.archive.org/web/20201112195107/https://github.com/signalapp/Signal-Desktop/releases/tag/v1.35.1 |url-status=live }}</ref> The instant messaging function includes sending text, voice notes, [[Digital image|images]], [[Engineering:Video|videos]], and [[Computer file|other files]].<ref name="arbitrary-file-types" /> Communication may be one-to-one between users or may involve group messaging.

The application uses a [[Centralized computing|centralized computing]] architecture and is [[Cross-platform software|cross-platform software]]. It is developed by the non-profit [[Organization:Signal Foundation|Signal Foundation]] and its subsidiary Signal Messenger LLC. Signal's software is [[Free and open-source software|free and open-source]]. Its [[Mobile app|mobile]] [[Client (computing)|clients]], [[Desktop metaphor|desktop]] client, and [[Server (computing)|server]] are all published under the [[Software:GNU Affero General Public License|AGPL-3.0-only]] [[Software license|license]].<ref name="signal-ios-github" /><ref name="signal-android-github" /><ref name="signal-desktop-github"/><ref name="Signal-Server" />{{efn|name="mobile license"|The [[Software:IOS|iOS]] and [[Software:Android (operating system)|Android]] clients were formerly published under the [[Software:GNU General Public License|GPL-3.0-only]] license, and were updated to AGPL in 2022 and 2023 respectively.<ref name="signal-ios-license-update">{{cite web |url=https://github.com/signalapp/Signal-iOS/commit/9865d398f70aeb1888c1c537f29b043690a570b2 |title=Update top-level LICENSE file to AGPL |website=GitHub.com |publisher=Signal |date=1 November 2022 |access-date=10 May 2023 |archive-date=28 August 2023 |archive-url=https://web.archive.org/web/20230828005533/https://github.com/signalapp/Signal-iOS/commit/9865d398f70aeb1888c1c537f29b043690a570b2 |url-status=live }}</ref><ref name="signal-android-license-update">{{cite web |url=https://github.com/signalapp/Signal-Android/commit/ace47c61b1aa4c32b8468343615e3e7288915dea |title=Update top-level LICENSE file to AGPL |website=GitHub.com |publisher=Signal |date=5 May 2023 |access-date=10 May 2023 |archive-date=14 August 2023 |archive-url=https://web.archive.org/web/20230814182331/https://github.com/signalapp/Signal-Android/commit/ace47c61b1aa4c32b8468343615e3e7288915dea |url-status=live }}</ref>}}{{efn|name=SpamDetection|The source code for [[Spamming|spam]] detection is not public.<ref>{{cite web |last1=O'Leary |first1=Jim |title=Improving first impressions on Signal |url=https://signal.org/blog/keeping-spam-off-signal/ |website=Signal Blog |access-date=3 November 2021 |date=1 November 2021 |archive-date=24 September 2023 |archive-url=https://web.archive.org/web/20230924194458/https://signal.org/blog/keeping-spam-off-signal/ |url-status=live }}</ref>}} The official Android app generally uses the proprietary [[Software:Google Play Services|Google Play Services]], although it is designed to work without them. Signal is also distributed for [[Software:IOS|iOS]] and desktop programs for [[Software:Windows|Windows]], [[Software:MacOS|macOS]], and [[Software:Linux|Linux]]. Registration for desktop use requires an iOS or Android device.<ref name="Softpedia-2021">{{cite web |url=https://www.softpedia.com/get/Internet/Chat/Instant-Messaging/Signal-Desktop.shtml |title=Signal Desktop |last=Ciobica |first=Vladimir |date=26 May 2021 |website=Softpedia |access-date=28 May 2021 |archive-date=2 June 2021 |archive-url=https://web.archive.org/web/20210602212341/https://www.softpedia.com/get/Internet/Chat/Instant-Messaging/Signal-Desktop.shtml |url-status=live }}</ref><ref name="VPNpro-2021">{{cite web |url=https://vpnpro.com/messaging-apps/signal-review/ |title=Signal messaging app review 2021 |last=Youngren |first=Jan |date=19 January 2021 |website=VPNpro |access-date=28 May 2021 |archive-date=2 June 2021 |archive-url=https://web.archive.org/web/20210602212529/https://vpnpro.com/messaging-apps/signal-review/ |url-status=live }}</ref>

Signal uses mobile [[Telephone number|telephone number]]s as identifiers for users. It secures all communications with [[End-to-end encryption|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.<ref name="Frosch 2016"/><ref name="Schröder-2016"/>

The [[Engineering:Nonprofit organization|non-profit]] Signal Foundation was launched in February 2018 with initial funding of $50&nbsp;million from Brian Acton.<ref name="Greenberg20180221"/> {{As of|2022|January}}, the platform had approximately 40&nbsp;million monthly active users. {{As of|2021|May}}, it was downloaded more than 105 million times.<ref name="Curry-2022">{{Cite news |last=Curry |first=David |date=11 January 2022 |title=Signal Revenue & Usage Statistics (2022) |language=en-US |work=Business of Apps |url=https://www.businessofapps.com/data/signal-statistics/ |access-date=13 January 2022 |archive-date=26 January 2021 |archive-url=https://web.archive.org/web/20210126020455/https://www.businessofapps.com/data/signal-statistics/ |url-status=live }}</ref><ref>{{Cite web |date=28 January 2021 |title=Signal Private Messenger - Apps on Google Play |url=https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms&hl=en_US&gl=US |access-date=28 January 2021 |publisher=Google |language=en |archive-date=11 June 2021 |archive-url=https://web.archive.org/web/20210611140028/https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms&hl=en_US&gl=US |url-status=live }}</ref>

Until the feature was removed in 2023, the [[Software:Android (operating system)|Android]] version was also optionally capable of functioning as an [[SMS]] [[Mobile app|app]].<ref name="Frosch 2016"/><ref name="Nina-signal-2022-10-12">{{cite web |author1=nina-signal |title=Removing SMS support from Signal Android (soon) |url=https://signal.org/blog/sms-removal-android/ |website=signal.org |date=12 October 2022 |access-date=28 November 2022 |archive-date=27 November 2022 |archive-url=https://web.archive.org/web/20221127000227/https://signal.org/blog/sms-removal-android/ |url-status=live }}</ref>

==History==

===2010–2013: Origins===
Signal is the successor of the RedPhone encrypted voice calling app and the [[Software:TextSecure|TextSecure]] encrypted texting program. The beta versions of RedPhone and TextSecure were first launched in May 2010 by [[Company:Whisper Systems|Whisper Systems]],<ref name="whispersystems-2010-05-25">{{cite web |url=http://www.whispersys.com/updates.html |title=Announcing the public beta |date=25 May 2010 |archive-url=https://web.archive.org/web/20100530011131/http://www.whispersys.com/updates.html |archive-date=30 May 2010 |publisher=Whisper Systems |access-date=22 January 2015}}</ref> a startup company co-founded by security researcher Moxie Marlinspike and roboticist Stuart Anderson.<ref name="Garling-2011-12-20" /><ref>{{cite magazine |url=http://investing.businessweek.com/research/stocks/private/snapshot.asp?privcapId=141104009 |archive-url=https://archive.today/20140304095526/http://investing.businessweek.com/research/stocks/private/snapshot.asp?privcapId=141104009 |url-status=dead |archive-date=4 March 2014 |title=Company Overview of Whisper Systems Inc. |magazine=Bloomberg Businessweek |access-date=4 March 2014}}</ref> Whisper Systems also produced a firewall and tools for encrypting other forms of data.<ref name="Garling-2011-12-20" /><ref name="Greenberg-2010-05-25" /> All of these were [[Proprietary software|proprietary]] enterprise mobile security software and were only available for Android.

In November 2011, Whisper Systems announced that it had been acquired by [[Company:Twitter|Twitter]]. Neither company disclosed the financial terms of the deal.<ref name="Cheredar-2011-11-28" /> The acquisition was done "primarily so that Mr. Marlinspike could help the then-startup improve its security".<ref name="Yadron2015">{{cite news |last1=Yadron |first1=Danny |title=Moxie Marlinspike: The Coder Who Encrypted Your Texts |url=https://www.wsj.com/articles/moxie-marlinspike-the-coder-who-encrypted-your-texts-1436486274 |access-date=10 July 2015 |work=The Wall Street Journal |date=9 July 2015 |archive-date=12 July 2015 |archive-url=https://web.archive.org/web/20150712035634/https://www.wsj.com/articles/moxie-marlinspike-the-coder-who-encrypted-your-texts-1436486274 |url-status=live}}</ref> Shortly after the acquisition, Whisper Systems' RedPhone service was made unavailable.<ref name="Greenberg-2011-11-28" /> 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.<ref name="Garling-2011-11-28" />

Twitter released TextSecure as [[Free and open-source software|free and open-source software]] under the [[Software:GNU General Public License|GPLv3]] license in December 2011.<ref name="Garling-2011-12-20"/><ref name="Aniszczyk-2011-12-20" /><ref name="whispersystems-2011-12-20">{{cite web |url=http://www.whispersys.com/updates.html |title=TextSecure is now Open Source! |archive-url=https://web.archive.org/web/20120106024504/http://www.whispersys.com/updates.html |archive-date=6 January 2012 |date=20 December 2011 |publisher=Whisper Systems |access-date=22 January 2015}}</ref><ref name="Pachal-2011-12-20" /> RedPhone was also released under the same license in July 2012.<ref name="whispersystems-2012-07-18">{{cite web |url=http://www.whispersys.com/updates.html |title=RedPhone is now Open Source! |archive-url=https://web.archive.org/web/20120731143138/http://www.whispersys.com/updates.html |archive-date=31 July 2012 |date=18 July 2012 |publisher=Whisper Systems |access-date=22 January 2015}}</ref> Marlinspike later left Twitter and founded Open Whisper Systems as a collaborative Open Source project for the continued development of TextSecure and RedPhone.<ref name="Greenberg-2014-07-29" /><ref name="welcome" />

===2013–2018: Open Whisper Systems===

Open Whisper Systems' website was launched in January 2013.<ref name="welcome" />

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.<ref name="Donohue-2014">{{cite web |date=24 February 2014 |first=Brian |last=Donohue |url=https://threatpost.com/textsecure-sheds-sms-in-latest-version/104456 |title=TextSecure Sheds SMS in Latest Version |website=Threatpost |access-date=14 July 2016 |archive-date=15 February 2017 |archive-url=https://web.archive.org/web/20170215020451/https://threatpost.com/textsecure-sheds-sms-in-latest-version/104456/ |url-status=live}}</ref> Toward the end of July 2014, they announced plans to merge the RedPhone and TextSecure applications as Signal.<ref name="Mimoso-2014-07-29" /> This announcement coincided with the initial release of Signal as a RedPhone counterpart for [[Software:IOS|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 application|web client]].<ref name="Mimoso-2014-07-29" /> Signal was the first iOS app to enable end-to-end encrypted voice calls for free.<ref name="Greenberg-2014-07-29" /><ref name="Evans-2014-07-29" /> TextSecure compatibility was added to the iOS application in March 2015.<ref name="Lee-2015-03-02" /><ref name="Geuss-2015-03-03" />

{{multiple image
 | width = 60
 | align = left
 | image1            = TextSecure Blue Icon.png
 | caption1          = Signal Android icon, 2015–2017
 | image2            = Signal Blue Icon.png
 | caption2          = Signal icon, 2015–2020
}}
From its launch in May 2010<ref name="whispersystems-2010-05-25" /> until March 2015, the Android version of Signal (then called TextSecure) included support for encrypted SMS/[[Multimedia Messaging Service|MMS]] messaging.<ref>{{cite web |url=https://whispersystems.org/blog/goodbye-encrypted-sms/ |title=Saying goodbye to encrypted SMS/MMS |author=Open Whisper Systems |date=6 March 2015 |access-date=24 March 2016 |archive-date=24 August 2017 |archive-url=https://web.archive.org/web/20170824085458/https://whispersystems.org/blog/goodbye-encrypted-sms/ |url-status=live}}</ref> From version 2.7.0 onward, the Android application only supported sending and receiving encrypted messages via the data channel.<ref name="Rottermanner-2015-p3"/> Reasons for this included security flaws of SMS/MMS and problems with the [[Key exchange|key exchange]].<ref name="Rottermanner-2015-p3">{{harvnb|Rottermanner|Kieseberg|Huber|Schmiedecker|2015|p=3}}</ref> Open Whisper Systems' abandonment of SMS/MMS encryption prompted some users to create a [[Software:Fork (software development)|fork]] named Silence (initially called SMSSecure<ref name="github383">{{cite web |url=https://github.com/SilenceIM/Silence/pull/383 |author=BastienLQ |title=Change the name of SMSSecure |website=GitHub |publisher=SilenceIM |type=pull request |date=20 April 2016 |access-date=27 August 2016 |archive-date=23 February 2020 |archive-url=https://web.archive.org/web/20200223121439/https://github.com/SilenceIM/Silence/pull/383 |url-status=live}}</ref>) that is meant solely for the exchange of encrypted SMS and MMS messages.<ref name="Heise-April-2015">{{cite web |title=TextSecure-Fork bringt SMS-Verschlüsselung zurück |url=http://www.heise.de/security/meldung/TextSecure-Fork-bringt-SMS-Verschluesselung-zurueck-2595471.html |website=Heise |access-date=29 July 2015 |language=de |date=2 April 2015 |archive-date=19 November 2018 |archive-url=https://web.archive.org/web/20181119024003/https://www.heise.de/security/meldung/TextSecure-Fork-bringt-SMS-Verschluesselung-zurueck-2595471.html |url-status=live}}</ref><ref name="derstandard">{{cite web |title=SMSSecure: TextSecure-Abspaltung belebt SMS-Verschlüsselung wieder |url=http://derstandard.at/2000013841576/SMSSecure-TextSecure-Abspaltung-belebt-SMS-Verschluesselung-wieder |website=Der Standard |access-date=1 August 2015 |language=de |date=3 April 2015 |archive-date=20 November 2018 |archive-url=https://web.archive.org/web/20181120012146/https://derstandard.at/2000013841576/SMSSecure-TextSecure-Abspaltung-belebt-SMS-Verschluesselung-wieder |url-status=live}}</ref>

In November 2015, the TextSecure and RedPhone applications on Android were merged to become Signal for Android.<ref name="Greenberg-2015-11-2">{{cite magazine |last1=Greenberg |first1=Andy |title=Signal, the Snowden-Approved Crypto App, Comes to Android |url=https://www.wired.com/2015/11/signals-snowden-approved-phone-crypto-app-comes-to-android/ |magazine=Wired |access-date=19 March 2016 |date=2 November 2015 |archive-date=26 January 2018 |archive-url=https://web.archive.org/web/20180126050207/https://www.wired.com/2015/11/signals-snowden-approved-phone-crypto-app-comes-to-android/ |url-status=live}}</ref> A month later, Open Whisper Systems announced Signal Desktop, a Chrome app that could link with a Signal mobile client.<ref name="Motherboard-2015-12-02">{{cite web |last1=Franceschi-Bicchierai |first1=Lorenzo |title=Snowden's Favorite Chat App Is Coming to Your Computer |url=http://motherboard.vice.com/read/signal-snowdens-favorite-chat-app-is-coming-to-your-computer |website=Motherboard |publisher=Vice Media LLC |access-date=4 December 2015 |date=2 December 2015 |archive-date=16 December 2016 |archive-url=https://web.archive.org/web/20161216025211/http://motherboard.vice.com/read/signal-snowdens-favorite-chat-app-is-coming-to-your-computer |url-status=live}}</ref> At launch, the app could only be linked with the Android version of Signal.<ref name="Coldewey-2016-04-07">{{cite web |last1=Coldewey |first1=Devin |title=Now's your chance to try Signal's desktop Chrome app |url=https://techcrunch.com/2016/04/07/nows-your-chance-to-try-signals-desktop-chrome-app/ |website=TechCrunch |publisher=AOL Inc. |access-date=5 May 2016 |date=7 April 2016 |archive-date=21 October 2019 |archive-url=https://web.archive.org/web/20191021173212/http://techcrunch.com/2016/04/07/nows-your-chance-to-try-signals-desktop-chrome-app/ |url-status=live}}</ref> On 26 September 2016, Open Whisper Systems announced that Signal Desktop could now be linked with the iOS version of Signal as well.<ref name="signal-desktop-ios">{{cite web |last1=Marlinspike |first1=Moxie |title=Desktop support comes to Signal for iPhone |url=https://whispersystems.org/blog/signal-desktop-ios/ |publisher=Open Whisper Systems |access-date=26 September 2016 |date=26 September 2016 |archive-date=7 July 2017 |archive-url=https://web.archive.org/web/20170707095128/https://whispersystems.org/blog/signal-desktop-ios/ |url-status=live}}</ref> On 31 October 2017, Open Whisper Systems announced that the Chrome app was [[Deprecation|deprecated]].<ref name="standalone-signal-desktop"/> At the same time, they announced the release of a standalone desktop client (based on the [[Software:Electron (software framework)|Electron]] [[Software framework|framework]]<ref name="signal-desktop-github" />) for [[Software:Microsoft Windows|Windows]], [[Software:MacOS|macOS]] and certain [[Software:Linux distribution|Linux distributions]].<ref name="standalone-signal-desktop"/><ref>{{cite web |last1=Coldewey |first1=Devin |title=Signal escapes the confines of the browser with a standalone desktop app |url=https://techcrunch.com/2017/10/31/signal-escapes-the-confines-of-the-browser-with-a-standalone-desktop-app/ |website=TechCrunch |publisher=Oath Tech Network |access-date=31 October 2017 |date=31 October 2017 |archive-date=14 May 2019 |archive-url=https://web.archive.org/web/20190514155929/https://techcrunch.com/2017/10/31/signal-escapes-the-confines-of-the-browser-with-a-standalone-desktop-app/ |url-status=live}}</ref>

On 4 October 2016, 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 2016.<ref name="Perlroth-2016-10-04">{{cite web |last1=Perlroth |first1=Nicole |last2=Benner |first2=Katie |title=Subpoenas and Gag Orders Show Government Overreach, Tech Companies Argue |url=https://www.nytimes.com/2016/10/05/technology/subpoenas-and-gag-orders-show-government-overreach-tech-companies-argue.html |website=The New York Times |access-date=4 October 2016 |date=4 October 2016 |archive-date=24 January 2020 |archive-url=https://web.archive.org/web/20200124010809/https://www.nytimes.com/2016/10/05/technology/subpoenas-and-gag-orders-show-government-overreach-tech-companies-argue.html |url-status=live}}</ref><ref name="Kaufman-2016-10-04">{{cite web |last1=Kaufman |first1=Brett Max |title=New Documents Reveal Government Effort to Impose Secrecy on Encryption Company |url=https://www.aclu.org/blog/free-future/new-documents-reveal-government-effort-impose-secrecy-encryption-company |publisher=American Civil Liberties Union |type=Blog post |access-date=4 October 2016 |date=4 October 2016 |archive-date=25 July 2017 |archive-url=https://web.archive.org/web/20170725085001/https://www.aclu.org/blog/free-future/new-documents-reveal-government-effort-impose-secrecy-encryption-company |url-status=live}}</ref><ref name="OWS-2016-10-04">{{cite web |title=Grand jury subpoena for Signal user data, Eastern District of Virginia |url=https://whispersystems.org/bigbrother/eastern-virginia-grand-jury/ |publisher=Open Whisper Systems |access-date=4 October 2016 |date=4 October 2016 |archive-date=29 August 2017 |archive-url=https://web.archive.org/web/20170829091039/https://whispersystems.org/bigbrother/eastern-virginia-grand-jury/ |url-status=live}}</ref> Only one of the two phone numbers was registered on Signal, and because of how the service is designed, OWS was only able to provide "the time the user's account had been created and the last time it had connected to the service".<ref name="Kaufman-2016-10-04"/><ref name="Perlroth-2016-10-04"/> Along with the subpoena, OWS received a [[Social:Gag order|gag order]] requiring OWS not to tell anyone about the subpoena for one year.<ref name="Perlroth-2016-10-04"/> OWS approached the ACLU, and they were able to lift part of the gag order after challenging it in court.<ref name="Perlroth-2016-10-04"/> OWS said it was the first time they had received a subpoena, and that they were committed to treat "any future requests the same way".<ref name="OWS-2016-10-04"/>

In March 2017, Open Whisper Systems transitioned Signal's calling system from RedPhone to [[Software:WebRTC|WebRTC]], also adding the ability to make video calls with the mobile apps.<ref name="signal-video-calls-beta"/><ref name="signal-video-calls" /><ref name="Mott-2017-03-14"/>

===Since 2018: Signal Technology Foundation===

On 21 February 2018, Moxie Marlinspike and [[Software:WhatsApp|WhatsApp]] co-founder Brian Acton announced the formation of the Signal Technology Foundation, a [[Organization:501(c) organization|501(c)(3) nonprofit organization]] whose mission is "to support, accelerate, and broaden Signal's mission of making private communication accessible and ubiquitous".<ref name="signal-foundation">{{cite web |last1=Marlinspike |first1=Moxie |last2=Acton |first2=Brian |title=Signal Foundation |url=https://signal.org/blog/signal-foundation/ |website=Signal.org |access-date=21 February 2018 |date=21 February 2018 |archive-date=16 February 2020 |archive-url=https://web.archive.org/web/20200216021349/https://signal.org/blog/signal-foundation/ |url-status=live}}</ref><ref name="Greenberg20180221"/> Acton started the foundation with $50&nbsp;million in funding and became the foundation's executive chairman after leaving WhatsApp's parent company Facebook in September 2017.<ref name="Greenberg20180221">{{cite magazine |last1=Greenberg |first1=Andy |title=WhatsApp Co-Founder Puts $50M Into Signal To Supercharge Encrypted Messaging |url=https://www.wired.com/story/signal-foundation-whatsapp-brian-acton/ |magazine=Wired |access-date=21 February 2018 |date=21 February 2018 |archive-date=22 February 2018 |archive-url=https://web.archive.org/web/20180222022309/https://www.wired.com/story/signal-foundation-whatsapp-brian-acton/ |url-status=live}}</ref> Marlinspike continued as Signal Messenger's first [[Social:Chief executive officer|CEO]].<ref name="signal-foundation"/> {{As of|2020}}, Signal ran entirely on donations, as a [[Engineering:Nonprofit organization|nonprofit]].<ref name="thenewyorker-2020-10-19"/>

Between November 2019 and February 2020, Signal added [[Engineering:IPad|iPad]] support, view-once images and videos, [[Sticker (messaging)|sticker]]s, and reactions.<ref name="Greenberg2020">{{cite magazine |last1=Greenberg |first1=Andy |title=Signal Is Finally Bringing Its Secure Messaging to the Masses |url=https://www.wired.com/story/signal-encrypted-messaging-features-mainstream/ |access-date=15 February 2020 |magazine=Wired |date=14 February 2020 |archive-date=14 February 2020 |archive-url=https://web.archive.org/web/20200214231514/https://www.wired.com/story/signal-encrypted-messaging-features-mainstream/ |url-status=live}}</ref> They also announced plans for a new group messaging system and an "experimental method for storing encrypted contacts in the cloud."<ref name="Greenberg2020" />

Signal was reportedly popularized in the United States during the George Floyd protests. Heightened awareness of police monitoring led protesters to use the platform to communicate. Black Lives Matter organizers had used the platform "for several years".<ref name=Nierenberg>{{Cite news |last=Nierenberg |first=Amelia |date=12 June 2020 |title=Signal Downloads Are Way Up Since the Protests Began |work={{wipe|The New York Times}} |url=https://www.nytimes.com/2020/06/11/style/signal-messaging-app-encryption-protests.html |url-status=live |archive-date=25 June 2020 |archive-url=https://web.archive.org/web/20200625005732/https://www.nytimes.com/2020/06/11/style/signal-messaging-app-encryption-protests.html}}</ref><ref name="thenewyorker-2020-10-19"/> During the first week of June, the encrypted messaging app was downloaded over five times more than it had been during the week prior to the [[Biography:Murder of George Floyd|murder of George Floyd]].<ref name=Nierenberg/> In June 2020, Signal Foundation announced a new feature that enables users to blur faces in photos, in response to increased federal efforts to monitor protesters.<ref name="thenewyorker-2020-10-19"/><ref>{{Cite web |last=Lyngaas |first=Sean |date=4 June 2020 |title=Signal aims to boost protesters' phone security at George Floyd demonstrations with face-blurring tool |url=https://www.cyberscoop.com/george-floyd-protest-phone-security/ |url-status=live |archive-url=https://web.archive.org/web/20200614014517/https://www.cyberscoop.com/george-floyd-protest-phone-security/ |archive-date=14 June 2020 |access-date=14 June 2020 |website=CyberScoop |language=en}}</ref>

On 7 January 2021, Signal saw a surge in new user registrations, which temporarily overwhelmed Signal's capacity to deliver account verification messages.<ref name="Hardwick-2021-01-08"/> CNN and MacRumors linked the surge with a [[Software:WhatsApp|WhatsApp]] privacy policy change and a Signal endorsement by [[Biography:Elon Musk|Elon Musk]] and Edward Snowden via Twitter.<ref name="Hardwick-2021-01-08">{{Cite web |last=Hardwick |first=Tim |title=Encrypted Messaging App Signal Sees Surge in Popularity Following WhatsApp Privacy Policy Update |url=https://www.macrumors.com/2021/01/08/messaging-app-signal-sees-surge-signups/ |access-date=9 January 2021 |website=MacRumors |date=8 January 2021 |language=en |archive-date=9 January 2021 |archive-url=https://web.archive.org/web/20210109082738/https://www.macrumors.com/2021/01/08/messaging-app-signal-sees-surge-signups/ |url-status=live}}</ref><ref>{{cite news |last1=Duffy |first1=Clare |date=12 January 2021 |title=Why messaging app Signal is surging in popularity right now |publisher=CNN |url=https://www.cnn.com/2021/01/12/tech/signal-growth-whatsapp-confusion/index.html |access-date=12 January 2021 |archive-date=13 January 2021 |archive-url=https://web.archive.org/web/20210113133528/https://www.cnn.com/2021/01/12/tech/signal-growth-whatsapp-confusion/index.html |url-status=live}}</ref> The surge was also tied to the far-right attack on the United States Capitol.<ref>{{cite web |last1=Chau |first1=David |title=Donald Trump supporters embrace Signal, Telegram and other 'free speech' apps |url=https://www.abc.net.au/news/2021-01-20/donald-trump-social-media-apps-free-speech-privacy/13071206 |website=ABC News |date=20 January 2021 |access-date=12 May 2023 |archive-date=13 December 2023 |archive-url=https://web.archive.org/web/20231213150315/https://www.abc.net.au/news/2021-01-20/donald-trump-social-media-apps-free-speech-privacy/13071206 |url-status=live }}</ref> International newspapers reported similar trends in the [[Place:United Arab Emirates|United Arab Emirates]].<ref>{{Cite news |last=Cabral |first=Alvin R. |title=UAE WhatsApp users look for other messaging platforms over new terms |url=https://www.khaleejtimes.com/technology/uae-whatsapp-users-look-for-other-messaging-platforms-over-new-terms |access-date=10 January 2021 |website=Khaleej Times |language=en |archive-date=17 January 2021 |archive-url=https://web.archive.org/web/20210117005233/https://www.khaleejtimes.com/technology/uae-whatsapp-users-look-for-other-messaging-platforms-over-new-terms |url-status=live}}</ref> {{wipe|Reuters}} reported that more than 100,000&nbsp;people had installed Signal between 7 and 8 January.<ref>{{Cite news |date=8 January 2021 |title=Signal, Telegram see demand spike as new WhatsApp terms stir debate |language=en |agency={{wipe|Reuters}} |url=https://www.reuters.com/article/us-whatsapp-users-idUSKBN29D1TY |access-date=10 January 2021 |archive-date=17 January 2021 |archive-url=https://web.archive.org/web/20210117005232/https://www.reuters.com/article/us-whatsapp-users-idUSKBN29D1TY |url-status=live}}</ref>

Between 12 and 14 January 2021, the number of Signal installations listed on Google Play increased from over 10 million to over 50 million.<ref>[[:File:Signal Messenger downloads between 2021-01-12 and 2021-01-14.png|Screenshot]] showing 40 million downloads between 12 and 14 January 2021.</ref><ref>{{cite web |title=Signal Private Messenger - Apps on Google Play |url=https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms |publisher=Google Play |access-date=18 January 2021 |date=12 January 2021 |archive-url=https://web.archive.org/web/20210112141222/https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms |archive-date=12 January 2021}}</ref><ref>{{cite web |title=Signal Private Messenger - Apps on Google Play |url=https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms |access-date=18 January 2021 |date=14 January 2021 |archive-url=https://web.archive.org/web/20210114073330/https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms |archive-date=14 January 2021}}</ref><ref>{{cite web |url=https://www.androidpolice.com/2021/01/14/signal-hits-50-million-installs-on-play-store-amid-whatsapp-privacy-concerns/ |title=Signal hits 50 million installs on Play Store amid WhatsApp privacy concerns |publisher=Android Police |date=14 January 2021 |access-date=20 January 2021 |archive-date=16 March 2021 |archive-url=https://web.archive.org/web/20210316161056/https://www.androidpolice.com/2021/01/14/signal-hits-50-million-installs-on-play-store-amid-whatsapp-privacy-concerns/ |url-status=live }}</ref>
On 15 January 2021, due to the surge of new users, Signal was overwhelmed with the new traffic and was down for all users.<ref>{{Cite web |title=Signal Status |url=https://status.signal.org/ |url-status=live |archive-url=https://web.archive.org/web/20210115221438/https://status.signal.org/ |archive-date=15 January 2021 |access-date=15 January 2021 |website=status.signal.org}}</ref><ref>{{Cite news |date=15 January 2021 |title=WhatsApp changes: Signal messaging platform stops working as downloads surge |language=en-GB |work=BBC News |url=https://www.bbc.com/news/technology-55684595 |access-date=16 January 2021 |archive-date=16 January 2021 |archive-url=https://web.archive.org/web/20210116092644/https://www.bbc.com/news/technology-55684595 |url-status=live}}</ref> On the afternoon of 16 January, Signal announced via Twitter that service had been restored.<ref>{{Cite tweet |user=SignalApp |number=1350595202872823809 |title=Signal is back! |date=16 January 2021}}</ref>

On 10 January 2022, Moxie Marlinspike announced that he was stepping down from his role as CEO of Signal Messenger.<ref name="Marlinspike-2022-01-10"/> He continues to remain on the Signal Foundation's [[Organization:Board of directors|board of directors]] and Brian Acton has volunteered to serve as interim CEO during the search for a new CEO.<ref name="Marlinspike-2022-01-10">{{cite web |last1=Marlinspike |first1=Moxie |title=New year, new CEO |url=https://signal.org/blog/new-year-new-ceo/ |website=signal.org |publisher=Signal Messenger |access-date=10 January 2022 |date=10 January 2022 |archive-date=16 December 2022 |archive-url=https://web.archive.org/web/20221216031722/https://signal.org/blog/new-year-new-ceo/ |url-status=live }}</ref>

In August 2022, Signal notified 1900 users that their data had been affected by the [[Company:Twilio|Twilio]] breach including user phone numbers and SMS verification codes.<ref name="Twilio-2022-08-15">{{cite web |last1=Page |first1=Carly |title=Signal says 1,900 users' phone numbers exposed by Twilio breach |url=https://techcrunch.com/2022/08/15/signal-phone-number-exposed-twilio/ |website=techcrunch.com |publisher=TechCrunch |access-date=16 August 2022 |date=15 August 2022 |archive-date=13 December 2023 |archive-url=https://web.archive.org/web/20231213150314/https://techcrunch.com/2022/08/15/signal-phone-number-exposed-twilio/ |url-status=live }}</ref> At least one journalist had his account re-registered to a device he did not control as a result of the attack.<ref name="Lorenzo-2022-08-17">{{cite news |last1=Franceschi-Bicchierai |first1=Lorenzo |title=How a Third-Party SMS Service Was Used to Take Over Signal Accounts |url=https://www.vice.com/en/article/qjkvxv/how-a-third-party-sms-service-was-used-to-take-over-signal-accounts |access-date=19 August 2022 |work=VICE |date=17 August 2022 |language=en |archive-date=18 August 2022 |archive-url=https://web.archive.org/web/20220818194205/https://www.vice.com/en/article/qjkvxv/how-a-third-party-sms-service-was-used-to-take-over-signal-accounts |url-status=live }}</ref>

In September 2022 Signal Messaging LLC announced that AI researcher and noted critic of big tech [[Biography:Meredith Whittaker|Meredith Whittaker]] would fill the newly created position of President.<ref>{{Cite web |date=6 September 2022 |title=Signal Has a New President and She Has a Plan |url=https://gizmodo.com/meredith-whittaker-new-president-of-signal-messaging-1849501653 |access-date=23 June 2023 |website=Gizmodo |language=en |archive-date=23 June 2023 |archive-url=https://web.archive.org/web/20230623171238/https://gizmodo.com/meredith-whittaker-new-president-of-signal-messaging-1849501653 |url-status=live }}</ref>

===Usage===
{{Graph:Chart
|width=480
|height=150
|type=line
|xType=date
|xAxisFormat=%b %Y
|xAxisAngle=-40
|yGrid=
|x=2010-05-01, 2020-12-01, 2022-01-12
|y1=0, 20, 40
|xAxisTitle=Signal timeline (May 2010 – January 2022)
|yAxisTitle=Monthly active users (million)
}}

Signal's userbase started in May 2010, when its predecessor [[Software:TextSecure|TextSecure]] was launched by [[Company:Whisper Systems|Whisper Systems]].<ref name="whispersystems-2010-05-25"/> According to App Annie, Signal had approximately 20 million monthly active users at the end of December 2020.<ref>{{cite web |last1=Singh |first1=Manish |title=Signal's Brian Acton talks about exploding growth, monetization and WhatsApp data-sharing outrage |url=https://techcrunch.com/2021/01/12/signal-brian-acton-talks-about-exploding-growth-monetization-and-whatsapp-data-sharing-outrage/ |website=TechCrunch |publisher=Yahoo |access-date=13 January 2022 |date=13 January 2021 |archive-date=13 January 2022 |archive-url=https://web.archive.org/web/20220113135343/https://techcrunch.com/2021/01/12/signal-brian-acton-talks-about-exploding-growth-monetization-and-whatsapp-data-sharing-outrage/ |url-status=live }}</ref> In January 2022, the BBC reported that Signal was used by over 40 million people.<ref>{{cite web |title=Moxie Marlinspike leaves encrypted-messaging app Signal |url=https://www.bbc.com/news/technology-59937614 |website=BBC News |publisher=BBC |access-date=13 January 2022 |date=12 January 2022 |archive-date=12 January 2022 |archive-url=https://web.archive.org/web/20220112173823/https://www.bbc.com/news/technology-59937614 |url-status=live }}</ref>

=== Developers and funding ===
{{Main|Organization:Signal Foundation}}
The development of Signal and its predecessors at [[Organization:Open Whisper Systems|Open Whisper Systems]] was funded by a combination of consulting contracts, donations and grants.<ref name="ONeill-2017-01-03">{{cite web |last1=O'Neill |first1=Patrick |date=3 January 2017 |title=How Tor and Signal can maintain the fight for freedom in Trump's America |url=https://www.cyberscoop.com/tor-signal-funding-donald-trump-steve-bannon-encryption/ |url-status=live |archive-url=https://web.archive.org/web/20170917033354/https://www.cyberscoop.com/tor-signal-funding-donald-trump-steve-bannon-encryption/ |archive-date=17 September 2017 |access-date=16 September 2017 |website=CyberScoop |publisher=Scoop News Group}}</ref> The Freedom of the Press Foundation acted as Signal's [[Finance:Fiscal sponsorship|fiscal sponsor]].<ref name="signal-foundation" /><ref>{{cite web |last1=Timm |first1=Trevor |date=8 December 2016 |title=Freedom of the Press Foundation's new look, and our plans to protect press freedom for 2017 |url=https://freedom.press/news/freedom-press-foundations-new-look-and-our-plans-protect-press-freedom-2017/ |url-status=live |archive-url=https://web.archive.org/web/20170202042832/https://freedom.press/news/freedom-press-foundations-new-look-and-our-plans-protect-press-freedom-2017/ |archive-date=2 February 2017 |access-date=25 January 2017 |website=Freedom of the Press Foundation}}</ref><ref name="pressfreedomfoundation" /> Between 2013 and 2016, the project received grants from the Knight Foundation,<ref name="knightfoundation" /> the Shuttleworth Foundation,<ref name="shuttleworthfoundation" /> and almost $3&nbsp;million from the US government–sponsored Open Technology Fund.<ref name="opentechfund" /> Signal is now developed by Signal Messenger LLC, a software company founded by Moxie Marlinspike and Brian Acton in 2018, which is wholly owned by a tax-exempt nonprofit corporation called the [[Organization:Signal Foundation|Signal Technology Foundation]], also created by them in 2018. The Foundation was funded with an initial loan of $50&nbsp;million from Acton, "to support, accelerate, and broaden Signal's mission of making private communication accessible and ubiquitous".<ref name="signal-foundation" /><ref name="Greenberg20180221" /><ref name="propublica">{{cite web |date=9 May 2013 |title=Signal Technology Foundation |url=https://projects.propublica.org/nonprofits/organizations/824506840 |url-status=live |archive-url=https://web.archive.org/web/20210109161310/https://projects.propublica.org/nonprofits/organizations/824506840 |archive-date=9 January 2021 |access-date=7 June 2019 |website=Nonprofit Explorer |publisher=Pro Publica Inc.}}</ref> All of the organization's products are published as [[Free and open-source software|free and open-source software]].

In November 2023, Meredith Whittaker revealed that she expected the annual cost of running Signal to reach $50 million in 2025, with the current cost estimated around $40 million.<ref>{{cite web |last1=Greenberg |first1=Andy |title=Running Signal Will Soon Cost $50 Million a Year |url=https://www.wired.com/story/signal-operating-costs/ |website=Wired |date=16 November 2023 |access-date=10 December 2023 |archive-date=10 December 2023 |archive-url=https://web.archive.org/web/20231210081135/https://www.wired.com/story/signal-operating-costs/ |url-status=live }}</ref>

==Features==
Signal provides one-to-one and group<ref name="Group Calls">{{cite web |last1=Davenport |first1=Corbin |title=Signal messaging now supports encrypted group video calls |url=https://www.androidpolice.com/2020/12/14/signal-messaging-now-supports-encrypted-group-video-calls/ |website=Android Police |access-date=15 December 2020 |date=14 December 2020 |archive-date=14 December 2020 |archive-url=https://web.archive.org/web/20201214193352/https://www.androidpolice.com/2020/12/14/signal-messaging-now-supports-encrypted-group-video-calls/ |url-status=live}}</ref> voice and video<ref name="Mott-2017-03-14" /> calls with up to forty participants on iOS, Android, and desktop platforms.<ref>{{cite news |last1=Vonau |first1=Manuel |title=Signal significantly ups its video group call limit, surpassing WhatsApp |url=https://www.androidpolice.com/signal-significantly-ups-its-video-group-call-limit-surpassing-whatsapp/ |access-date=16 December 2021 |work=Android Police |date=16 December 2021 |archive-date=16 December 2021 |archive-url=https://web.archive.org/web/20211216101311/https://www.androidpolice.com/signal-significantly-ups-its-video-group-call-limit-surpassing-whatsapp/ |url-status=live }}</ref><ref>{{cite web |last1=Thatcher |first1=Peter |title=How to build large-scale end-to-end encrypted group video calls |url=https://signal.org/blog/how-to-build-encrypted-group-calls/ |website=signal.org |publisher=Signal Foundation |access-date=16 December 2021 |date=15 December 2021 |archive-date=15 December 2021 |archive-url=https://web.archive.org/web/20211215215210/https://signal.org/blog/how-to-build-encrypted-group-calls/ |url-status=live }}</ref> The calls are carried via the devices' wired or wireless (carrier or WiFi) data connections.<ref name="Evans-2014-07-29" /> The application can send text messages, documents files,<ref name="arbitrary-file-types">{{cite tweet |author=Signal |user=signalapp |number=859125874901135360 |date=1 May 2017 |title=Today's Signal release for Android, iOS, and Desktop includes the ability to send arbitrary file types. |access-date=5 November 2018}}</ref> voice notes, pictures, [[GIF]]s,<ref name="giphy-update">{{cite web |last1=Lund |first1=Joshua |title=Expanding Signal GIF search |url=https://signal.org/blog/signal-and-giphy-update/ |publisher=Open Whisper Systems |access-date=9 November 2017 |date=1 November 2017 |archive-date=23 September 2019 |archive-url=https://web.archive.org/web/20190923133209/https://signal.org/blog/signal-and-giphy-update/ |url-status=live}}</ref> and video messages. The platform also supports group messaging.

All communication sessions between Signal users are automatically [[End-to-end encryption|end-to-end encrypted]] (the encryption [[Key (cryptography)|keys]] are generated and stored on the devices, and not on servers).<ref name="eff-2014-11-04" /> To verify that a correspondent is really the person that they claim to be, Signal users can compare key fingerprints (or scan QR codes) [[Out-of-band data|out-of-band]].<ref name="Rottermanner-2015-p5"/> The platform employs a [[Trust on first use|trust-on-first-use]] mechanism to notify the user if a correspondent's key changes.<ref name="Rottermanner-2015-p5"/>

Until 2023, on Android users could opt into making Signal the default SMS/MMS application, allowing them to send and receive unencrypted SMS messages in addition to the standard end-to-end encrypted Signal messages.<ref name="Donohue-2014"/> Users could then use the same application to communicate with contacts who do not have Signal.<ref name="Donohue-2014"/> As of October 2022, this feature has been deprecated due to safety and security concerns, and was removed in 2023.<ref>{{Cite web |last=Clark |first=Mitchell |date=12 October 2022 |title=Signal is 'starting to phase out SMS support' from its Android app |url=https://www.theverge.com/2022/10/12/23400896/signal-sms-support-ending-android-simplification |access-date=13 October 2022 |website=The Verge |language=en-US |archive-date=12 October 2022 |archive-url=https://web.archive.org/web/20221012233521/https://www.theverge.com/2022/10/12/23400896/signal-sms-support-ending-android-simplification |url-status=live }}</ref><ref name="Nina-signal-2022-10-12" />

TextSecure allowed the user to set a passphrase that encrypted the local message database and the user's encryption keys.<ref name="Rottermanner-2015-p9">{{harvnb|Rottermanner|Kieseberg|Huber|Schmiedecker|2015|p=9}}</ref> This did not encrypt the user's contact database or message timestamps.<ref name="Rottermanner-2015-p9"/> The Signal applications on Android and iOS can be locked with the phone's pin, passphrase, or biometric authentication.<ref name="screen-lock">{{cite web |url=https://support.signal.org/hc/en-us/articles/360007059572-Screen-Lock |title=Screen Lock |website=support.signal.org |publisher=Signal |date=n.d. |access-date=22 November 2018 |archive-date=15 February 2020 |archive-url=https://web.archive.org/web/20200215110134/https://support.signal.org/hc/en-us/articles/360007059572-Screen-Lock |url-status=live}}</ref> The user can define a "screen lock timeout" interval, providing an additional protection mechanism in case the phone is lost or stolen.<ref name="Rottermanner-2015-p5">{{harvnb|Rottermanner|Kieseberg|Huber|Schmiedecker|2015|p=5}}</ref><ref name="screen-lock"/>

Signal has a feature for scheduling messages.<ref>{{Cite web |url=https://support.signal.org/hc/en-us/articles/5365881590682-Schedule-a-Message-on-Signal-Android |title=Schedule a Message on Signal Android – Signal Support |access-date=18 May 2023 |archive-date=18 May 2023 |archive-url=https://web.archive.org/web/20230518110700/https://support.signal.org/hc/en-us/articles/5365881590682-Schedule-a-Message-on-Signal-Android |url-status=live }}</ref> In addition, timers may be attached to messages<ref name="Greenberg-2016-10-11"/> to automatically delete the messages from both the sender's and the receivers' devices.<ref name="Greenberg-2016-10-11">{{cite magazine |last1=Greenberg |first1=Andy |title=Signal, the Cypherpunk App of Choice, Adds Disappearing Messages |url=https://www.wired.com/2016/10/signal-cypherpunk-app-choice-adds-disappearing-messages/ |magazine=Wired |access-date=11 October 2016 |date=11 October 2016 |archive-date=12 October 2016 |archive-url=https://web.archive.org/web/20161012075856/https://www.wired.com/2016/10/signal-cypherpunk-app-choice-adds-disappearing-messages/ |url-status=live}}</ref> The time period for keeping the message may be between five seconds and one week,<ref name="Greenberg-2016-10-11"/> and begins for each recipient once they have read their copy of the message.<ref name="disappearing-messages">{{cite web |last1=Marlinspike |first1=Moxie |title=Disappearing messages for Signal |url=https://www.whispersystems.org/blog/disappearing-messages/ |publisher=Open Whisper Systems |access-date=11 October 2016 |date=11 October 2016 |archive-date=13 June 2017 |archive-url=https://web.archive.org/web/20170613112209/https://whispersystems.org/blog/disappearing-messages/ |url-status=live}}</ref> The developers stressed that this is meant to be "a collaborative feature for conversations where all participants want to automate minimal data hygiene, not for situations where the recipient is an adversary".<ref name="Greenberg-2016-10-11"/><ref name="disappearing-messages"/>

Signal's app icon may be changed from with a variety of colour themes for customization and the application name can also be customized.<ref>{{cite web |url=https://github.com/signalapp/Signal-Android/commit/c963e99dca76d8d334c7dfbf0d062305daebfaa2 |title=Introduce the ability to change the app icon. · signalapp/Signal-Android@c963e99 |website=[[GitHub]] |access-date=19 May 2023 |archive-date=19 May 2023 |archive-url=https://web.archive.org/web/20230519163414/https://github.com/signalapp/Signal-Android/commit/c963e99dca76d8d334c7dfbf0d062305daebfaa2 |url-status=live }}</ref> Upcoming features includes hiding spoilers<ref>{{cite web |url=https://github.com/signalapp/Signal-Desktop/commit/d9d820e72aeaf2175cee658e3325d58b63f17082 |title=Support for receiving formatted messages · signalapp/Signal-Desktop@d9d820e |website=[[GitHub]] |access-date=18 May 2023 |archive-date=18 May 2023 |archive-url=https://web.archive.org/web/20230518110659/https://github.com/signalapp/Signal-Desktop/commit/d9d820e72aeaf2175cee658e3325d58b63f17082 |url-status=live }}</ref> as well adding other users via QR code.<ref>{{cite web |url=https://github.com/signalapp/Signal-Android/commit/855e194baaa50de00f90bd7cb7e3f4ac10b10b9c |title=Add initial username link screen + QR code generation. · signalapp/Signal-Android@855e194 |website=[[GitHub]] |access-date=18 May 2023 |archive-date=18 May 2023 |archive-url=https://web.archive.org/web/20230518110700/https://github.com/signalapp/Signal-Android/commit/855e194baaa50de00f90bd7cb7e3f4ac10b10b9c |url-status=live }}</ref>

Signal excludes users' messages from non-encrypted cloud backups by default.<ref name="Lee-2016-06-22"/>

Signal allows users to automatically blur faces of people in photos to protect identities.<ref>{{Cite web |last=O'Flaherty |first=Kate |title=Signal Will Now Blur Protesters' Faces: Here's How It Works |url=https://www.forbes.com/sites/kateoflahertyuk/2020/06/04/secure-messaging-app-signal-launches-new-feature-to-protect-protestors-identities/ |access-date=5 June 2020 |website=Forbes |language=en |archive-date=4 June 2020 |archive-url=https://web.archive.org/web/20200604123732/https://www.forbes.com/sites/kateoflahertyuk/2020/06/04/secure-messaging-app-signal-launches-new-feature-to-protect-protestors-identities/ |url-status=live}}</ref><ref>{{Cite web |last=Vincent |first=James |date=4 June 2020 |title=Signal announces new face-blurring tool for Android and iOS |url=https://www.theverge.com/2020/6/4/21280112/signal-face-blurring-tool-ios-android-update |access-date=5 June 2020 |website=The Verge |language=en |archive-date=4 June 2020 |archive-url=https://web.archive.org/web/20200604123950/https://www.theverge.com/2020/6/4/21280112/signal-face-blurring-tool-ios-android-update |url-status=live}}</ref>

Signal includes a [[Finance:Cryptocurrency wallet|cryptocurrency wallet]] functionality for storing, sending and receiving in-app payments.<ref name="payments-support">{{cite web |title=In-app Payments |url=https://support.signal.org/hc/en-us/articles/360057625692-In-app-Payments |website=support.signal.org |publisher=Signal Messenger |access-date=12 January 2022 |archive-date=9 January 2022 |archive-url=https://web.archive.org/web/20220109072526/https://support.signal.org/hc/en-us/articles/360057625692-In-app-Payments |url-status=live }}</ref> Apart from certain regions and countries,<ref name="payments-support"/> the feature was enabled globally in November 2021.<ref name="Greenberg-2022-01-06">{{cite magazine |last1=Greenberg |first1=Andy |title=Signal's Cryptocurrency Feature Has Gone Worldwide |url=https://www.wired.com/story/signal-mobilecoin-cryptocurrency-payments/ |magazine=Wired |access-date=12 January 2022 |date=6 January 2022 |archive-date=11 January 2022 |archive-url=https://web.archive.org/web/20220111220625/https://www.wired.com/story/signal-mobilecoin-cryptocurrency-payments/ |url-status=live }}</ref> {{As of|January 2022}}, the only supported payment method is [[Finance:MobileCoin|MobileCoin]].<ref name="payments-support"/>

==Limitations==
Signal requires that the user provide a telephone number for verification,<ref name="phone-number">{{cite web |first=Masha |last=Kolenkina |title=Will any phone number work? How do I get a verification number? |url=http://support.whispersystems.org/hc/en-us/articles/212476118-Will-any-phone-number-work-How-do-I-get-a-verification-number- |publisher=Open Whisper Systems |date=20 November 2015 |access-date=20 December 2015 |archive-date=16 January 2017 |archive-url=https://web.archive.org/web/20170116181641/https://support.whispersystems.org/hc/en-us/articles/212476118-Will-any-phone-number-work-How-do-I-get-a-verification-number- |url-status=live}}</ref> eliminating the need for user names or passwords and facilitating contact discovery (see below).<ref name=":0">{{Cite web |url=https://theintercept.com/2017/09/28/signal-tutorial-second-phone-number/ |title=How to Use Signal Without Giving Out Your Phone Number |last=Lee |first=Micah |date=28 September 2017 |website=The Intercept |language=en-US |access-date=25 February 2018 |archive-date=22 January 2020 |archive-url=https://web.archive.org/web/20200122040137/https://theintercept.com/2017/09/28/signal-tutorial-second-phone-number/ |url-status=live}}</ref> The number does not have to be the same as on the device's SIM card; it can also be a VoIP number<ref name="phone-number" /> or a landline as long as the user can receive the verification code and have a separate device to set up the software. A number can only be registered on one mobile device at a time.<ref name="multiple-devices">{{cite web |title=Troubleshooting multiple devices |url=https://support.signal.org/hc/en-us/articles/360007320451-Troubleshooting-multiple-devices#desktop_multiple_device |url-status=live |archive-url=https://web.archive.org/web/20191220053903/https://support.signal.org/hc/en-us/articles/360007320451-Troubleshooting-multiple-devices#desktop_multiple_device |archive-date=20 December 2019 |access-date=20 March 2019 |website=support.signal.org |publisher=Signal Messenger LLC}}</ref> Account registration requires an iOS or Android device.<ref name="Softpedia-2021"/><ref name="VPNpro-2021"/>

This mandatory connection to a telephone number (a feature Signal shares with [[Software:WhatsApp|WhatsApp]], [[Software:KakaoTalk|KakaoTalk]], and others) has been criticized as a "major issue" for privacy-conscious users who are not comfortable with giving out their private number.<ref name=":0" /> A workaround is to use a secondary phone number.<ref name=":0" /> The ability to choose a public, changeable username instead of sharing one's phone number is a widely-requested feature.<ref name=":0" /><ref>{{Cite web |url=https://github.com/signalapp/Signal-Android/issues/1085 |title=Allow different kinds of identifiers for registration · Issue #1085 · signalapp/Signal-Android |website=GitHub |language=en |access-date=25 February 2018 |archive-date=15 February 2020 |archive-url=https://web.archive.org/web/20200215082751/https://github.com/signalapp/Signal-Android/issues/1085 |url-status=live}}</ref><ref>{{cite web |title=Discussion: A proposal for alternative primary identifiers |url=https://community.signalusers.org/t/a-proposal-for-alternative-primary-identifiers/3023 |website=Signal Community |access-date=26 June 2020 |date=24 May 2018 |archive-date=28 June 2020 |archive-url=https://web.archive.org/web/20200628045054/https://community.signalusers.org/t/a-proposal-for-alternative-primary-identifiers/3023 |url-status=live}}</ref>

Using phone numbers as identifiers may also create security risks that arise from the possibility of an attacker taking over a phone number.<ref name=":0" /> A similar vulnerability was used to attack at least one user in August 2022, though the attack was performed via the provider of Signal's SMS services, not any user's provider.<ref name="Twilio-2022-08-15" /> The threat of this attack can be mitigated by enabling Signal's Registration Lock feature, a form of [[Multi-factor authentication|two-factor authentication]] that requires the user to enter a PIN to register the phone number on a new device.<ref>{{Cite web |last=Lovejoy |first=Ben |date=21 May 2020 |title=Signal registration lock stops your account being hijacked on a new phone |url=https://9to5mac.com/2020/05/21/signal-registration-lock/ |access-date=27 May 2023 |website=9 to 5 Mac |archive-date=13 December 2023 |archive-url=https://web.archive.org/web/20231213150314/https://9to5mac.com/2020/05/21/signal-registration-lock/ |url-status=live }}</ref>

When linking Signal Desktop to a mobile device, the conversations history will not be synced; only the new messages will be shown on Signal Desktop.<ref>{{Cite web |last=Fedewa |first=Joe |title=How to Use Signal on Your Desktop Computer |url=https://www.howtogeek.com/710253/how-to-use-signal-on-desktop/ |access-date=31 May 2022 |website=How-To Geek |date=27 January 2021 |language=en-US |archive-date=25 June 2022 |archive-url=https://web.archive.org/web/20220625045542/https://www.howtogeek.com/710253/how-to-use-signal-on-desktop/ |url-status=live }}</ref>

==Usability==
In July 2016, the [[Organization:Internet Society|Internet Society]] published a user study that assessed the ability of Signal users to detect and deter [[Man-in-the-middle attack|man-in-the-middle attack]]s.<ref name="Schröder-2016">{{harvnb|Schröder|Huber|Wind|Rottermanner|2016}}</ref> The study concluded that 21 out of 28 participants failed to correctly compare [[Public key fingerprint|public key fingerprint]]s in order to verify the identity of other Signal users, and that most of these users believed they had succeeded, while they had actually failed.<ref name="Schröder-2016"/> Four months later, Signal's user interface was updated to make verifying the identity of other Signal users simpler.<ref name="safety-number-updates">{{cite web |last1=Marlinspike |first1=Moxie |title=Safety number updates |url=https://whispersystems.org/blog/safety-number-updates/ |publisher=Open Whisper Systems |access-date=17 July 2017 |date=17 November 2016 |archive-date=17 July 2017 |archive-url=https://web.archive.org/web/20170717080736/https://whispersystems.org/blog/safety-number-updates/ |url-status=live}}</ref>

In 2023, the French government is pushing for the adoption of a European encrypted messaging alternative to Signal and WhatsApp named Olvid as their secured platform for communications.<ref>{{cite news |last1=Elizabeth |first1=Pineau |title=Stop using WhatsApp, get Paris-made alternative, French PM tells ministers |url=https://www.reuters.com/world/europe/stop-using-whatsapp-get-paris-made-alternative-french-pm-tells-ministers-2023-11-29/ |agency=Reuters |date=29 November 2023 |access-date=3 December 2023 |archive-date=1 December 2023 |archive-url=https://web.archive.org/web/20231201144238/https://www.reuters.com/world/europe/stop-using-whatsapp-get-paris-made-alternative-french-pm-tells-ministers-2023-11-29/ |url-status=live }}</ref>

==Architecture==
===Encryption protocols===
{{Main|Signal Protocol}}
Signal messages are encrypted with the Signal Protocol (formerly known as the TextSecure Protocol). The protocol combines the [[Double Ratchet Algorithm]], prekeys, and an Extended Triple [[Diffie–Hellman key exchange|Diffie–Hellman]] (X3DH) handshake.<ref>{{harvnb|Unger|Dechand|Bonneau|Fahl|2015|p=241}}</ref><ref>{{cite web |last1=Marlinspike |first1=Moxie |last2=Perrin |first2=Trevor |title=The X3DH Key Agreement Protocol |url=https://signal.org/docs/specifications/x3dh/ |website=signal.org |access-date=24 December 2020 |archive-date=23 November 2020 |archive-url=https://web.archive.org/web/20201123155217/https://signal.org/docs/specifications/x3dh/ |url-status=live}}</ref> It uses [[Curve25519]], AES-256, and HMAC-SHA256 as [[Cryptographic primitive|primitives]].<ref name="Frosch 2016">{{harvnb|Frosch|Mainka|Bader|Bergsma|2016}}</ref> The protocol provides confidentiality, integrity, [[Authenticated encryption|authentication]], participant consistency, destination validation, [[Forward secrecy|forward secrecy]], backward secrecy ({{Aka}} future secrecy), causality preservation, message unlinkability, [[Deniable authentication|message repudiation]], participation repudiation, and [[Asynchronous communication|asynchronicity]].<ref name="Unger-2015-p239"/> It does not provide anonymity preservation, and requires servers for the relaying of messages and storing of public key material.<ref name="Unger-2015-p239">{{harvnb|Unger|Dechand|Bonneau|Fahl|2015|p=239}}</ref>

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.<ref name="Unger-2015-p239"/> 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.<ref name="Unger-2015-p239"/>

In October 2014, researchers from [[Organization:Ruhr University Bochum|Ruhr University Bochum]] published an analysis of the Signal Protocol.<ref name="Frosch 2016"/> Among other findings, they presented an [[Unknown key-share attack|unknown key-share attack]] on the protocol, but in general, they found that it was secure.<ref name="Pauli-2014-11-03" /> In October 2016, researchers from UK's [[Organization:University of Oxford|University of Oxford]], Queensland University of Technology in Australia, and Canada's McMaster University published a formal analysis of the protocol.<ref name="Brook-2016-11-10"/><ref name="Cohn-Gordon-2016"/> They concluded that the protocol was cryptographically sound.<ref name="Brook-2016-11-10">{{cite web |last1=Brook |first1=Chris |title=Signal Audit Reveals Protocol Cryptographically Sound |url=https://threatpost.com/signal-audit-reveals-protocol-cryptographically-sound/121892/ |website=Threatpost |publisher=Kaspersky Lab |access-date=11 November 2016 |date=10 November 2016 |archive-date=14 February 2017 |archive-url=https://web.archive.org/web/20170214222434/https://threatpost.com/signal-audit-reveals-protocol-cryptographically-sound/121892/ |url-status=live}}</ref><ref name="Cohn-Gordon-2016">{{harvnb|Cohn-Gordon|Cremers|Dowling|Garratt|2016}}</ref> In July 2017, researchers from [[Organization:Ruhr University Bochum|Ruhr University Bochum]] found during another analysis of group messengers a purely theoretic attack against the group protocol of Signal: A user who knows the secret group ID of a group (due to having been a group member previously or stealing it from a member's device) can become a member of the group. Since the group ID cannot be guessed and such member changes are displayed to the remaining members, this attack is likely to be difficult to carry out without being detected.<ref>{{Cite journal |last1=Rösler |first1=Paul |last2=Mainka |first2=Christian |last3=Schwenk |first3=Jörg |year=2017 |title=More is Less: On the End-to-End Security of Group Chats in Signal, WhatsApp, and Threema |journal=3rd IEEE European Symposium on Security and Privacy |url=https://eprint.iacr.org/2017/713.pdf |access-date=1 November 2019 |archive-date=19 November 2019 |archive-url=https://web.archive.org/web/20191119140438/https://eprint.iacr.org/2017/713.pdf |url-status=live}}</ref>

{{As of|August 2018}}, the Signal Protocol has been implemented into [[Software:WhatsApp|WhatsApp]], [[Software:Facebook Messenger|Facebook Messenger]], [[Software:Skype|Skype]],<ref name="skype">{{cite web |last1=Lund |first1=Joshua |title=Signal partners with Microsoft to bring end-to-end encryption to Skype |url=https://signal.org/blog/skype-partnership/ |publisher=Open Whisper Systems |access-date=17 January 2018 |date=11 January 2018 |archive-date=2 February 2020 |archive-url=https://web.archive.org/web/20200202152037/https://signal.org/blog/skype-partnership/ |url-status=live}}</ref> and [[Software:Google Allo|Google Allo]],<ref name="allo">{{cite web |url=https://signal.org/blog/allo/ |title=Open Whisper Systems partners with Google on end-to-end encryption for Allo |last=Marlinspike |first=Moxie |publisher=Open Whisper Systems |date=18 May 2016 |access-date=22 August 2018 |archive-date=22 January 2020 |archive-url=https://web.archive.org/web/20200122102117/https://signal.org/blog/allo/ |url-status=live}}</ref> making it possible for the conversations of "more than a billion people worldwide" to be end-to-end encrypted.<ref>{{cite web |url=http://fortune.com/40-under-40/moxie-marlinspike-31/ |title=Moxie Marlinspike - 40 under 40 |year=2016 |website=Fortune |access-date=6 October 2016 |archive-date=3 February 2017 |archive-url=https://web.archive.org/web/20170203011735/http://fortune.com/40-under-40/moxie-marlinspike-31/ |url-status=live}}</ref> In Google Allo, Skype and Facebook Messenger, conversations are not encrypted with the Signal Protocol by default; they only offer end-to-end encryption in an optional mode.<ref name="Lee-2016-06-22"/><ref name="facebook-messenger">{{cite web |last1=Marlinspike |first1=Moxie |title=Facebook Messenger deploys Signal Protocol for end to end encryption |url=https://whispersystems.org/blog/facebook-messenger/ |publisher=Open Whisper Systems |access-date=10 May 2017 |date=8 July 2016 |archive-date=1 July 2017 |archive-url=https://web.archive.org/web/20170701124315/https://whispersystems.org/blog/facebook-messenger/ |url-status=live}}</ref><ref name="skype" /><ref>{{cite web |last1=Gebhart |first1=Gennie |title=Google's Allo Sends The Wrong Message About Encryption |url=https://www.eff.org/deeplinks/2016/09/googles-allo-sends-wrong-message-about-encryption |publisher=Electronic Frontier Foundation |access-date=20 August 2018 |date=3 October 2016 |archive-date=30 August 2018 |archive-url=https://web.archive.org/web/20180830024739/https://www.eff.org/deeplinks/2016/09/googles-allo-sends-wrong-message-about-encryption |url-status=live}}</ref>

Up until March 2017, Signal's voice calls were encrypted with [[Secure Real-time Transport Protocol|SRTP]] and the [[ZRTP]] key-agreement protocol, which was developed by Phil Zimmermann.<ref name="Greenberg-2014-07-29" /><ref name="RedPhone-Encryption-Protocols"/> In March 2017, Signal transitioned to a new [[Software:WebRTC|WebRTC]]-based calling system that introduced the ability to make video calls.<ref name="signal-video-calls">{{cite web |url=https://whispersystems.org/blog/signal-video-calls/ |title=Video calls for Signal out of beta |last1=Marlinspike |first1=Moxie |date=13 March 2017 |website=Signal Blog |publisher=[[Organization:Open Whisper Systems|Open Whisper Systems]] |access-date=7 April 2017 |archive-date=15 March 2017 |archive-url=https://web.archive.org/web/20170315175109/https://whispersystems.org/blog/signal-video-calls/ |url-status=live}}</ref> Signal's voice and video calling functionalities use the Signal Protocol channel for authentication instead of ZRTP.<ref name="Greenberg-2017-02-14">{{cite magazine |last1=Greenberg |first1=Andy |title=The Best Encrypted Chat App Now Does Video Calls Too |url=https://www.wired.com/2017/02/encryption-app-signal-enables-video-calls-new-privacy-tradeoff/ |magazine=Wired |access-date=15 February 2017 |date=14 February 2017 |archive-date=15 February 2017 |archive-url=https://web.archive.org/web/20170215021847/https://www.wired.com/2017/02/encryption-app-signal-enables-video-calls-new-privacy-tradeoff/ |url-status=live}}</ref><ref name="signal-video-calls-beta">{{cite web |last1=Marlinspike |first1=Moxie |title=Video calls for Signal now in public beta |url=https://whispersystems.org/blog/signal-video-calls-beta/ |publisher=Open Whisper Systems |access-date=15 February 2017 |date=14 February 2017 |archive-date=15 March 2017 |archive-url=https://web.archive.org/web/20170315184106/https://whispersystems.org/blog/signal-video-calls-beta// |url-status=live}}</ref><ref name="Mott-2017-03-14"/>

====Authentication====
To verify that a correspondent is really the person that they claim to be, Signal users can compare key fingerprints (or scan QR codes) [[Out-of-band data|out-of-band]].<ref name="Rottermanner-2015-p5"/> The platform employs a [[Trust on first use|trust on first use]] mechanism in order to notify the user if a correspondent's key changes.<ref name="Rottermanner-2015-p5"/>

===Local storage===
After receiving and decrypting messages, the application stored them locally on each device in a [[Software:SQLite|SQLite]] database that is encrypted with SQLCipher.<ref name="Ganor-2020-12-10"/> The cryptographic key for this database is also stored locally and can be accessed if the device is unlocked.<ref name="Ganor-2020-12-10"/><ref name="Green-2020-12-10">{{cite tweet |author=Matthew D. Green |user=matthew_d_green |number=1337106648016547843 |date=10 December 2020 |title=Someone asked me what this Cellebrite post meant, and whether it's a big deal for Signal. From what I can see it just means Cellebrite can read your texts if they have your (unlocked) phone, which, duh. |access-date=22 December 2020}}</ref> In December 2020, [[Company:Cellebrite|Cellebrite]] published a blog post announcing that one of their products could now access this key and use it to "decrypt the Signal app".<ref name="Ganor-2020-12-10">{{cite web |last1=Ganor |first1=Alon |title=Cellebrite's New Solution for Decrypting the Signal App |publisher=Cellebrite |access-date=22 December 2020 |url=https://www.cellebrite.com/en/blog/cellebrites-new-solution-for-decrypting-the-signal-app/ |archive-url=https://web.archive.org/web/20201210150311/https://www.cellebrite.com/en/blog/cellebrites-new-solution-for-decrypting-the-signal-app/ |archive-date=10 December 2020 |date=10 December 2020}}</ref><ref name="Snowden-2020-12-15">{{cite tweet |author=Edward Snowden |user=Snowden |number=1338859880384368641 |date=15 December 2020 |title=No, Cellebrite cannot decrypt Signal communications. What they sell is a forensic device cops connect to insecure, unlockable phones to download a bunch of popular apps' data more easily than doing it manually. They just added Signal to that app list. That's it. There's no magic. |access-date=22 December 2020}}</ref> Technology reporters later published articles about how Cellebrite had claimed to have the ability to "break into the Signal app" and "crack Signal's encryption".<ref>{{cite web |last1=Benjakob |first1=Omer |title=Israeli Phone-hacking Firm Claims It Can Now Break Into Encrypted Signal App |url=https://www.haaretz.com/israel-news/tech-news/.premium-israeli-spy-tech-firm-says-it-can-break-into-signal-app-previously-considered-safe-1.9368581 |work=Haaretz |access-date=22 December 2020 |date=14 December 2020 |archive-date=21 December 2020 |archive-url=https://web.archive.org/web/20201221224157/https://www.haaretz.com/israel-news/tech-news/.premium-israeli-spy-tech-firm-says-it-can-break-into-signal-app-previously-considered-safe-1.9368581 |url-status=live}}</ref><ref>{{Cite news |last=Wakefield |first=Jane |date=22 December 2020 |title=Signal: Cellebrite claimed to have cracked chat app's encryption |language=en-GB |work=BBC News |url=https://www.bbc.com/news/technology-55412230 |access-date=22 December 2020 |archive-date=24 December 2020 |archive-url=https://web.archive.org/web/20201224104727/https://www.bbc.com/news/technology-55412230 |url-status=live}}</ref> This latter interpretation was rejected by several experts,<ref>{{unbulleted list|{{cite tweet |author=Matthew D. Green |user=matthew_d_green |number=1337106648016547843 |date=10 December 2020 |title=Someone asked me what this Cellebrite post meant, and whether it's a big deal for Signal. From what I can see it just means Cellebrite can read your texts if they have your (unlocked) phone, which, duh. |access-date=22 December 2020}}|{{cite tweet |author=Edward Snowden |user=Snowden |number=1338859880384368641 |date=15 December 2020 |title=No, Cellebrite cannot decrypt Signal communications. What they sell is a forensic device cops connect to insecure, unlockable phones to download a bunch of popular apps' data more easily than doing it manually. They just added Signal to that app list. That's it. There's no magic. |access-date=22 December 2020}}|{{cite web |author1=Bruce Schneier |title=Cellebrite Can Break Signal |url=https://www.schneier.com/blog/archives/2020/12/cellebrite-can-break-signal.html |website=Schneier on Security |access-date=24 December 2020 |date=21 December 2020 |quote=I finally got the chance to read all of this more carefully, and it seems that all Cellebrite is doing is reading the texts off of a phone they can already access. To [''sic''] this has nothing to do with Signal at all. So: never mind. |archive-date=24 December 2020 |archive-url=https://web.archive.org/web/20201224002854/https://www.schneier.com/blog/archives/2020/12/cellebrite-can-break-signal.html |url-status=live}}|{{cite tweet |author=John Scott-Railton (Citizen Lab) |user=jsrailton |number=1341421365371559938 |date=22 December 2020 |title=Dangerous misunderstanding spreading that @Cellebrite_UFED can  'crack' & intercept @signalapp. This is FALSE. Modest reality: Cellebrite claimed to extract the Signal chat database from an unlocked Android in their possession. |access-date=24 December 2020}} }}</ref> as well as representatives from Signal, who said the original post by Cellebrite had been about accessing data on "an unlocked Android phone in their physical possession" and that they "could have just opened the app to look at the messages".<ref name="Marlinspike-2020-12-23">{{cite web |last1=Marlinspike |first1=Moxie |title=No, Cellebrite cannot 'break Signal encryption.' |url=https://signal.org/blog/cellebrite-and-clickbait/ |website=signal.org |publisher=Signal Messenger |access-date=24 December 2020 |date=23 December 2020 |archive-date=24 December 2020 |archive-url=https://web.archive.org/web/20201224002643/https://signal.org/blog/cellebrite-and-clickbait/ |url-status=live}}</ref><ref>{{cite tweet |author=Signal |user=signalapp |number=1339362791153487874 |date=17 December 2020 |title=No, Haaretz was duped. The original blog post was about accessing data on an unlocked Android phone in their physical possession. They could have just opened the app to look at the messages. |access-date=22 December 2020}}</ref> Similar extraction tools also exist for iOS devices and Signal Desktop.<ref>{{cite news |last1=Betz |first1=Bradford |title=Court documents show FBI may have tool to access private Signal messages on locked iPhones |url=https://www.foxbusiness.com/technology/fbi-tool-access-private-signal-messages-locked-iphones |access-date=14 February 2021 |work=FOXBusiness |date=9 February 2021 |archive-date=14 February 2021 |archive-url=https://web.archive.org/web/20210214003438/https://www.foxbusiness.com/technology/fbi-tool-access-private-signal-messages-locked-iphones |url-status=live }}</ref><ref>{{Cite web |last=Abrams |first=Lawrence |date=23 October 2018 |title=Signal Desktop Leaves Message Decryption Key in Plain Sight |url=https://www.bleepingcomputer.com/news/security/signal-desktop-leaves-message-decryption-key-in-plain-sight/ |url-status=live |archive-url=https://archive.today/20190327174001/https://www.bleepingcomputer.com/news/security/signal-desktop-leaves-message-decryption-key-in-plain-sight/ |archive-date=27 March 2019 |access-date=19 March 2021 |website=BleepingComputer |language=en-us}}</ref>

===Servers===
Signal relies on [[Centralized computing|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 [[Key exchange|exchange]] of users' [[Public-key cryptography|public keys]]. By default, Signal's voice and video calls are [[Peer-to-peer|peer-to-peer]].<ref name="Mott-2017-03-14"/> 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 address]]es.<ref name="Mott-2017-03-14"/>

====Contact discovery====
The servers store registered users' phone numbers, public key material and push tokens which are necessary for setting up calls and transmitting messages.<ref name="privacy-policy">{{cite web |title=Privacy Policy |url=https://signal.org/legal/#privacy-policy |publisher=Signal Messenger LLC. |access-date=24 June 2018 |date=25 May 2018 |archive-date=24 June 2018 |archive-url=https://web.archive.org/web/20180624175800/https://signal.org/legal/#privacy-policy |url-status=live}}</ref> In order to determine which contacts are also Signal users, [[Cryptographic hash function|cryptographic hashes]] of the user's contact numbers are periodically transmitted to the server.<ref name="contact-discovery" /> The server then checks to see if those match any of the SHA256 hashes of registered users and tells the client if any matches are found.<ref name="contact-discovery" /> The hashed numbers are thereafter discarded from the server.<ref name="privacy-policy"/> In 2014, Moxie Marlinspike wrote that it is easy to calculate a map of all possible hash inputs to hash outputs and reverse the mapping because of the limited preimage space (the set of all possible hash inputs) of phone numbers, and that a "practical privacy preserving contact discovery remains an unsolved problem."<ref name="Rottermanner-2015-p4">{{harvnb|Rottermanner|Kieseberg|Huber|Schmiedecker|2015|p=4}}</ref><ref name="contact-discovery">{{cite web |url=https://whispersystems.org/blog/contact-discovery/ |title=The Difficulty Of Private Contact Discovery |author=Moxie Marlinspike |date=3 January 2013 |publisher=Open Whisper Systems |access-date=14 January 2016 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304015510/https://whispersystems.org/blog/contact-discovery/ |url-status=live}}</ref> In September 2017, Signal's developers announced that they were working on a way for the Signal client applications to "efficiently and scalably determine whether the contacts in their address book are Signal users without revealing the contacts in their address book to the Signal service."<ref name="private-contact-discovery">{{cite web |last1=Marlinspike |first1=Moxie |title=Technology preview: Private contact discovery for Signal |url=https://signal.org/blog/private-contact-discovery/ |publisher=Open Whisper Systems |access-date=28 September 2017 |date=26 September 2017 |archive-date=28 September 2017 |archive-url=https://web.archive.org/web/20170928043707/https://signal.org/blog/private-contact-discovery/ |url-status=live}}</ref><ref>{{cite magazine |last1=Greenberg |first1=Andy |title=Signal Has a Fix for Apps' Contact-Leaking Problem |url=https://www.wired.com/story/signal-contact-lists-private-secure-enclave |magazine=Wired |access-date=28 September 2017 |date=26 September 2017 |archive-date=27 September 2017 |archive-url=https://web.archive.org/web/20170927143335/https://www.wired.com/story/signal-contact-lists-private-secure-enclave |url-status=live}}</ref>

====Metadata====
All client-server communications are protected by [[Transport Layer Security|TLS]].<ref name="RedPhone-Encryption-Protocols">{{cite web |url=https://github.com/WhisperSystems/RedPhone/wiki/Encryption-Protocols |archive-url=https://web.archive.org/web/20150905192057/https://github.com/WhisperSystems/RedPhone/wiki/Encryption-Protocols |archive-date=5 September 2015 |title=Encryption Protocols |website=GitHub |last=Marlinspike |first=Moxie |date=17 July 2012 |access-date=8 January 2016}}</ref><ref>{{harvnb|Frosch|Mainka|Bader|Bergsma|2016|p=7}}</ref> Signal's developers have asserted that their servers do not keep logs about who called whom and when.<ref name="Brandom-2014-07-29" /> In June 2016, Marlinspike told ''The Intercept'' that "the closest piece of information to metadata that the Signal server stores is the last time each user connected to the server, and the precision of this information is reduced to the day, rather than the hour, minute, and second".<ref name="Lee-2016-06-22">{{cite web |last1=Lee |first1=Micah |title=Battle of the Secure Messaging Apps: How Signal Beats WhatsApp |url=https://theintercept.com/2016/06/22/battle-of-the-secure-messaging-apps-how-signal-beats-whatsapp/ |website=The Intercept |publisher=First Look Media |access-date=6 September 2016 |date=22 June 2016 |archive-date=19 February 2017 |archive-url=https://web.archive.org/web/20170219051224/https://theintercept.com/2016/06/22/battle-of-the-secure-messaging-apps-how-signal-beats-whatsapp/ |url-status=live}}</ref>

The group messaging mechanism is designed so that the servers do not have access to the membership list, group title, or group icon.<ref name="Rottermanner-2015-p3"/> Instead, the creation, updating, joining, and leaving of groups is done by the clients, which deliver pairwise messages to the participants in the same way that one-to-one messages are delivered.<ref name="private-groups">{{cite web |url=https://whispersystems.org/blog/private-groups/ |title=Private Group Messaging |publisher=Open Whisper Systems |author=Moxie Marlinspike |date=5 May 2014 |access-date=9 July 2014 |archive-date=14 July 2014 |archive-url=https://web.archive.org/web/20140714133409/https://whispersystems.org/blog/private-groups/ |url-status=live}}</ref><ref name="the-new-textsecure">{{cite web |url=https://whispersystems.org/blog/the-new-textsecure/ |title=The New TextSecure: Privacy Beyond SMS |author=Moxie Marlinspike |date=24 February 2014 |publisher=Open Whisper Systems |access-date=26 February 2014 |archive-date=24 February 2014 |archive-url=https://web.archive.org/web/20140224200749/https://whispersystems.org/blog/the-new-textsecure/ |url-status=live}}</ref>

====Federation====
Signal's server architecture was [[Engineering:Federation (information technology)|federated]] between December 2013 and February 2016. In December 2013, it was announced that the messaging protocol Signal uses had successfully been integrated into the Android-based open-source operating system [[Software:CyanogenMod|CyanogenMod]].<ref>{{cite news |author=Andy Greenberg |url=https://www.forbes.com/sites/andygreenberg/2013/12/09/ten-million-more-android-users-text-messages-will-soon-be-encrypted-by-default/ |title=Ten Million More Android Users' Text Messages Will Soon Be Encrypted By Default |work=Forbes |date=9 December 2013 |access-date=28 February 2014 |archive-date=4 March 2014 |archive-url=https://web.archive.org/web/20140304100716/http://www.forbes.com/sites/andygreenberg/2013/12/09/ten-million-more-android-users-text-messages-will-soon-be-encrypted-by-default/ |url-status=live}}</ref><ref>{{cite news |author=Seth Schoen |url=https://www.eff.org/deeplinks/2013/12/2013-review-encrypting-web-takes-huge-leap-forward |title=2013 in Review: Encrypting the Web Takes A Huge Leap Forward |publisher=Electronic Frontier Foundation |date=28 December 2013 |access-date=1 March 2014 |archive-date=1 February 2014 |archive-url=https://web.archive.org/web/20140201093013/https://www.eff.org/deeplinks/2013/12/2013-review-encrypting-web-takes-huge-leap-forward |url-status=live}}</ref><ref name="cyanogen-integration" /> Since CyanogenMod 11.0, the client logic was contained in a system app called WhisperPush. According to Signal's developers, the Cyanogen team ran their own Signal messaging server for WhisperPush clients, which federated with the main server, so that both clients could exchange messages with each other.<ref name="cyanogen-integration">{{cite news |author=Moxie Marlinspike |url=https://whispersystems.org/blog/cyanogen-integration/ |title=TextSecure, Now With 10 Million More Users |publisher=Open Whisper Systems |date=9 December 2013 |access-date=28 February 2014 |archive-date=25 February 2014 |archive-url=https://web.archive.org/web/20140225042302/https://www.whispersystems.org/blog/cyanogen-integration/ |url-status=live}}</ref> The WhisperPush source code was available under the GPLv3 license.<ref>{{cite web |url=https://github.com/CyanogenMod/android_external_whispersystems_WhisperPush |title=android_external_whispersystems_WhisperPush |author=CyanogenMod |website=GitHub |date=7 January 2014 |access-date=26 March 2015 |archive-date=28 June 2015 |archive-url=https://web.archive.org/web/20150628011341/https://github.com/CyanogenMod/android_external_whispersystems_WhisperPush |url-status=live}}</ref> In February 2016, the CyanogenMod team discontinued WhisperPush and recommended that its users switch to Signal.<ref>{{cite web |last1=Sinha |first1=Robin |title=CyanogenMod to Shutter WhisperPush Messaging Service on February 1 |url=http://gadgets.ndtv.com/mobiles/news/cyanogenmod-to-shutter-whisperpush-messaging-service-on-february-1-792064 |website=Gadgets360 |publisher=NDTV |access-date=23 January 2016 |date=20 January 2016 |archive-date=11 October 2016 |archive-url=https://web.archive.org/web/20161011034310/http://gadgets.ndtv.com/mobiles/news/cyanogenmod-to-shutter-whisperpush-messaging-service-on-february-1-792064 |url-status=live}}</ref> In May 2016, Moxie Marlinspike wrote that federation with the CyanogenMod servers had degraded the user experience and held back development, and that their servers will probably not federate with other servers again.<ref name="Edge-2016-05-18"/>

In May 2016, Moxie Marlinspike requested that a third-party client called LibreSignal not use the Signal service or the Signal name.<ref name="Edge-2016-05-18">{{cite web |url=https://lwn.net/Articles/687294/ |last=Edge |first=Jake |title=The perils of federated protocols |website=LWN.net |date=18 May 2016 |access-date=5 July 2016 |archive-date=14 September 2016 |archive-url=https://web.archive.org/web/20160914124807/https://lwn.net/Articles/687294/ |url-status=live}}</ref> As a result, on 24 May 2016 the LibreSignal project posted that the project was "abandoned".<ref>{{cite web |last=Le Bihan |first=Michel |url=https://github.com/LibreSignal/LibreSignal/blob/master/README.md |title=README.md |website=GitHub |publisher=LibreSignal |date=24 May 2016 |access-date=6 November 2016 |archive-date=27 June 2017 |archive-url=https://web.archive.org/web/20170627105032/https://github.com/LibreSignal/LibreSignal/blob/master/README.md |url-status=live}}</ref> The functionality provided by LibreSignal was subsequently incorporated into Signal by Marlinspike.<ref>{{Cite web |url=https://github.com/signalapp/Signal-Android/commit/1669731329bcc32c84e33035a67a2fc22444c24b |title=Support for using Signal without Play Services · signalapp/Signal-Android@1669731 |website=GitHub |language=en |access-date=3 January 2020 |archive-date=15 February 2020 |archive-url=https://web.archive.org/web/20200215092119/https://github.com/signalapp/Signal-Android/commit/1669731329bcc32c84e33035a67a2fc22444c24b |url-status=live}}</ref>

===Licensing===
The complete [[Source code|source code]] of the Signal clients for Android, iOS and desktop is available on [[GitHub]] under a [[Free software license|free software license]].<ref name="signal-ios-github" /><ref name="signal-android-github" /><ref name="signal-desktop-github" /> This enables interested parties to examine the code and help the developers verify that everything is behaving as expected. It also allows advanced users to compile their own copies of the applications and compare them with the versions that are distributed by Signal Messenger. In March 2016, Moxie Marlinspike wrote that, apart from some shared libraries that are not compiled with the project build due to a lack of Gradle&nbsp;NDK support, Signal for Android is reproducible.<ref>{{cite web |last1=Marlinspike |first1=Moxie |title=Reproducible Signal builds for Android |url=https://whispersystems.org/blog/reproducible-android/ |publisher=Open Whisper Systems |access-date=31 March 2016 |date=31 March 2016 |archive-date=15 May 2017 |archive-url=https://web.archive.org/web/20170515171632/https://whispersystems.org/blog/reproducible-android/ |url-status=live}}</ref> Signal's servers are partially open source, but the server software's anti-spam component is proprietary and closed source due to security concerns.<ref name="Signal-Server" /><ref>{{Cite web |title=Improving first impressions on Signal |url=https://signal.org/blog/keeping-spam-off-signal/ |access-date=5 December 2021 |website=Signal Messenger |language=en |archive-date=24 September 2023 |archive-url=https://web.archive.org/web/20230924194458/https://signal.org/blog/keeping-spam-off-signal/ |url-status=live }}</ref>

==Reception==
===Security===
In October 2014, the [[Organization:Electronic Frontier Foundation|Electronic Frontier Foundation]] (EFF) included Signal in their updated surveillance self-defense guide.<ref name="eff-2014-10-23" /> In November 2014, Signal received a perfect score on the EFF's secure messaging scorecard;<ref name="eff-2014-11-04" /> it received points for having communications encrypted in transit, having communications encrypted with keys the provider does not have access to ([[End-to-end encryption|end-to-end encryption]]), making it possible for users to independently verify their correspondents' identities, having past communications secure if the keys are stolen ([[Forward secrecy|forward secrecy]]), having the code open to independent review ([[Open-source software|open source]]), having the security designs well-documented, and having a recent independent security audit.<ref name="eff-2014-11-04" /> At the time, "[[Software:ChatSecure|ChatSecure]] + [[Software:Orbot|Orbot]]", [[Software:Pidgin|Pidgin]] (with [[Software:Off-the-Record Messaging|OTR]]), [[Software:Silent Circle|Silent Phone]], and [[Software:Telegram|Telegram]]'s optional "secret chats" also received seven out of seven points on the scorecard.<ref name="eff-2014-11-04" />

Former [[Organization:National Security Agency|NSA]] contractor Edward Snowden has endorsed Signal on multiple occasions.<ref name="Motherboard-2015-12-02"/> In his keynote speech at SXSW in March 2014, he praised Signal's predecessors (TextSecure and RedPhone) for their ease of use.<ref name="Eddy-2014-03-11" /><ref name="thenewyorker-2014-10-11" /> In December 2014, {{lang|de|Der Spiegel}} leaked slides from an internal NSA presentation dating to June 2012 in which the NSA deemed Signal's encrypted voice calling component (RedPhone) on its own as a "major threat" to its mission of accessing users' private data, and when used in conjunction with other privacy tools such as Cspace, [[Software:Tor (anonymity network)|Tor]], [[Software:Tails (operating system)|Tails]], and [[Software:TrueCrypt|TrueCrypt]] was ranked as "catastrophic" and led to a "near-total loss/lack of insight to target communications [and] presence".<ref name="spiegel-staff-2014-12-28" /><ref name="spiegel-media-2014-12-28" />

Following the [[Organization:2016 Democratic National Committee email leak|2016 Democratic National Committee email leak]], it was reported by ''Vanity Fair'' that Marc Elias (the general counsel for Hillary Clinton's presidential campaign) had instructed DNC staffers to exclusively use Signal when saying anything negative about Republican presidential nominee Donald Trump.<ref name="Bilton-2016-08-26">{{cite magazine |last1=Bilton |first1=Nick |title=How the Clinton Campaign Is Foiling the Kremlin |url=http://www.vanityfair.com/news/2016/08/how-the-clinton-campaign-is-foiling-the-kremlin |magazine=Vanity Fair |access-date=1 September 2016 |date=26 August 2016 |archive-date=29 August 2016 |archive-url=https://web.archive.org/web/20160829230558/http://www.vanityfair.com/news/2016/08/how-the-clinton-campaign-is-foiling-the-kremlin |url-status=live}}</ref><ref name="Blake-2016-08-27">{{cite web |last1=Blake |first1=Andrew |title=Democrats warned to use encryption weeks before email leaks |url=http://www.washingtontimes.com/news/2016/aug/27/dems-urged-encrypt-their-communications-weeks-prio/ |website=The Washington Times |publisher=The Washington Times, LLC |access-date=1 September 2016 |date=27 August 2016 |archive-date=1 September 2016 |archive-url=https://web.archive.org/web/20160901141312/http://www.washingtontimes.com/news/2016/aug/27/dems-urged-encrypt-their-communications-weeks-prio/ |url-status=live}}</ref>

In March 2017, Signal was approved by the sergeant at arms of the U.S. Senate for use by senators and their staff.<ref>{{cite web |last1=Whittaker |first1=Zack |title=In encryption push, Senate staff can now use Signal for secure messaging |url=http://www.zdnet.com/article/in-encryption-push-senate-approves-signal-for-encrypted-messaging/ |publisher=ZDNet |access-date=20 July 2017 |date=16 May 2017 |archive-date=19 July 2017 |archive-url=https://web.archive.org/web/20170719021847/http://www.zdnet.com/article/in-encryption-push-senate-approves-signal-for-encrypted-messaging/ |url-status=live}}</ref><ref>{{cite web |last1=Wyden |first1=Ron |title=Ron Wyden letter on Signal encrypted messaging |url=https://www.documentcloud.org/documents/3723701-Ron-Wyden-letter-on-Signal-encrypted-messaging.html |website=Documentcloud |publisher=Zack Whittaker, ZDNet |access-date=20 July 2017 |date=9 May 2017 |archive-date=6 June 2017 |archive-url=https://web.archive.org/web/20170606171205/https://www.documentcloud.org/documents/3723701-Ron-Wyden-letter-on-Signal-encrypted-messaging.html |url-status=live}}</ref>

On 27 September 2019, Natalie Silvanovich, a security engineer working in [[Company:Google|Google]]'s vulnerability research team at [[Project Zero]], disclosed how a bug in the [[Software:Android (operating system)|Android]] Signal client could let an attacker spy on a user without their knowledge.<ref>{{cite web |url=https://saltdna.com/news/signal%27s-messenger-eavesdropping-exploit-saltdna-cto-shares-his-views |title=Signal's Messenger Eavesdropping Exploit - SaltDNA CTO Shares His Views |archive-url=https://web.archive.org/web/20200811034319/https://saltdna.com/news/signal's-messenger-eavesdropping-exploit-saltdna-cto-shares-his-views |archive-date=11 August 2020 |url-status=dead |access-date=24 September 2021}}</ref> The bug allowed an attacker to phone a target device, mute the call, and the call would complete - keeping the audio open but without the owner being aware of that (however they would still be aware of a ring and / or a vibration from the initial call).<ref>{{Cite web |url=https://bugs.chromium.org/p/project-zero/issues/detail?id=1943 |title=1943 - project-zero - Project Zero - Monorail |website=bugs.chromium.org |access-date=3 March 2021 |archive-date=28 March 2021 |archive-url=https://web.archive.org/web/20210328084042/https://bugs.chromium.org/p/project-zero/issues/detail?id=1943 |url-status=live }}</ref> The bug was fixed the same day that it was reported and patched in release 4.47.7 of the app for Android.<ref>{{cite web |url=https://www.forbes.com/sites/daveywinder/2019/10/05/signal-messenger-eavesdropping-exploit-confirmedwhat-you-need-to-know |title=Signal Messenger Eavesdropping Exploit Confirmed—What You Need to Know |website={{wipe|Forbes}} |access-date=3 March 2021 |archive-date=17 February 2021 |archive-url=https://web.archive.org/web/20210217104318/https://www.forbes.com/sites/daveywinder/2019/10/05/signal-messenger-eavesdropping-exploit-confirmedwhat-you-need-to-know/ |url-status=live }}</ref>

In February 2020, the European Commission recommended that its staff use Signal.<ref name="Politico-European-Commission-Signal-2020-02-20">{{cite web |title=EU Commission to staff: Switch to Signal messaging app |url=https://www.politico.eu/article/eu-commission-to-staff-switch-to-signal-messaging-app/ |website=Politico EU |date=20 February 2020 |access-date=20 February 2020 |archive-date=20 February 2020 |archive-url=https://web.archive.org/web/20200220204534/https://www.politico.eu/article/eu-commission-to-staff-switch-to-signal-messaging-app/ |url-status=live}}</ref> Following the George Floyd protests, which began in May 2020, Signal was downloaded 121,000&nbsp;times in the U.S. between 25 May and 4 June.<ref>{{Cite web |last=Molla |first=Rani |date=3 June 2020 |title=From Citizen to Signal, the most popular apps right now reflect America's protests |url=https://www.vox.com/recode/2020/6/3/21278558/protest-apps-signal-citizen-twitter-instagram-george-floyd |access-date=7 June 2020 |website=Vox |language=en |archive-date=7 June 2020 |archive-url=https://web.archive.org/web/20200607045045/https://www.vox.com/recode/2020/6/3/21278558/protest-apps-signal-citizen-twitter-instagram-george-floyd |url-status=live}}</ref> In July 2020, Signal became the most downloaded app in [[Place:Hong Kong|Hong Kong]] on both the Apple App Store and the Google Play Store after the passage of the Hong Kong national security law.<ref>{{Cite web |last=Lee |first=Timothy B. |date=8 July 2020 |title=Hong Kong downloads of Signal surge as residents fear crackdown |url=https://arstechnica.com/tech-policy/2020/07/hong-kong-downloads-of-signal-surge-as-residents-fear-crackdown/ |access-date=12 July 2020 |website=Ars Technica |language=en-us |archive-date=11 July 2020 |archive-url=https://web.archive.org/web/20200711174108/https://arstechnica.com/tech-policy/2020/07/hong-kong-downloads-of-signal-surge-as-residents-fear-crackdown/ |url-status=live}}</ref>

{{As of|2021|January}}, Signal is a contact method for securely providing tips to major news outlets such as ''{{wipe|The Washington Post}}'',<ref>{{Cite news |title=How to share documents and news tips with Washington Post journalists. |url=https://www.washingtonpost.com/anonymous-news-tips/ |url-status=live |access-date=17 January 2021 |newspaper=The Washington Post |language=en |archive-date=4 June 2020 |archive-url=https://web.archive.org/web/20200604180541/https://www.washingtonpost.com/anonymous-news-tips/}}</ref> ''{{wipe|The Guardian}}'',<ref>{{Cite web |last1=Hoyland |first1=Luke |last2=Fenn |first2=Chris |title=Contact the Guardian securely |url=http://www.theguardian.com/help/ng-interactive/2017/mar/17/contact-the-guardian-securely |access-date=17 January 2021 |website=The Guardian |language=en |archive-date=7 January 2021 |archive-url=https://web.archive.org/web/20210107215254/https://www.theguardian.com/help/ng-interactive/2017/mar/17/contact-the-guardian-securely |url-status=live}}</ref> ''{{wipe|The New York Times}}'',<ref>{{Cite news |date=14 December 2016 |title=Tips |language=en-US |work=The New York Times |url=https://www.nytimes.com/tips |url-status=live |access-date=17 January 2021 |issn=0362-4331 |archive-date=9 June 2020 |archive-url=https://web.archive.org/web/20200609200034/https://www.nytimes.com/tips}}</ref> and ''{{wipe|The Wall Street Journal}}''.<ref>{{Cite web |title=WSJ.com Secure Drop |url=https://www.wsj.com/tips |url-status=live |access-date=17 January 2021 |website=The Wall Street Journal |archive-date=24 July 2020 |archive-url=https://web.archive.org/web/20200724052433/https://www.wsj.com/tips}}</ref>

Candiru claims the ability to capture data from Signal Private Messenger with their spyware, at a fee of €500.000.<ref name="Citizen Lab">{{Cite journal |date=15 July 2021 |title=Hooking Candiru: Another Mercenary Spyware Vendor Comes into Focus |url=https://citizenlab.ca/2021/07/hooking-candiru-another-mercenary-spyware-vendor-comes-into-focus/ |access-date=20 July 2021 |website=The Citizen Lab |language=en-US |last1=Marczak |first1=Bill |last2=Scott-Railton |first2=John |last3=Berdan |first3=Kristin |last4=Razzak |first4=Bahr Abdul |last5=Deibert |first5=Ron |archive-date=29 May 2023 |archive-url=https://web.archive.org/web/20230529224455/https://citizenlab.ca/2021/07/hooking-candiru-another-mercenary-spyware-vendor-comes-into-focus/ |url-status=live }}</ref>

On 9 August 2022, Ismail Sabri Yaakob, the Prime Minister of Malaysia, reported that his Signal account was "hacked" and infiltrated by a third party, sending out messages and impersonating the politician. No details were disclosed regarding the method used to gain access to the account.<ref>{{cite web |title=PM lodges report over hacked Telegram, Signal accounts |url=https://www.thestar.com.my/news/nation/2022/08/09/pm-lodges-report-over-hacked-telegram-signal-accounts |work=The Star |access-date=9 August 2022 |archive-date=9 August 2022 |archive-url=https://web.archive.org/web/20220809012705/https://www.thestar.com.my/news/nation/2022/08/09/pm-lodges-report-over-hacked-telegram-signal-accounts |url-status=live }}</ref>

===In-app payments===
In April 2021, Signal announced the addition of a [[Finance:Cryptocurrency wallet|cryptocurrency wallet]] feature that would allow users to send and receive payments in [[Finance:MobileCoin|MobileCoin]].<ref>{{cite magazine |last1=Greenberg |first1=Andy |title=Signal Adds a Payments Feature—With a Privacy-Focused Cryptocurrency |url=https://www.wired.com/story/signal-mobilecoin-payments-messaging-cryptocurrency/ |magazine=Wired |access-date=12 January 2022 |date=6 April 2021 |archive-date=8 April 2021 |archive-url=https://web.archive.org/web/20210408155920/https://www.wired.com/story/signal-mobilecoin-payments-messaging-cryptocurrency/ |url-status=live }}</ref> This received criticism from security expert Bruce Schneier, who had previously praised the software. Schneier stated that this would bloat the client and attract unwanted attention from the authorities.<ref>{{Cite web |date=7 April 2021 |title=Signal Adds Cryptocurrency Support |url=https://www.schneier.com/blog/archives/2021/04/wtf-signal-adds-cryptocurrency-support.html |access-date=8 April 2021 |website=Schneier on Security |archive-date=8 April 2021 |archive-url=https://web.archive.org/web/20210408015642/https://www.schneier.com/blog/archives/2021/04/wtf-signal-adds-cryptocurrency-support.html |url-status=live }}</ref> The wallet functionality was initially only available in certain countries, but was later enabled globally in November 2021.<ref name="Greenberg-2022-01-06"/>

===Blocking===
[[File:Countries_where_Signal_provides_domain_fronting.svg|thumb|400px|{{legend|#346733|Countries where Signal's domain fronting is enabled by default|border=1px #555 solid}}{{legend|#8E0000|Countries where Signal is blocked (March 2021)|border=1px #555 solid}}]]
In December 2016, [[Place:Egypt|Egypt]] blocked access to Signal.<ref>{{cite web |last1=Cox |first1=Joseph |title=Signal Claims Egypt Is Blocking Access to Encrypted Messaging App |url=https://motherboard.vice.com/en_us/article/nz755w/signal-claims-egypt-is-blocking-access-to-encrypted-messaging-app |website=Motherboard |publisher=Vice Media LLC |access-date=20 July 2017 |date=19 December 2016 |archive-date=29 June 2017 |archive-url=https://web.archive.org/web/20170629080022/https://motherboard.vice.com/en_us/article/nz755w/signal-claims-egypt-is-blocking-access-to-encrypted-messaging-app |url-status=live}}</ref> In response, Signal's developers added [[Social:Domain fronting|domain fronting]] to their service.<ref name="doodles-stickers-censorship"/> This allows Signal users in a specific country to circumvent censorship by making it look like they are connecting to a different internet-based service.<ref name="doodles-stickers-censorship">{{cite web |last1=Marlinspike |first1=Moxie |title=Doodles, stickers, and censorship circumvention for Signal Android |url=https://whispersystems.org/blog/doodles-stickers-censorship/ |publisher=Open Whisper Systems |access-date=20 July 2017 |date=21 December 2016 |archive-date=28 December 2016 |archive-url=https://web.archive.org/web/20161228221206/https://whispersystems.org/blog/doodles-stickers-censorship/ |url-status=live}}</ref><ref name="Greenberg-2016-12-21">{{cite magazine |last1=Greenberg |first1=Andy |title=Encryption App 'Signal' Fights Censorship with a Clever Workaround |url=https://www.wired.com/2016/12/encryption-app-signal-fights-censorship-clever-workaround/ |magazine=Wired |access-date=20 July 2017 |date=21 December 2016 |archive-date=11 July 2017 |archive-url=https://web.archive.org/web/20170711183009/https://www.wired.com/2016/12/encryption-app-signal-fights-censorship-clever-workaround/ |url-status=live}}</ref> {{As of|May 2022}}, Signal's domain fronting is enabled by default in Egypt, [[Place:United Arab Emirates|UAE]], Oman, Qatar, [[Religion:Iran|Iran]], Cuba, Uzbekistan and {{wipe|Ukraine}}.<ref>{{cite web |title=SignalServiceNetworkAccess.kt |url=https://github.com/signalapp/Signal-Android/blob/main/app/src/main/java/org/thoughtcrime/securesms/push/SignalServiceNetworkAccess.kt |access-date=17 May 2022 |website=GitHub |publisher=Signal Foundation |archive-date=17 May 2022 |archive-url=https://web.archive.org/web/20220517023825/https://github.com/signalapp/Signal-Android/blob/main/app/src/main/java/org/thoughtcrime/securesms/push/SignalServiceNetworkAccess.kt |url-status=live }}</ref>

{{As of|January 2018}}, Signal was blocked in Iran.<ref name="Frenkel-2018-1-2">{{cite web |last1=Frenkel |first1=Sheera |title=Iranian Authorities Block Access to Social Media Tools |url=https://www.nytimes.com/2018/01/02/technology/iran-protests-social-media.html |website=The New York Times |access-date=15 January 2018 |date=2 January 2018 |archive-date=16 January 2018 |archive-url=https://web.archive.org/web/20180116135145/https://www.nytimes.com/2018/01/02/technology/iran-protests-social-media.html |url-status=live}}</ref><ref name="issue-7311">{{cite web |title=Domain Fronting for Iran #7311 |url=https://github.com/WhisperSystems/Signal-Android/issues/7311 |website=GitHub |access-date=15 January 2018 |date=1 January 2018}}</ref> Signal's domain fronting feature relies on the [[Software:Google App Engine|Google App Engine]] (GAE) service.<ref name="issue-7311"/><ref name="Frenkel-2018-1-2"/> This does not work in Iran because Google has blocked Iranian access to GAE in order to comply with U.S. sanctions.<ref name="Frenkel-2018-1-2"/><ref>{{cite web |last1=Brandom |first1=Russell |title=Iran blocks encrypted messaging apps amid nationwide protests |url=https://www.theverge.com/2018/1/2/16841292/iran-telegram-block-encryption-protest-google-signal |website=The Verge  |access-date=23 March 2018 |date=2 January 2018 |archive-date=22 March 2018 |archive-url=https://web.archive.org/web/20180322015257/https://www.theverge.com/2018/1/2/16841292/iran-telegram-block-encryption-protest-google-signal |url-status=live}}</ref>

In early 2018, [[Software:Google App Engine|Google App Engine]] made an internal change to stop domain fronting for all countries. Due to this issue, Signal made a public change to use Amazon CloudFront for domain fronting. However, [[Company:Amazon Web Services|AWS]] also announced that they would be making changes to their service to prevent domain fronting. As a result, Signal said that they would start investigating new methods/approaches.<ref>{{Cite web |last=Marlinspike |first=Moxie |title=A letter from Amazon |url=https://signal.org/blog/looking-back-on-the-front/ |website=signal.org |publisher=Open Whisper Systems |date=1 May 2018 |access-date=10 January 2019 |archive-date=3 January 2019 |archive-url=https://web.archive.org/web/20190103125121/https://signal.org/blog/looking-back-on-the-front/ |url-status=live}}</ref><ref>{{cite web |last1=Gallagher |first1=Sean |title=Amazon blocks domain fronting, threatens to shut down Signal's account |url=https://arstechnica.com/information-technology/2018/05/amazon-blocks-domain-fronting-threatens-to-shut-down-signals-account/ |website=Ars Technica |access-date=23 January 2019 |date=2 May 2018 |archive-date=24 January 2019 |archive-url=https://web.archive.org/web/20190124041528/https://arstechnica.com/information-technology/2018/05/amazon-blocks-domain-fronting-threatens-to-shut-down-signals-account/ |url-status=live}}</ref> Signal switched from AWS back to Google in April 2019.<ref>{{cite web |last1=Parrelli |first1=Greyson |title=Attempt to resolve connectivity problems for some users. |url=https://github.com/signalapp/Signal-Android/commit/9aed2343c141b1b3809f7eaccce61c22e342fda7 |website=GitHub |publisher=Signal Messenger LLC |access-date=2 May 2019 |date=4 April 2019 |archive-date=17 January 2021 |archive-url=https://web.archive.org/web/20210117005234/https://github.com/signalapp/Signal-Android/commit/9aed2343c141b1b3809f7eaccce61c22e342fda7 |url-status=live}}</ref>

In January 2021, Iran removed the app from app stores,<ref>{{Cite news |date=15 January 2021 |title=حذف سیگنال از فروشگاه‌های نرم‌افزار آنلاین در ایران |url=https://www.bbc.com/persian/iran-55679732 |access-date=17 January 2021 |website=BBC News فارسی |language=fa |archive-date=17 January 2021 |archive-url=https://web.archive.org/web/20210117075004/https://www.bbc.com/persian/iran-55679732 |url-status=live }}</ref><ref>{{Cite news |title=پیام‌رسان سیگنال به "دستور کمیته فیلترینگ" از فروشگاه‌های آنلاین در ایران حذف شد |url=https://www.radiofarda.com/a/signal-app-iran-government/31047041.html |access-date=17 January 2021 |website=رادیو فردا |language=fa |publisher=Radio Farda |archive-date=13 December 2023 |archive-url=https://web.archive.org/web/20231213150316/https://www.radiofarda.com/a/signal-app-iran-government/31047041.html |url-status=live }}</ref> and blocked Signal.<ref>{{cite news |title=Signal Blocked by Iran as Encrypted Messaging App's Popularity Explodes |url=https://gizmodo.com/signal-blocked-by-iran-as-encrypted-messaging-apps-popu-1846141987 |first=Rhett |last=Jones |website=Gizmodo |date=27 January 2021 |access-date=28 January 2021 |archive-date=28 January 2021 |archive-url=https://web.archive.org/web/20210128111300/https://gizmodo.com/signal-blocked-by-iran-as-encrypted-messaging-apps-popu-1846141987 |url-status=live }}</ref> Signal was later blocked by China in March 2021.<ref>{{cite web |last1=Rita |first1=Liao |title=Rising encrypted app Signal is down in China |url=https://techcrunch.com/2021/03/15/signal-is-down-in-china/ |website=TechCrunch |publisher=Verizon Media |access-date=16 March 2021 |date=15 March 2021 |archive-date=16 March 2021 |archive-url=https://web.archive.org/web/20210316022725/https://techcrunch.com/2021/03/15/signal-is-down-in-china/ |url-status=live }}</ref>

===Audience===
==== Use by activists ====
In 2020, the app was used for coordination and communication by protesters during the George Floyd protests as they relied on the app's end-to-end encryption to share information securely.<ref>{{cite web |last1=Nierenberg |first1=Amelia |title=Signal Downloads Are Way Up Since the Protests Began |url=https://www.nytimes.com/2020/06/11/style/signal-messaging-app-encryption-protests.html |website=The New York Times |date=11 June 2020 |access-date=12 May 2023 |archive-date=25 June 2020 |archive-url=https://web.archive.org/web/20200625005732/https://www.nytimes.com/2020/06/11/style/signal-messaging-app-encryption-protests.html |url-status=live }}</ref>

In March 2021, the {{wipe|United Nations}} recommended Myanmar residents use Signal and [[Software:Proton Mail|Proton Mail]] to pass and preserve evidence of human rights violations committed during the 2021 coup.<ref>{{Cite news |last=Nebehay |first=Stephanie |date=17 March 2021 |title=U.N. team seeks evidence linking Myanmar military leaders to crimes |language=en |agency=Reuters |url=https://www.reuters.com/article/us-myanmar-politics-un-crimes-idUSKBN2B90XV |access-date=24 March 2021 |archive-date=25 March 2021 |archive-url=https://web.archive.org/web/20210325105220/https://www.reuters.com/article/us-myanmar-politics-un-crimes-idUSKBN2B90XV |url-status=live }}</ref>

====Controversial use====
Signal's [[Terms of service|terms of service]] state that the product must not be used to violate the law.<ref name="Newton-2021-01-26"/> According to a former employee, Signal's leadership at the time told him they would say something "if and when people start abusing Signal or doing things that we think are terrible".<ref name="Newton-2021-01-26"/> In January 2021, the position of Signal's leadership was to take a "hands-off approach to moderation" as the company's employees are not able to read user messages and the Signal Foundation "don't want to be a media company".<ref name="Newton-2021-01-26">{{Cite web |last=Newton |first=Casey |title=🚨 The battle inside Signal |url=https://www.platformer.news/p/-the-battle-inside-signal |date=26 January 2021 |access-date=17 May 2022 |website=platformer.news |language=en |archive-date=13 December 2023 |archive-url=https://web.archive.org/web/20231213150315/https://www.platformer.news/p/-the-battle-inside-signal |url-status=live }}</ref><ref> Cohn-Gordon et al. 2016 </ref>

In 2016, authorities in {{wipe|India}} arrested members of a suspected ISIS-affiliated terrorist cell that communicated via Signal.<ref>{{cite news |last1=Tripathi |first1=Rahul |date=11 April 2016 |title=Dangerous Signal: This encrypted app is helping ISIS members in India to communicate |work=The Times of India |url=https://telecom.economictimes.indiatimes.com/news/dangerous-signal-this-encrypted-app-is-helping-isis-members-in-india-to-communicate/51774226 |access-date=13 January 2021 |quote=A group of terrorist suspects in India said to be inspired by Islamic State wanted to emulate US whistleblower Edward Snowden and use encrypted communication tool Signal to stay in touch, it was revealed in interrogation by the National Investigation Agency (NIA). |archive-date=30 September 2020 |archive-url=https://web.archive.org/web/20200930225330/https://telecom.economictimes.indiatimes.com/news/dangerous-signal-this-encrypted-app-is-helping-isis-members-in-india-to-communicate/51774226 |url-status=live}}</ref> 

Extremist right-wing militias and white nationalists use Signal for organizing their actions, including the Unite the Right II rally in 2018.<ref>{{cite news |last1=Frenkel |first1=Sheera |quote=In the days since rioters stormed Capitol Hill, fringe groups like armed militias, QAnon conspiracy theorists and far-right supporters of President Trump have vowed to continue their fight in hundreds of conversations on a range of internet platforms. Some of the organizers have moved to encrypted messaging apps like Telegram and Signal, which cannot be as easily monitored as social media platforms. |title=Fringe Groups Splinter Online After Facebook and Twitter Bans |url=https://www.nytimes.com/2021/01/11/technology/fringe-groups-splinter-online-after-facebook-and-twitter-bans.html |access-date=13 January 2021 |work={{wipe|The New York Times}} |date=11 January 2021 |archive-date=12 January 2021 |archive-url=https://web.archive.org/web/20210112141817/https://www.nytimes.com/2021/01/11/technology/fringe-groups-splinter-online-after-facebook-and-twitter-bans.html |url-status=live}}</ref><ref>{{cite news |last1=Glaser |quote=This year, Kessler and his fellow white nationalist co-organizers switched much of their rally planning throughout the summer to private groups on Facebook Messenger and the encrypted texting app Signal... |first1=April |title=How White Supremacists Planned Their Rally in D.C. |url=https://slate.com/technology/2018/08/how-white-supremacists-planned-their-rally-in-d-c-and-what-it-tells-us-about-the-strength-of-their-movement.html |access-date=13 January 2021 |work=Slate |date=11 August 2018 |archive-date=24 December 2020 |archive-url=https://web.archive.org/web/20201224231522/https://slate.com/technology/2018/08/how-white-supremacists-planned-their-rally-in-d-c-and-what-it-tells-us-about-the-strength-of-their-movement.html |url-status=live}}</ref><ref>{{cite news |quote=Telegram and Signal are far more stable and secure and could prove more enduring homes and recruitment stations for far-right groups. |last1=Daly |first1=Kye |title=The online far right is moving underground |url=https://www.axios.com/the-online-far-right-is-moving-underground-e429d45d-1b30-46e0-82a3-6e240bf44fef.html |access-date=12 January 2021 |work=Axios |date=11 January 2021 |archive-date=12 January 2021 |archive-url=https://web.archive.org/web/20210112144650/https://www.axios.com/the-online-far-right-is-moving-underground-e429d45d-1b30-46e0-82a3-6e240bf44fef.html |url-status=live}}</ref><ref>{{cite news |last1=Glaser |first1=April |title=White Supremacists Still Have a Safe Space Online |url=https://slate.com/technology/2018/10/discord-safe-space-white-supremacists.html |quote=A year later, in the runup to an ultimately barely attended sequel to Unite the Right in D.C., organizers appeared to stay off the platform, opting instead to discuss logistics over Facebook Messenger and the encrypted texting app Signal. |access-date=12 January 2021 |work=Slate |date=9 October 2018 |archive-date=6 January 2021 |archive-url=https://web.archive.org/web/20210106084707/https://slate.com/technology/2018/10/discord-safe-space-white-supremacists.html |url-status=live}}</ref>

The claim that Signal is used to fund terrorist or criminal activities is the justification for [[Place:Turkey|Turkey]] to criminalize the app for the general population, which Abdullah Bozkurt claims is a way the "government abuses its counterterrorism laws to punish critics, opponents and dissidents."<ref>{{Cite web |date=2021-07-08 |title=Secure messaging app Signal has been effectively criminalized in Turkey - Nordic Monitor |url=https://nordicmonitor.com/2021/07/secure-messaging-app-signal-has-been-effectively-criminalised-in-turkey/ |access-date=2023-12-25 |website=nordicmonitor.com |language=en-US |archive-date=25 December 2023 |archive-url=https://web.archive.org/web/20231225233656/https://nordicmonitor.com/2021/07/secure-messaging-app-signal-has-been-effectively-criminalised-in-turkey/ |url-status=live }}</ref><ref>{{Cite web |title=Turkey's new media law is bad news – but don't report it |url=https://www.brookings.edu/articles/turkeys-new-media-law-is-bad-news-but-dont-report-it/ |access-date=2023-12-25 |website=Brookings |language=en-US |archive-date=25 December 2023 |archive-url=https://web.archive.org/web/20231225233306/https://www.brookings.edu/articles/turkeys-new-media-law-is-bad-news-but-dont-report-it/ |url-status=live }}</ref>

== See also ==
* [[Software:Comparison of cross-platform instant messaging clients|Comparison of cross-platform instant messaging clients]]
* [[Software:Comparison of VoIP software|Comparison of VoIP software]]
* [[Internet privacy]]
* List of video telecommunication services and product brands
* [[Secure communication]]
{{Clear}}

== Notes ==
{{notelist}}

==References==
{{reflist|refs=

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<ref name="Lee-2015-03-02">{{cite web |first=Micah |last=Lee |url=https://firstlook.org/theintercept/2015/03/02/signal-iphones-encrypted-messaging-app-now-supports-text/ |title=You Should Really Consider Installing Signal, an Encrypted Messaging App for iPhone |website=The Intercept |date=2 March 2015 |access-date=3 March 2015 |archive-date=3 March 2015 |archive-url=https://web.archive.org/web/20150303052746/https://firstlook.org/theintercept/2015/03/02/signal-iphones-encrypted-messaging-app-now-supports-text/ |url-status=live}}</ref>

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<ref name="eff-2014-10-23">{{cite web |url=https://ssd.eff.org/en/module/communicating-others |publisher=Electronic Frontier Foundation |title=Surveillance Self-Defense. Communicating with Others |date=23 October 2014 |access-date=25 January 2015 |archive-date=8 November 2014 |archive-url=https://web.archive.org/web/20141108170218/https://ssd.eff.org/en/module/communicating-others |url-status=live}}</ref>

<ref name="eff-2014-11-04">{{cite web |url=https://www.eff.org/node/82654 |publisher=Electronic Frontier Foundation |title=Secure Messaging Scorecard. Which apps and tools actually keep your messages safe? |date=4 November 2014 |access-date=27 August 2016 |archive-date=28 July 2016 |archive-url=https://web.archive.org/web/20160728011841/https://www.eff.org/node/82654 |url-status=live}}</ref>

<ref name="signal-ios-github">{{cite web |url=https://github.com/signalapp/Signal-iOS |title=Signal-iOS |author=Signal |website=GitHub |access-date=10 May 2023 |archive-date=11 November 2014 |archive-url=https://web.archive.org/web/20141111134037/https://github.com/WhisperSystems/Signal-iOS |url-status=live}}</ref>

<ref name="signal-android-github">{{cite web |url=https://github.com/signalapp/Signal-Android |title=Signal-Android |author=Signal |website=GitHub |access-date=10 May 2023 |archive-date=30 December 2015 |archive-url=https://web.archive.org/web/20151230221156/https://github.com/WhisperSystems/Signal-Android |url-status=live}}</ref>

<ref name="signal-desktop-github">{{cite web |url=https://github.com/signalapp/Signal-Desktop |title=Signal-Desktop |author=Signal |website=GitHub |access-date=10 May 2023 |archive-date=8 April 2016 |archive-url=https://web.archive.org/web/20160408205540/https://github.com/WhisperSystems/Signal-Desktop |url-status=live}}</ref>

<ref name="Signal-Server">{{cite web |url=https://github.com/signalapp/Signal-Server |title=Signal-Server |author=Signal |website=GitHub |access-date=10 May 2023 |archive-date=28 December 2016 |archive-url=https://web.archive.org/web/20161228222103/https://github.com/WhisperSystems/Signal-Server |url-status=live}}</ref>

<ref name="Greenberg-2014-07-29">{{cite magazine |url=https://www.wired.com/2014/07/free-encrypted-calling-finally-comes-to-the-iphone/ |title=Your iPhone Can Finally Make Free, Encrypted Calls |magazine=Wired |first=Andy |last=Greenberg |date=29 July 2014 |access-date=18 January 2015 |archive-date=18 January 2015 |archive-url=https://web.archive.org/web/20150118152718/http://www.wired.com/2014/07/free-encrypted-calling-finally-comes-to-the-iphone/ |url-status=live}}</ref>

<ref name="Evans-2014-07-29">{{cite web |url=https://techcrunch.com/2014/07/29/talk-private-to-me-free-worldwide-encrypted-voice-calls-with-signal/ |title=Talk Private To Me: Free, Worldwide, Encrypted Voice Calls With Signal For iPhone |work=TechCrunch |publisher=AOL |first=Jon |last=Evans |date=29 July 2014 |access-date=25 June 2017 |archive-date=4 June 2016 |archive-url=https://web.archive.org/web/20160604000058/http://techcrunch.com/2014/07/29/talk-private-to-me-free-worldwide-encrypted-voice-calls-with-signal/ |url-status=live}}</ref>

<ref name="Mimoso-2014-07-29">{{cite web |url=http://threatpost.com/new-signal-app-brings-encrypted-calling-to-iphone/107491 |title=New Signal App Brings Encrypted Calling to iPhone |publisher=Threatpost |first=Michael |last=Mimoso |date=29 July 2014 |access-date=25 January 2015 |archive-date=18 January 2015 |archive-url=https://web.archive.org/web/20150118143554/http://threatpost.com/new-signal-app-brings-encrypted-calling-to-iphone/107491 |url-status=live}}</ref>

<ref name="pressfreedomfoundation">{{cite web |url=https://freedom.press/crowdfunding/signal/ |title=Signal |publisher=Freedom of the Press Foundation |access-date=31 January 2018 |archive-date=20 March 2018 |archive-url=https://web.archive.org/web/20180320152018/https://freedom.press/crowdfunding/signal/ |url-status=live}}</ref>

<ref name="knightfoundation">{{cite web |title=TextSecure |url=http://www.knightfoundation.org/grants/201499909/ |publisher=Knight Foundation |access-date=5 January 2015 |archive-date=9 January 2021 |archive-url=https://web.archive.org/web/20210109161315/https://knightfoundation.org/grants/201499909/ |url-status=live}}</ref>

<ref name="shuttleworthfoundation">{{cite web |title=Moxie Marlinspike |url=https://www.shuttleworthfoundation.org/alumni/moxie-marlinspike |publisher=Shuttleworth Foundation |access-date=14 January 2015 |url-status=dead |archive-url=https://archive.today/20161115142945/https://www.shuttleworthfoundation.org/alumni/moxie-marlinspike |archive-date=15 November 2016}}</ref>

<ref name="opentechfund">{{cite web |title=Open Whisper Systems |url=https://www.opentech.fund/results/supported-projects/open-whisper-systems/ |publisher=Open Technology Fund |access-date=1 March 2019 |archive-date=2 March 2019 |archive-url=https://web.archive.org/web/20190302024657/https://www.opentech.fund/results/supported-projects/open-whisper-systems/ |url-status=live}}</ref>

<ref name="Brandom-2014-07-29">{{cite web |last1=Brandom |first1=Russell |title=Signal brings painless encrypted calling to iOS |url=https://www.theverge.com/2014/7/29/5945547/signal-brings-painless-encrypted-calling-whisper-systems-moxie-marlinspike |website=The Verge |access-date=26 January 2015 |date=29 July 2014 |archive-date=3 February 2015 |archive-url=https://web.archive.org/web/20150203221418/http://www.theverge.com/2014/7/29/5945547/signal-brings-painless-encrypted-calling-whisper-systems-moxie-marlinspike |url-status=live}}</ref>

<ref name="Eddy-2014-03-11">{{cite web |date=11 March 2014 |first=Max |last=Eddy |url=http://securitywatch.pcmag.com/security/321511-snowden-to-sxsw-here-s-how-to-keep-the-nsa-out-of-your-stuff |title=Snowden to SXSW: Here's How To Keep The NSA Out Of Your Stuff |publisher=PC Magazine: SecurityWatch |access-date=16 March 2014 |archive-date=16 March 2014 |archive-url=https://web.archive.org/web/20140316122922/http://securitywatch.pcmag.com/security/321511-snowden-to-sxsw-here-s-how-to-keep-the-nsa-out-of-your-stuff |url-status=live}}</ref>

<ref name="thenewyorker-2014-10-11">{{cite web |url=https://www.youtube.com/watch?v=fidq3jow8bc |title=The Virtual Interview: Edward Snowden - The New Yorker Festival |work=The New Yorker |via=YouTube |date=11 October 2014 |access-date=24 May 2015 |archive-date=9 January 2021 |archive-url=https://web.archive.org/web/20210109161401/https://www.youtube.com/watch?v=fidq3jow8bc |url-status=live}}</ref>

<ref name="thenewyorker-2020-10-19">{{cite magazine |url=https://www.newyorker.com/magazine/2020/10/26/taking-back-our-privacy |title=Taking Back Our Privacy |last=Wiener |first=Anna |magazine=The New Yorker |date=19 October 2020 |access-date=27 October 2020 |archive-date=27 October 2020 |archive-url=https://web.archive.org/web/20201027010827/https://www.newyorker.com/magazine/2020/10/26/taking-back-our-privacy |url-status=live}}</ref>

<ref name="Mott-2017-03-14">{{cite web |last1=Mott |first1=Nathaniel |title=Signal's Encrypted Video Calling For iOS, Android Leaves Beta |url=http://www.tomshardware.com/news/signal-encrypted-video-calling-ios-android,33898.html |website=Tom's Hardware |publisher=Purch Group, Inc. |date=14 March 2017 |access-date=14 March 2017}}</ref>


}}

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{{Refend}}

==External links==
{{Cryptographic software}}
{{Instant messaging}}

[[Category:Cross-platform software]]
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{{Sourceattribution|Signal (software)|1}}