Privacy-focused encrypted messaging app
Signal is an open-source, encrypted messaging service for instant messaging, voice calls, and video calls. The instant messaging function includes sending text, voice notes, images, videos, and other files. Communication may be one-to-one between users or may involve group messaging.
The application uses a centralized computing architecture and is cross-platform software. It is developed by the non-profit Signal Foundation and its subsidiary Signal Messenger LLC. Signal's software is free and open-source. Its mobile clients, desktop client, and server are all published under the AGPL-3.0-only license. The official Android app generally uses the proprietary Google Play Services, although it is designed to be able to work without them. Signal is also distributed for iOS and desktop programs for Windows, macOS, and Linux. Registration for desktop use requires an iOS or Android device.
Signal uses mobile telephone numbers to register and manage user accounts, though configurable usernames were added in March 2024 to allow users to hide their phone numbers from other users. After removing support for SMS on Android in 2023, the app now secures all communications with end-to-end encryption. The client software includes mechanisms by which users can independently verify the identity of their contacts and the integrity of the data channel.
The non-profit Signal Foundation was launched in February 2018 with initial funding of $50 million from WhatsApp co-founder Brian Acton. As of January 2022[ [update]], the platform had approximately 40 million monthly active users. As of May 2021[ [update]], it was downloaded more than 105 million times.
History.
2010–2013: Origins.
Signal is the successor of the RedPhone encrypted voice calling app and the TextSecure encrypted texting program. The beta versions of RedPhone and TextSecure were first launched in May 2010 by Whisper Systems, a startup company co-founded by security researcher Moxie Marlinspike and roboticist Stuart Anderson. Whisper Systems also produced a firewall and tools for encrypting other forms of data. All of these were proprietary enterprise mobile security software and were only available for Android.
In November 2011, Whisper Systems announced that it had been acquired by Twitter. Neither company disclosed the financial terms of the deal. The acquisition was done "primarily so that Mr. Marlinspike could help the then-startup improve its security". Shortly after the acquisition, Whisper Systems' RedPhone service was made unavailable. Some criticized the removal, arguing that the software was "specifically targeted [to help] people under repressive regimes" and that it left people like the Egyptians in "a dangerous position" during the events of the Egyptian revolution of 2011.
Twitter released TextSecure as free and open-source software under the GPLv3 license in December 2011. RedPhone was also released under the same license in July 2012. Marlinspike later left Twitter and founded Open Whisper Systems as a collaborative Open Source project for the continued development of TextSecure and RedPhone.
2013–2018: Open Whisper Systems.
Open Whisper Systems' website was launched in January 2013.
In February 2014, Open Whisper Systems introduced the second version of their TextSecure Protocol (now Signal Protocol), which added end-to-end encrypted group chat and instant messaging capabilities to TextSecure. Toward the end of July 2014, they announced plans to merge the RedPhone and TextSecure applications as Signal. This announcement coincided with the initial release of Signal as a RedPhone counterpart for iOS. The developers said that their next steps would be to provide TextSecure instant messaging capabilities for iOS, unify the RedPhone and TextSecure applications on Android, and launch a web client. Signal was the first iOS app to enable end-to-end encrypted voice calls for free. TextSecure compatibility was added to the iOS application in March 2015.
From its launch in May 2010 until March 2015, the Android version of Signal (then called TextSecure) included support for encrypted SMS/MMS messaging. From version 2.7.0 onward, the Android application only supported sending and receiving encrypted messages via the data channel. Reasons for this included security flaws of SMS/MMS and problems with the key exchange. Open Whisper Systems' abandonment of SMS/MMS encryption prompted some users to create a fork named Silence (initially called SMSSecure) that is meant solely for the exchange of encrypted SMS and MMS messages.
In November 2015, the TextSecure and RedPhone applications on Android were merged to become Signal for Android. A month later, Open Whisper Systems announced Signal Desktop, a Chrome app that could link with a Signal mobile client. At launch, the app could only be linked with the Android version of Signal. On 26 September 2016, Open Whisper Systems announced that Signal Desktop could now be linked with the iOS version of Signal as well. On 31 October 2017, Open Whisper Systems announced that the Chrome app was deprecated. At the same time, they announced the release of a standalone desktop client (based on the Electron framework) for Windows, macOS and certain Linux distributions.
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. 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". Along with the subpoena, OWS received a gag order requiring OWS not to tell anyone about the subpoena for one year. OWS approached the ACLU, and they were able to lift part of the gag order after challenging it in court. OWS said it was the first time they had received a subpoena, and that they were "committed to treating any future requests the same way".
In March 2017, Open Whisper Systems transitioned Signal's calling system from RedPhone to WebRTC, also adding the ability to make video calls with the mobile apps.
Since 2018: Signal Technology Foundation.
On 21 February 2018, Moxie Marlinspike and WhatsApp co-founder Brian Acton announced the formation of the Signal Technology Foundation, a 501(c)(3) nonprofit organization whose mission is "to support, accelerate, and broaden Signal's mission of making private communication accessible and ubiquitous". Acton started the foundation with $50 million in funding and became the foundation's executive chairman after leaving WhatsApp's parent company Facebook in September 2017. Marlinspike continued as Signal Messenger's first CEO. As of 2020[ [update]], Signal ran entirely on donations, as a nonprofit.
Between November 2019 and February 2020, Signal added iPad support, view-once images and videos, stickers, and reactions. They also announced plans for a new group messaging system and an "experimental method for storing encrypted contacts in the cloud."
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". 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 murder of George Floyd. 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.
On 7 January 2021, Signal saw a surge in new user registrations, which temporarily overwhelmed Signal's capacity to deliver account verification messages. CNN and MacRumors linked the surge with a WhatsApp privacy policy change and a Signal endorsement by Elon Musk and Edward Snowden via Twitter. The surge was also tied to the attack on the United States Capitol. International newspapers reported similar trends in the United Arab Emirates. Reuters reported that more than 100,000 people had installed Signal between 7 and 8 January.
Between 12 and 14 January 2021, the number of Signal installations listed on Google Play increased from over 10 million to over 50 million.
On 15 January 2021, due to the surge of new users, Signal was overwhelmed with the new traffic and was down for all users. On the afternoon of 16 January, Signal announced via Twitter that service had been restored.
On 10 January 2022, Moxie Marlinspike announced that he was stepping down from his role as CEO of Signal Messenger. He continues to remain on the Signal Foundation's board of directors and Brian Acton has volunteered to serve as interim CEO during the search for a new CEO.
In August 2022, Signal notified 1900 users that their data had been affected by the Twilio breach including user phone numbers and SMS verification codes. At least one journalist had his account re-registered to a device he did not control as a result of the attack.
In September 2022 Signal Messaging LLC announced that AI researcher and noted critic of big tech Meredith Whittaker would fill the newly created position of President.
Usage.
Signal's userbase started in May 2010, when its predecessor TextSecure was launched by Whisper Systems. According to App Annie, Signal had approximately 20 million monthly active users at the end of December 2020. In January 2022, the BBC reported that Signal was used by over 40 million people.
Developers and funding.
The development of Signal and its predecessors at Open Whisper Systems was funded by a combination of consulting contracts, donations and grants. The Freedom of the Press Foundation acted as Signal's fiscal sponsor. Between 2013 and 2016, the project received grants from the Knight Foundation, the Shuttleworth Foundation, and almost $3 million from the US government–sponsored Open Technology Fund. 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 Signal Technology Foundation, also created by them in 2018. The Foundation was funded with an initial loan of $50 million from Acton, "to support, accelerate, and broaden Signal's mission of making private communication accessible and ubiquitous". All of the organization's products are published as 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.
Features.
Signal provides one-to-one and group voice and video calls with up to forty participants on iOS, Android, and desktop platforms. The calls are carried via the devices' wired or wireless (carrier or WiFi) data connections. The application can send text messages, documents files, voice notes, pictures, stickers, GIFs, and video messages. The platform also supports group messaging.
All communication sessions between Signal users are automatically end-to-end encrypted (the encryption keys are generated and stored on the devices, and not on servers). 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. The platform employs a trust-on-first-use mechanism to notify the user if a correspondent's key changes.
Until 2023, 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. Users could then use the same application to communicate with contacts who do not have Signal. As of October 2022, this feature has been deprecated due to safety and security concerns, and was removed in 2023.
TextSecure allowed the user to set a passphrase that encrypted the local message database and the user's encryption keys. This did not encrypt the user's contact database or message timestamps. The Signal applications on Android and iOS can be locked with the phone's pin, passphrase, or biometric authentication. The user can define a "screen lock timeout" interval, where Signal will re-encrypt the messages after a certain amount of time, providing an additional protection mechanism in case the phone is lost or stolen.
Signal has a feature for scheduling messages. In addition, timers may be attached to messages to automatically delete the messages from both the sender's and the receivers' devices. The time period for keeping the message may be between five seconds and one week, and begins for each recipient once they have read their copy of the message. 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".
Signal's app icon may be changed with a variety of colour themes for customization and to hide the app. The application name can also be customized. Messages can have effects like spoilers and italics, and users can add each other via QR code.
Signal excludes users' messages from non-encrypted cloud