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Wikipedia

Signal (messaging app)

April 12, 2024

"Signal Messenger" redirects here. For the company, see Signal Messenger LLC. For its parent organization, see Signal Technology Foundation. For protocol, see Signal Protocol.

Signal is an encrypted messaging service for instant messaging, voice, and video calls.[13][14] The instant messaging function includes sending text, voice notes, images, videos, and other files.[15] Communication may be one-to-one between users or may involve group messaging.

Signal
Developer(s)
Initial release29 July 2014; 9 years ago (2014-07-29)[1][2]
Stable release(s)
Android7.2.4[3]  / 3 April 2024; 6 days ago (3 April 2024)
iOS7.4[4]  / 27 March 2024; 13 days ago (27 March 2024)
Desktop7.4.0[5]  / 28 March 2024; 12 days ago (28 March 2024)
Preview release(s)
Android (Beta)7.3.0[6]  / 3 April 2024; 6 days ago (3 April 2024)
iOS (Beta)7.4.0.9[4]  / 27 March 2024; 13 days ago (27 March 2024)
Desktop (Beta)7.5.0-beta.1[7]  / 4 April 2024; 5 days ago (4 April 2024)
Repository
  • github.com/signalapp/Signal-Android
Operating system
TypeEncrypted voice calling, video calling and instant messaging
LicenseAGPL-3.0-only[9][10][11][12][a][b]
Websitesignal.org 

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.[10][9][11][12][a][b] The official Android app generally uses the proprietary Google Play Services, although it is designed to work without them. Signal is also distributed for iOS and desktop programs for Windows, macOS, and Linux. Registration for desktop use requires an iOS or Android device.[19][20]

Signal uses mobile telephone numbers as identifiers for users. It secures all communications with end-to-end encryption. The client software includes mechanisms by which users can independently verify the identity of their contacts and the integrity of the data channel.[21][22]

The non-profit Signal Foundation was launched in February 2018 with initial funding of $50 million from Brian Acton.[23] As of January 2022, the platform had approximately 40 million monthly active users. As of May 2021, it was downloaded more than 105 million times.[24][25]

Until the feature was removed in 2023, the Android version was also optionally capable of functioning as an SMS app.[21][26]

History edit

2010–2013: Origins edit

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,[52] a startup company co-founded by security researcher Moxie Marlinspike and roboticist Stuart Anderson.[53][54] Whisper Systems also produced a firewall and tools for encrypting other forms of data.[53][55] 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.[56] The acquisition was done "primarily so that Mr. Marlinspike could help the then-startup improve its security".[57] Shortly after the acquisition, Whisper Systems' RedPhone service was made unavailable.[58] 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.[59]

Twitter released TextSecure as free and open-source software under the GPLv3 license in December 2011.[53][60][61][62] RedPhone was also released under the same license in July 2012.[63] Marlinspike later left Twitter and founded Open Whisper Systems as a collaborative Open Source project for the continued development of TextSecure and RedPhone.[1][64]

2013–2018: Open Whisper Systems edit

See also: Open Whisper Systems

Open Whisper Systems' website was launched in January 2013.[64]

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.[65] Toward the end of July 2014, they announced plans to merge the RedPhone and TextSecure applications as Signal.[66] 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.[66] Signal was the first iOS app to enable end-to-end encrypted voice calls for free.[1][67] TextSecure compatibility was added to the iOS application in March 2015.[68][69]

 
Signal Android icon, 2015–2017
 
Signal icon, 2015–2020

From its launch in May 2010[52] until March 2015, the Android version of Signal (then called TextSecure) included support for encrypted SMS/MMS messaging.[70] From version 2.7.0 onward, the Android application only supported sending and receiving encrypted messages via the data channel.[71] Reasons for this included security flaws of SMS/MMS and problems with the key exchange.[71] Open Whisper Systems' abandonment of SMS/MMS encryption prompted some users to create a fork named Silence (initially called SMSSecure[72]) that is meant solely for the exchange of encrypted SMS and MMS messages.[73][74]

In November 2015, the TextSecure and RedPhone applications on Android were merged to become Signal for Android.[75] A month later, Open Whisper Systems announced Signal Desktop, a Chrome app that could link with a Signal mobile client.[76] At launch, the app could only be linked with the Android version of Signal.[77] On 26 September 2016, Open Whisper Systems announced that Signal Desktop could now be linked with the iOS version of Signal as well.[78] On 31 October 2017, Open Whisper Systems announced that the Chrome app was deprecated.[8] At the same time, they announced the release of a standalone desktop client (based on the Electron framework[11]) for Windows, macOS and certain Linux distributions.[8][79]

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.[80][81][82] 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".[81][80] Along with the subpoena, OWS received a gag order requiring OWS not to tell anyone about the subpoena for one year.[80] OWS approached the ACLU, and they were able to lift part of the gag order after challenging it in court.[80] 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".[82]

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.[83][84][13]

Since 2018: Signal Technology Foundation edit

See also: 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".[85][23] 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.[23] Marlinspike continued as Signal Messenger's first CEO.[85] As of 2020, Signal ran entirely on donations, as a nonprofit.[86]

Between November 2019 and February 2020, Signal added iPad support, view-once images and videos, stickers, and reactions.[87] They also announced plans for a new group messaging system and an "experimental method for storing encrypted contacts in the cloud."[87]

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".[88][86] 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.[88] 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.[86][89]

On 7 January 2021, Signal saw a surge in new user registrations, which temporarily overwhelmed Signal's capacity to deliver account verification messages.[90] CNN and MacRumors linked the surge with a WhatsApp privacy policy change and a Signal endorsement by Elon Musk and Edward Snowden via Twitter.[90][91] The surge was also tied to the far-right attack on the United States Capitol.[92] International newspapers reported similar trends in the United Arab Emirates.[93] Reuters reported that more than 100,000 people had installed Signal between 7 and 8 January.[94]

Between 12 and 14 January 2021, the number of Signal installations listed on Google Play increased from over 10 million to over 50 million.[95][96][97][98] On 15 January 2021, due to the surge of new users, Signal was overwhelmed with the new traffic and was down for all users.[99][100] On the afternoon of 16 January, Signal announced via Twitter that service had been restored.[101]

On 10 January 2022, Moxie Marlinspike announced that he was stepping down from his role as CEO of Signal Messenger.[102] 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.[102]

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.[103] At least one journalist had his account re-registered to a device he did not control as a result of the attack.[104]

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.[105]

Usage edit

Signal's userbase started in May 2010, when its predecessor TextSecure was launched by Whisper Systems.[52] According to App Annie, Signal had approximately 20 million monthly active users at the end of December 2020.[106] In January 2022, the BBC reported that Signal was used by over 40 million people.[107]

Developers and funding edit

Main article: Signal Foundation

The development of Signal and its predecessors at Open Whisper Systems was funded by a combination of consulting contracts, donations and grants.[108] The Freedom of the Press Foundation acted as Signal's fiscal sponsor.[85][109][110] Between 2013 and 2016, the project received grants from the Knight Foundation,[111] the Shuttleworth Foundation,[112] and almost $3 million from the US government–sponsored Open Technology Fund.[113] 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".[85][23][114] 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.[115]

Features edit

Signal provides one-to-one and group[116] voice and video[13] calls with up to forty participants on iOS, Android, and desktop platforms.[117][118] The calls are carried via the devices' wired or wireless (carrier or WiFi) data connections.[67] The application can send text messages, documents files,[15] voice notes, pictures, GIFs,[119] 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).[120] 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.[121] The platform employs a trust-on-first-use mechanism to notify the user if a correspondent's key changes.[121]

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.[65] Users could then use the same application to communicate with contacts who do not have Signal.[65] As of October 2022, this feature has been deprecated due to safety and security concerns, and was removed in 2023.[122][26]

TextSecure allowed the user to set a passphrase that encrypted the local message database and the user's encryption keys.[123] This did not encrypt the user's contact database or message timestamps.[123] The Signal applications on Android and iOS can be locked with the phone's pin, passphrase, or biometric authentication.[124] The user can define a "screen lock timeout" interval, providing an additional protection mechanism in case the phone is lost or stolen.[121][124]

Signal has a feature for scheduling messages.[125] In addition, timers may be attached to messages[126] to automatically delete the messages from both the sender's and the receivers' devices.[126] The time period for keeping the message may be between five seconds and one week,[126] and begins for each recipient once they have read their copy of the message.[127] 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".[126][127]

Signal's app icon may be changed from with a variety of colour themes for customization and the application name can also be customized.[128] Upcoming features includes hiding spoilers[129] as well adding other users via QR code.[130]

Signal excludes users' messages from non-encrypted cloud backups by default.[131]

Signal allows users to automatically blur faces of people in photos to protect identities.[132][133]

Signal includes a cryptocurrency wallet functionality for storing, sending and receiving in-app payments.[134] Apart from certain regions and countries,[134] the feature was enabled globally in November 2021.[135] As of January 2022, the only supported payment method is MobileCoin.[134]

In February 2024, Signal added a username feature to the beta version of the app. This is a privacy feature that allows users to communicate with others without having to share their telephone number.[136][137]

Limitations edit

Signal requires that the user provide a telephone number for verification,[138] eliminating the need for user names or passwords and facilitating contact discovery (see below).[139] The number does not have to be the same as on the device's SIM card; it can also be a VoIP number[138] 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.[140] Account registration requires an iOS or Android device.[19][20]

This mandatory connection to a telephone number (a feature Signal shares with WhatsApp, KakaoTalk, and others) has been criticized as a "major issue" for privacy-conscious users who are not comfortable with giving out their private number.[139] A workaround is to use a secondary phone number.[139] The ability to choose a public, changeable username instead of sharing one's phone number is a widely-requested feature.[139][141][142] This feature was added to the beta version of Signal in February 2024.[143]

Using phone numbers as identifiers may also create security risks that arise from the possibility of an attacker taking over a phone number.[139] 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.[103] The threat of this attack can be mitigated by enabling Signal's Registration Lock feature, a form of two-factor authentication that requires the user to enter a PIN to register the phone number on a new device.[144]

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.[145]

Usability edit

In July 2016, the Internet Society published a user study that assessed the ability of Signal users to detect and deter man-in-the-middle attacks.[22] The study concluded that 21 out of 28 participants failed to correctly compare public key fingerprints 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.[22] Four months later, Signal's user interface was updated to make verifying the identity of other Signal users simpler.[146]

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.[147]

Architecture edit

Encryption protocols edit

Main article: 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 (X3DH) handshake.[148][149] It uses Curve25519, AES-256, and HMAC-SHA256 as primitives.[21] The protocol provides confidentiality, integrity, authentication, participant consistency, destination validation, forward secrecy, backward secrecy (a.k.a. future secrecy), causality preservation, message unlinkability, message repudiation, participation repudiation, and asynchronicity.[150] It does not provide anonymity preservation, and requires servers for the relaying of messages and storing of public key material.[150]

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.[150] 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.[150]

In October 2014, researchers from Ruhr University Bochum published an analysis of the Signal Protocol.[21] Among other findings, they presented an unknown key-share attack on the protocol, but in general, they found that it was secure.[151] In October 2016, researchers from UK's University of Oxford, Queensland University of Technology in Australia, and Canada's McMaster University published a formal analysis of the protocol.[152][153] They concluded that the protocol was cryptographically sound.[152][153] In July 2017, researchers from 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.[154]

As of August 2018, the Signal Protocol has been implemented into WhatsApp, Facebook Messenger, Skype,[155] and Google Allo,[156] making it possible for the conversations of "more than a billion people worldwide" to be end-to-end encrypted.[157] 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.[131][158][155][159]

Up until March 2017, Signal's voice calls were encrypted with SRTP and the ZRTP key-agreement protocol, which was developed by Phil Zimmermann.[1][160] In March 2017, Signal transitioned to a new WebRTC-based calling system that introduced the ability to make video calls.[84] Signal's voice and video calling functionalities use the Signal Protocol channel for authentication instead of ZRTP.[161][83][13]

Authentication edit

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.[121] The platform employs a trust on first use mechanism in order to notify the user if a correspondent's key changes.[121]

Local storage edit

After receiving and decrypting messages, the application stored them locally on each device in a SQLite database that is encrypted with SQLCipher.[162] The cryptographic key for this database is also stored locally and can be accessed if the device is unlocked.[162][163] In December 2020, Cellebrite published a blog post announcing that one of their products could now access this key and use it to "decrypt the Signal app".[162][164] 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".[165][166] This latter interpretation was rejected by several experts,[167] 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".[168][169] Similar extraction tools also exist for iOS devices and Signal Desktop.[170][171]

Servers edit

Signal relies on centralized servers that are maintained by Signal Messenger. In addition to routing Signal's messages, the servers also facilitate the discovery of contacts who are also registered Signal users and the automatic exchange of users' public keys. By default, Signal's voice and video calls are peer-to-peer.[13] If the caller is not in the receiver's address book, the call is routed through a server in order to hide the users' IP addresses.[13]

Contact discovery edit

The servers store registered users' phone numbers, public key material and push tokens which are necessary for setting up calls and transmitting messages.[172] In order to determine which contacts are also Signal users, cryptographic hashes of the user's contact numbers are periodically transmitted to the server.[173] 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.[173] The hashed numbers are thereafter discarded from the server.[172] 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."[174][173] 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."[175][176]

Metadata edit

All client-server communications are protected by TLS.[160][177] Signal's developers have asserted that their servers do not keep logs about who called whom and when.[178] 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".[131]

The group messaging mechanism is designed so that the servers do not have access to the membership list, group title, or group icon.[71] 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.[179][180]

Federation edit

Signal's server architecture was 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 CyanogenMod.[181][182][183] 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.[183] The WhisperPush source code was available under the GPLv3 license.[184] In February 2016, the CyanogenMod team discontinued WhisperPush and recommended that its users switch to Signal.[185] 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.[186]

In May 2016, Moxie Marlinspike requested that a third-party client called LibreSignal not use the Signal service or the Signal name.[186] As a result, on 24 May 2016 the LibreSignal project posted that the project was "abandoned".[187] The functionality provided by LibreSignal was subsequently incorporated into Signal by Marlinspike.[188]

Licensing edit

The complete source code of the Signal clients for Android, iOS and desktop is available on GitHub under a free software license.[10][9][11] 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 NDK support, Signal for Android is reproducible.[189] Signal's servers are partially open source, but the server software's anti-spam component is proprietary and closed source due to security concerns.[12][190]

Reception edit

Security edit

In October 2014, the Electronic Frontier Foundation (EFF) included Signal in their updated surveillance self-defense guide.[191] In November 2014, Signal received a perfect score on the EFF's secure messaging scorecard;[120] 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), making it possible for users to independently verify their correspondents' identities, having past communications secure if the keys are stolen (forward secrecy), having the code open to independent review (open source), having the security designs well-documented, and having a recent independent security audit.[120] At the time, "ChatSecure + Orbot", Pidgin (with OTR), Silent Phone, and Telegram's optional "secret chats" also received seven out of seven points on the scorecard.[120]

Former NSA contractor Edward Snowden has endorsed Signal on multiple occasions.[76] In his keynote speech at SXSW in March 2014, he praised Signal's predecessors (TextSecure and RedPhone) for their ease of use.[192][193] In December 2014, 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, Tor, Tails, and TrueCrypt was ranked as "catastrophic" and led to a "near-total loss/lack of insight to target communications [and] presence".[194][195]

Following the 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.[196][197]

In March 2017, Signal was approved by the sergeant at arms of the U.S. Senate for use by senators and their staff.[198][199]

On 27 September 2019, Natalie Silvanovich, a security engineer working in Google's vulnerability research team at Project Zero, disclosed how a bug in the Android Signal client could let an attacker spy on a user without their knowledge.[200] 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).[201] The bug was fixed the same day that it was reported and patched in release 4.47.7 of the app for Android.[202]

In February 2020, the European Commission recommended that its staff use Signal.[203] Following the George Floyd protests, which began in May 2020, Signal was downloaded 121,000 times in the U.S. between 25 May and 4 June.[204] In July 2020, Signal became the most downloaded app in Hong Kong on both the Apple App Store and the Google Play Store after the passage of the Hong Kong national security law.[205]

As of January 2021, Signal is a contact method for securely providing tips to major news outlets such as The Washington Post,[206] The Guardian,[207] The New York Times,[208] and The Wall Street Journal.[209]

Candiru claims the ability to capture data from Signal Private Messenger with their spyware, at a fee of €500.000.[210]

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.[211]

In-app payments edit

In April 2021, Signal announced the addition of a cryptocurrency wallet feature that would allow users to send and receive payments in MobileCoin.[212] 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.[213] The wallet functionality was initially only available in certain countries, but was later enabled globally in November 2021.[135]

Blocking edit

 
  Countries where Signal's domain fronting is enabled by default
  Countries where Signal is blocked (March 2021)

In December 2016, Egypt blocked access to Signal.[214] In response, Signal's developers added domain fronting to their service.[215] 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.[215][216] As of May 2022, Signal's domain fronting is enabled by default in Egypt, UAE, Oman, Qatar, Iran, Cuba, Uzbekistan and Ukraine.[217]

As of January 2018, Signal was blocked in Iran.[218][219] Signal's domain fronting feature relies on the Google App Engine (GAE) service.[219][218] This does not work in Iran because Google has blocked Iranian access to GAE in order to comply with U.S. sanctions.[218][220]

In early 2018, 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, 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.[221][222] Signal switched from AWS back to Google in April 2019.[223]

In January 2021, Iran removed the app from app stores,[224][225] and blocked Signal.[226] Signal was later blocked by China in March 2021.[227]

Audience edit

Use by activists edit

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.[228]

In March 2021, the United Nations recommended Myanmar residents use Signal and Proton Mail to pass and preserve evidence of human rights violations committed during the 2021 coup.[229]

Controversial use edit

Signal's terms of service state that the product must not be used to violate the law.[230] 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".[230] 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".[230][231]

In 2016, authorities in India arrested members of a suspected ISIS-affiliated terrorist cell that communicated via Signal.[232]

Extremist right-wing militias and white nationalists use Signal for organizing their actions, including the Unite the Right II rally in 2018.[233][234][235][236]

The claim that Signal is used to fund terrorist or criminal activities is the justification for 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."[237][238]

See also edit

Notes edit

  1. ^ a b The iOS and Android clients were formerly published under the GPL-3.0-only license, and were updated to AGPL in 2022 and 2023 respectively.[16][17]
  2. ^ a b The source code for spam detection is not public.[18]

References edit

  1. ^ a b c d Greenberg, Andy (29 July 2014). "Your iPhone Can Finally Make Free, Encrypted Calls". Wired. from the original on 18 January 2015. Retrieved 18 January 2015.
  2. ^ Marlinspike, Moxie (29 July 2014). "Free, Worldwide, Encrypted Phone Calls for iPhone". Open Whisper Systems. from the original on 31 August 2017. Retrieved 16 January 2017.
  3. ^ "Signal Private Messenger Android GitHub Repo". 3 April 2024.
  4. ^ a b "Signal Private Messenger iOS GitHub Repo". 27 March 2024.
  5. ^ "Signal Private Messenger Desktop GitHub Repo". 28 March 2024.
  6. ^ "Signal Private Messenger Android GitHub Repo". 3 April 2024.
  7. ^ "Signal Private Messenger Desktop GitHub Repo". 4 April 2024.
  8. ^ a b c d e "Installing Signal - Signal Support". from the original on 27 December 2023. Retrieved 17 March 2024.
  9. ^ a b c Signal. "Signal-Android". GitHub. from the original on 30 December 2015. Retrieved 10 May 2023.
  10. ^ a b c Signal. "Signal-iOS". GitHub. from the original on 11 November 2014. Retrieved 10 May 2023.
  11. ^ a b c d Signal. "Signal-Desktop". GitHub. from the original on 8 April 2016. Retrieved 10 May 2023.
  12. ^ a b c Signal. "Signal-Server". GitHub. from the original on 28 December 2016. Retrieved 10 May 2023.
  13. ^ a b c d e f Mott, Nathaniel (14 March 2017). "Signal's Encrypted Video Calling For iOS, Android Leaves Beta". Tom's Hardware. Purch Group, Inc. Retrieved 14 March 2017.
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Bibliography edit

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External links edit

 
Wikimedia Commons has media related to Signal (software).
  • Official website  

April 12, 2024
signal, messaging, signal, messenger, redirects, here, company, signal, messenger, parent, organization, signal, technology, foundation, protocol, signal, protocol, signal, encrypted, messaging, service, instant, messaging, voice, video, calls, instant, messag. Signal Messenger redirects here For the company see Signal Messenger LLC For its parent organization see Signal Technology Foundation For protocol see Signal Protocol Signal is an encrypted messaging service for instant messaging voice and video calls 13 14 The instant messaging function includes sending text voice notes images videos and other files 15 Communication may be one to one between users or may involve group messaging SignalDeveloper s Signal Technology Foundation Signal Messenger LLC and contributorsInitial release29 July 2014 9 years ago 2014 07 29 1 2 Stable release s Android7 2 4 3 3 April 2024 6 days ago 3 April 2024 iOS7 4 4 27 March 2024 13 days ago 27 March 2024 Desktop7 4 0 5 28 March 2024 12 days ago 28 March 2024 Preview release s Android Beta 7 3 0 6 3 April 2024 6 days ago 3 April 2024 iOS Beta 7 4 0 9 4 27 March 2024 13 days ago 27 March 2024 Desktop Beta 7 5 0 beta 1 7 4 April 2024 5 days ago 4 April 2024 Repositorygithub wbr com wbr signalapp wbr Signal AndroidOperating systemAndroid 5 0 or later iOS 13 or later Windows 10 and Windows 11 8 macOS 10 15 or later 8 Linux distributions supporting APT 8 FreeBSDTypeEncrypted voice calling video calling and instant messagingLicenseAGPL 3 0 only 9 10 11 12 a b Websitesignal wbr org 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 10 9 11 12 a b The official Android app generally uses the proprietary Google Play Services although it is designed to work without them Signal is also distributed for iOS and desktop programs for Windows macOS and Linux Registration for desktop use requires an iOS or Android device 19 20 Signal uses mobile telephone numbers as identifiers for users It secures all communications with end to end encryption The client software includes mechanisms by which users can independently verify the identity of their contacts and the integrity of the data channel 21 22 The non profit Signal Foundation was launched in February 2018 with initial funding of 50 million from Brian Acton 23 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 24 25 Until the feature was removed in 2023 the Android version was also optionally capable of functioning as an SMS app 21 26 Contents 1 History 1 1 2010 2013 Origins 1 2 2013 2018 Open Whisper Systems 1 3 Since 2018 Signal Technology Foundation 1 4 Usage 1 5 Developers and funding 2 Features 3 Limitations 4 Usability 5 Architecture 5 1 Encryption protocols 5 1 1 Authentication 5 2 Local storage 5 3 Servers 5 3 1 Contact discovery 5 3 2 Metadata 5 3 3 Federation 5 4 Licensing 6 Reception 6 1 Security 6 2 In app payments 6 3 Blocking 6 4 Audience 6 4 1 Use by activists 6 4 2 Controversial use 7 See also 8 Notes 9 References 10 Bibliography 11 External linksHistory edit2010 2013 Origins edit 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 52 a startup company co founded by security researcher Moxie Marlinspike and roboticist Stuart Anderson 53 54 Whisper Systems also produced a firewall and tools for encrypting other forms of data 53 55 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 56 The acquisition was done primarily so that Mr Marlinspike could help the then startup improve its security 57 Shortly after the acquisition Whisper Systems RedPhone service was made unavailable 58 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 59 Twitter released TextSecure as free and open source software under the GPLv3 license in December 2011 53 60 61 62 RedPhone was also released under the same license in July 2012 63 Marlinspike later left Twitter and founded Open Whisper Systems as a collaborative Open Source project for the continued development of TextSecure and RedPhone 1 64 2013 2018 Open Whisper Systems edit See also Open Whisper Systems Open Whisper Systems website was launched in January 2013 64 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 65 Toward the end of July 2014 they announced plans to merge the RedPhone and TextSecure applications as Signal 66 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 66 Signal was the first iOS app to enable end to end encrypted voice calls for free 1 67 TextSecure compatibility was added to the iOS application in March 2015 68 69 nbsp Signal Android icon 2015 2017 nbsp Signal icon 2015 2020 From its launch in May 2010 52 until March 2015 the Android version of Signal then called TextSecure included support for encrypted SMS MMS messaging 70 From version 2 7 0 onward the Android application only supported sending and receiving encrypted messages via the data channel 71 Reasons for this included security flaws of SMS MMS and problems with the key exchange 71 Open Whisper Systems abandonment of SMS MMS encryption prompted some users to create a fork named Silence initially called SMSSecure 72 that is meant solely for the exchange of encrypted SMS and MMS messages 73 74 In November 2015 the TextSecure and RedPhone applications on Android were merged to become Signal for Android 75 A month later Open Whisper Systems announced Signal Desktop a Chrome app that could link with a Signal mobile client 76 At launch the app could only be linked with the Android version of Signal 77 On 26 September 2016 Open Whisper Systems announced that Signal Desktop could now be linked with the iOS version of Signal as well 78 On 31 October 2017 Open Whisper Systems announced that the Chrome app was deprecated 8 At the same time they announced the release of a standalone desktop client based on the Electron framework 11 for Windows macOS and certain Linux distributions 8 79 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 80 81 82 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 81 80 Along with the subpoena OWS received a gag order requiring OWS not to tell anyone about the subpoena for one year 80 OWS approached the ACLU and they were able to lift part of the gag order after challenging it in court 80 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 82 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 83 84 13 Since 2018 Signal Technology Foundation edit See also 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 85 23 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 23 Marlinspike continued as Signal Messenger s first CEO 85 As of 2020 update Signal ran entirely on donations as a nonprofit 86 Between November 2019 and February 2020 Signal added iPad support view once images and videos stickers and reactions 87 They also announced plans for a new group messaging system and an experimental method for storing encrypted contacts in the cloud 87 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 88 86 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 88 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 86 89 On 7 January 2021 Signal saw a surge in new user registrations which temporarily overwhelmed Signal s capacity to deliver account verification messages 90 CNN and MacRumors linked the surge with a WhatsApp privacy policy change and a Signal endorsement by Elon Musk and Edward Snowden via Twitter 90 91 The surge was also tied to the far right attack on the United States Capitol 92 International newspapers reported similar trends in the United Arab Emirates 93 Reuters reported that more than 100 000 people had installed Signal between 7 and 8 January 94 Between 12 and 14 January 2021 the number of Signal installations listed on Google Play increased from over 10 million to over 50 million 95 96 97 98 On 15 January 2021 due to the surge of new users Signal was overwhelmed with the new traffic and was down for all users 99 100 On the afternoon of 16 January Signal announced via Twitter that service had been restored 101 On 10 January 2022 Moxie Marlinspike announced that he was stepping down from his role as CEO of Signal Messenger 102 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 102 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 103 At least one journalist had his account re registered to a device he did not control as a result of the attack 104 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 105 Usage edit Graphs are unavailable due to technical issues There is more info on Phabricator and on MediaWiki org Signal s userbase started in May 2010 when its predecessor TextSecure was launched by Whisper Systems 52 According to App Annie Signal had approximately 20 million monthly active users at the end of December 2020 106 In January 2022 the BBC reported that Signal was used by over 40 million people 107 Developers and funding edit Main article Signal Foundation The development of Signal and its predecessors at Open Whisper Systems was funded by a combination of consulting contracts donations and grants 108 The Freedom of the Press Foundation acted as Signal s fiscal sponsor 85 109 110 Between 2013 and 2016 the project received grants from the Knight Foundation 111 the Shuttleworth Foundation 112 and almost 3 million from the US government sponsored Open Technology Fund 113 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 85 23 114 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 115 Features editSignal provides one to one and group 116 voice and video 13 calls with up to forty participants on iOS Android and desktop platforms 117 118 The calls are carried via the devices wired or wireless carrier or WiFi data connections 67 The application can send text messages documents files 15 voice notes pictures GIFs 119 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 120 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 121 The platform employs a trust on first use mechanism to notify the user if a correspondent s key changes 121 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 65 Users could then use the same application to communicate with contacts who do not have Signal 65 As of October 2022 this feature has been deprecated due to safety and security concerns and was removed in 2023 122 26 TextSecure allowed the user to set a passphrase that encrypted the local message database and the user s encryption keys 123 This did not encrypt the user s contact database or message timestamps 123 The Signal applications on Android and iOS can be locked with the phone s pin passphrase or biometric authentication 124 The user can define a screen lock timeout interval providing an additional protection mechanism in case the phone is lost or stolen 121 124 Signal has a feature for scheduling messages 125 In addition timers may be attached to messages 126 to automatically delete the messages from both the sender s and the receivers devices 126 The time period for keeping the message may be between five seconds and one week 126 and begins for each recipient once they have read their copy of the message 127 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 126 127 Signal s app icon may be changed from with a variety of colour themes for customization and the application name can also be customized 128 Upcoming features includes hiding spoilers 129 as well adding other users via QR code 130 Signal excludes users messages from non encrypted cloud backups by default 131 Signal allows users to automatically blur faces of people in photos to protect identities 132 133 Signal includes a cryptocurrency wallet functionality for storing sending and receiving in app payments 134 Apart from certain regions and countries 134 the feature was enabled globally in November 2021 135 As of January 2022 update the only supported payment method is MobileCoin 134 In February 2024 Signal added a username feature to the beta version of the app This is a privacy feature that allows users to communicate with others without having to share their telephone number 136 137 Limitations editSignal requires that the user provide a telephone number for verification 138 eliminating the need for user names or passwords and facilitating contact discovery see below 139 The number does not have to be the same as on the device s SIM card it can also be a VoIP number 138 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 140 Account registration requires an iOS or Android device 19 20 This mandatory connection to a telephone number a feature Signal shares with WhatsApp KakaoTalk and others has been criticized as a major issue for privacy conscious users who are not comfortable with giving out their private number 139 A workaround is to use a secondary phone number 139 The ability to choose a public changeable username instead of sharing one s phone number is a widely requested feature 139 141 142 This feature was added to the beta version of Signal in February 2024 143 Using phone numbers as identifiers may also create security risks that arise from the possibility of an attacker taking over a phone number 139 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 103 The threat of this attack can be mitigated by enabling Signal s Registration Lock feature a form of two factor authentication that requires the user to enter a PIN to register the phone number on a new device 144 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 145 Usability editIn July 2016 the Internet Society published a user study that assessed the ability of Signal users to detect and deter man in the middle attacks 22 The study concluded that 21 out of 28 participants failed to correctly compare public key fingerprints 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 22 Four months later Signal s user interface was updated to make verifying the identity of other Signal users simpler 146 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 147 Architecture editEncryption protocols edit Main article 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 X3DH handshake 148 149 It uses Curve25519 AES 256 and HMAC SHA256 as primitives 21 The protocol provides confidentiality integrity authentication participant consistency destination validation forward secrecy backward secrecy a k a future secrecy causality preservation message unlinkability message repudiation participation repudiation and asynchronicity 150 It does not provide anonymity preservation and requires servers for the relaying of messages and storing of public key material 150 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 150 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 150 In October 2014 researchers from Ruhr University Bochum published an analysis of the Signal Protocol 21 Among other findings they presented an unknown key share attack on the protocol but in general they found that it was secure 151 In October 2016 researchers from UK s University of Oxford Queensland University of Technology in Australia and Canada s McMaster University published a formal analysis of the protocol 152 153 They concluded that the protocol was cryptographically sound 152 153 In July 2017 researchers from 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 154 As of August 2018 update the Signal Protocol has been implemented into WhatsApp Facebook Messenger Skype 155 and Google Allo 156 making it possible for the conversations of more than a billion people worldwide to be end to end encrypted 157 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 131 158 155 159 Up until March 2017 Signal s voice calls were encrypted with SRTP and the ZRTP key agreement protocol which was developed by Phil Zimmermann 1 160 In March 2017 Signal transitioned to a new WebRTC based calling system that introduced the ability to make video calls 84 Signal s voice and video calling functionalities use the Signal Protocol channel for authentication instead of ZRTP 161 83 13 Authentication edit 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 121 The platform employs a trust on first use mechanism in order to notify the user if a correspondent s key changes 121 Local storage edit After receiving and decrypting messages the application stored them locally on each device in a SQLite database that is encrypted with SQLCipher 162 The cryptographic key for this database is also stored locally and can be accessed if the device is unlocked 162 163 In December 2020 Cellebrite published a blog post announcing that one of their products could now access this key and use it to decrypt the Signal app 162 164 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 165 166 This latter interpretation was rejected by several experts 167 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 168 169 Similar extraction tools also exist for iOS devices and Signal Desktop 170 171 Servers edit Signal relies on centralized servers that are maintained by Signal Messenger In addition to routing Signal s messages the servers also facilitate the discovery of contacts who are also registered Signal users and the automatic exchange of users public keys By default Signal s voice and video calls are peer to peer 13 If the caller is not in the receiver s address book the call is routed through a server in order to hide the users IP addresses 13 Contact discovery edit The servers store registered users phone numbers public key material and push tokens which are necessary for setting up calls and transmitting messages 172 In order to determine which contacts are also Signal users cryptographic hashes of the user s contact numbers are periodically transmitted to the server 173 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 173 The hashed numbers are thereafter discarded from the server 172 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 174 173 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 175 176 Metadata edit All client server communications are protected by TLS 160 177 Signal s developers have asserted that their servers do not keep logs about who called whom and when 178 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 131 The group messaging mechanism is designed so that the servers do not have access to the membership list group title or group icon 71 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 179 180 Federation edit Signal s server architecture was 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 CyanogenMod 181 182 183 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 183 The WhisperPush source code was available under the GPLv3 license 184 In February 2016 the CyanogenMod team discontinued WhisperPush and recommended that its users switch to Signal 185 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 186 In May 2016 Moxie Marlinspike requested that a third party client called LibreSignal not use the Signal service or the Signal name 186 As a result on 24 May 2016 the LibreSignal project posted that the project was abandoned 187 The functionality provided by LibreSignal was subsequently incorporated into Signal by Marlinspike 188 Licensing edit The complete source code of the Signal clients for Android iOS and desktop is available on GitHub under a free software license 10 9 11 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 NDK support Signal for Android is reproducible 189 Signal s servers are partially open source but the server software s anti spam component is proprietary and closed source due to security concerns 12 190 Reception editSecurity edit In October 2014 the Electronic Frontier Foundation EFF included Signal in their updated surveillance self defense guide 191 In November 2014 Signal received a perfect score on the EFF s secure messaging scorecard 120 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 making it possible for users to independently verify their correspondents identities having past communications secure if the keys are stolen forward secrecy having the code open to independent review open source having the security designs well documented and having a recent independent security audit 120 At the time ChatSecure Orbot Pidgin with OTR Silent Phone and Telegram s optional secret chats also received seven out of seven points on the scorecard 120 Former NSA contractor Edward Snowden has endorsed Signal on multiple occasions 76 In his keynote speech at SXSW in March 2014 he praised Signal s predecessors TextSecure and RedPhone for their ease of use 192 193 In December 2014 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 Tor Tails and TrueCrypt was ranked as catastrophic and led to a near total loss lack of insight to target communications and presence 194 195 Following the 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 196 197 In March 2017 Signal was approved by the sergeant at arms of the U S Senate for use by senators and their staff 198 199 On 27 September 2019 Natalie Silvanovich a security engineer working in Google s vulnerability research team at Project Zero disclosed how a bug in the Android Signal client could let an attacker spy on a user without their knowledge 200 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 201 The bug was fixed the same day that it was reported and patched in release 4 47 7 of the app for Android 202 In February 2020 the European Commission recommended that its staff use Signal 203 Following the George Floyd protests which began in May 2020 Signal was downloaded 121 000 times in the U S between 25 May and 4 June 204 In July 2020 Signal became the most downloaded app in Hong Kong on both the Apple App Store and the Google Play Store after the passage of the Hong Kong national security law 205 As of January 2021 update Signal is a contact method for securely providing tips to major news outlets such as The Washington Post 206 The Guardian 207 The New York Times 208 and The Wall Street Journal 209 Candiru claims the ability to capture data from Signal Private Messenger with their spyware at a fee of 500 000 210 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 211 In app payments edit In April 2021 Signal announced the addition of a cryptocurrency wallet feature that would allow users to send and receive payments in MobileCoin 212 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 213 The wallet functionality was initially only available in certain countries but was later enabled globally in November 2021 135 Blocking edit nbsp Countries where Signal s domain fronting is enabled by default Countries where Signal is blocked March 2021 In December 2016 Egypt blocked access to Signal 214 In response Signal s developers added domain fronting to their service 215 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 215 216 As of May 2022 update Signal s domain fronting is enabled by default in Egypt UAE Oman Qatar Iran Cuba Uzbekistan and Ukraine 217 As of January 2018 update Signal was blocked in Iran 218 219 Signal s domain fronting feature relies on the Google App Engine GAE service 219 218 This does not work in Iran because Google has blocked Iranian access to GAE in order to comply with U S sanctions 218 220 In early 2018 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 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 221 222 Signal switched from AWS back to Google in April 2019 223 In January 2021 Iran removed the app from app stores 224 225 and blocked Signal 226 Signal was later blocked by China in March 2021 227 Audience edit Use by activists edit 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 228 In March 2021 the United Nations recommended Myanmar residents use Signal and Proton Mail to pass and preserve evidence of human rights violations committed during the 2021 coup 229 Controversial use edit Signal s terms of service state that the product must not be used to violate the law 230 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 230 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 230 231 In 2016 authorities in India arrested members of a suspected ISIS affiliated terrorist cell that communicated via Signal 232 Extremist right wing militias and white nationalists use Signal for organizing their actions including the Unite the Right II rally in 2018 233 234 235 236 The claim that Signal is used to fund terrorist or criminal activities is the justification for 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 237 238 See also edit nbsp Freedom of speech portal nbsp Free and open source software portal nbsp Telecommunications portalComparison of cross platform instant messaging clients Comparison of VoIP software Internet privacy List of video telecommunication services and product brands Secure communicationNotes edit a b The iOS and Android clients were formerly published under the GPL 3 0 only license and were updated to AGPL in 2022 and 2023 respectively 16 17 a b The source code for spam detection is not public 18 References edit a b c d Greenberg Andy 29 July 2014 Your iPhone Can Finally Make Free Encrypted Calls Wired Archived from the original on 18 January 2015 Retrieved 18 January 2015 Marlinspike Moxie 29 July 2014 Free Worldwide Encrypted Phone Calls for iPhone Open Whisper Systems Archived from the original on 31 August 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Wakefield Jane 22 December 2020 Signal Cellebrite claimed to have cracked chat app s encryption BBC News Archived from the original on 24 December 2020 Retrieved 22 December 2020 Matthew D Green matthew d green 10 December 2020 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 Tweet Retrieved 22 December 2020 via Twitter Edward Snowden Snowden 15 December 2020 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 Tweet Retrieved 22 December 2020 via Twitter Bruce Schneier 21 December 2020 Cellebrite Can Break Signal Schneier on Security Archived from the original on 24 December 2020 Retrieved 24 December 2020 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 John Scott Railton Citizen Lab jsrailton 22 December 2020 Dangerous misunderstanding spreading that Cellebrite UFED can crack amp intercept signalapp This is FALSE Modest reality Cellebrite claimed to extract the Signal chat database from an unlocked Android in their possession Tweet Retrieved 24 December 2020 via Twitter Marlinspike Moxie 23 December 2020 No Cellebrite cannot break Signal encryption signal org Signal Messenger Archived from the original on 24 December 2020 Retrieved 24 December 2020 Signal signalapp 17 December 2020 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 Tweet Retrieved 22 December 2020 via Twitter Betz Bradford 9 February 2021 Court documents show FBI may have tool to access private Signal messages on locked iPhones FOXBusiness Archived from the original on 14 February 2021 Retrieved 14 February 2021 Abrams Lawrence 23 October 2018 Signal Desktop Leaves Message Decryption Key in Plain Sight BleepingComputer Archived from the original on 27 March 2019 Retrieved 19 March 2021 a b Privacy Policy Signal Messenger LLC 25 May 2018 Archived from the original on 24 June 2018 Retrieved 24 June 2018 a b c Moxie Marlinspike 3 January 2013 The Difficulty Of Private Contact Discovery Open Whisper Systems Archived from the original on 4 March 2016 Retrieved 14 January 2016 Rottermanner et al 2015 p 4 Marlinspike Moxie 26 September 2017 Technology preview Private contact discovery for Signal Open Whisper Systems Archived from the original on 28 September 2017 Retrieved 28 September 2017 Greenberg Andy 26 September 2017 Signal Has a Fix for Apps Contact Leaking Problem Wired Archived from the original on 27 September 2017 Retrieved 28 September 2017 Frosch et al 2016 p 7 Brandom Russell 29 July 2014 Signal brings painless encrypted calling to iOS The Verge Archived from the original on 3 February 2015 Retrieved 26 January 2015 Moxie Marlinspike 5 May 2014 Private Group Messaging Open Whisper Systems Archived from the original on 14 July 2014 Retrieved 9 July 2014 Moxie Marlinspike 24 February 2014 The New TextSecure Privacy Beyond SMS Open Whisper Systems Archived from the original on 24 February 2014 Retrieved 26 February 2014 Andy Greenberg 9 December 2013 Ten Million More Android Users Text Messages Will Soon Be Encrypted By Default Forbes Archived from the original on 4 March 2014 Retrieved 28 February 2014 Seth Schoen 28 December 2013 2013 in Review Encrypting the Web Takes A Huge Leap Forward Electronic Frontier Foundation Archived from the original on 1 February 2014 Retrieved 1 March 2014 a b Moxie Marlinspike 9 December 2013 TextSecure Now With 10 Million More Users Open Whisper Systems Archived from the original on 25 February 2014 Retrieved 28 February 2014 CyanogenMod 7 January 2014 android external whispersystems WhisperPush GitHub Archived from the original on 28 June 2015 Retrieved 26 March 2015 Sinha Robin 20 January 2016 CyanogenMod to Shutter WhisperPush Messaging Service on February 1 Gadgets360 NDTV Archived from the original on 11 October 2016 Retrieved 17 March 2024 a b Edge Jake 18 May 2016 The perils of federated protocols LWN net Archived from the original on 14 September 2016 Retrieved 5 July 2016 Le Bihan Michel 24 May 2016 README md GitHub LibreSignal Archived from the original on 27 June 2017 Retrieved 6 November 2016 Support for using Signal without Play Services signalapp Signal Android 1669731 GitHub Archived from the original on 15 February 2020 Retrieved 3 January 2020 Marlinspike Moxie 31 March 2016 Reproducible Signal builds for Android Open Whisper Systems Archived from the original on 15 May 2017 Retrieved 31 March 2016 Improving first impressions on Signal Signal Messenger Archived from the original on 24 September 2023 Retrieved 5 December 2021 Surveillance Self Defense Communicating with Others Electronic Frontier Foundation 23 October 2014 Archived from the original on 8 November 2014 Retrieved 25 January 2015 Eddy Max 11 March 2014 Snowden to SXSW Here s How To Keep The NSA Out Of Your Stuff PC Magazine SecurityWatch Archived from the original on 16 March 2014 Retrieved 17 March 2024 The Virtual Interview Edward Snowden The New Yorker Festival The New Yorker 11 October 2014 Archived from the original on 9 January 2021 Retrieved 24 May 2015 via YouTube Prying Eyes Inside the NSA s War on Internet Security Der Spiegel 28 December 2014 Archived from the original on 24 January 2015 Retrieved 23 January 2015 Presentation from the SIGDEV Conference 2012 explaining which encryption protocols and techniques can be attacked and which not PDF Der Spiegel 28 December 2014 Archived from the original PDF on 8 October 2018 Retrieved 23 January 2015 Bilton Nick 26 August 2016 How the Clinton Campaign Is Foiling the Kremlin Vanity Fair Archived from the original on 29 August 2016 Retrieved 1 September 2016 Blake Andrew 27 August 2016 Democrats warned to use encryption weeks before email leaks The Washington Times The Washington Times LLC Archived from the original on 1 September 2016 Retrieved 1 September 2016 Whittaker Zack 16 May 2017 In encryption push Senate staff can now use Signal for secure messaging ZDNet Archived from the original on 19 July 2017 Retrieved 20 July 2017 Wyden Ron 9 May 2017 Ron Wyden letter on Signal encrypted messaging Documentcloud Zack Whittaker ZDNet Archived from the original on 6 June 2017 Retrieved 20 July 2017 Signal s Messenger Eavesdropping Exploit SaltDNA CTO Shares His Views Archived from the original on 11 August 2020 Retrieved 24 September 2021 1943 project zero Project Zero Monorail bugs chromium org Archived from the original on 28 March 2021 Retrieved 3 March 2021 Signal Messenger Eavesdropping Exploit Confirmed What You Need to Know Forbes Archived from the original on 17 February 2021 Retrieved 3 March 2021 EU Commission to staff Switch to Signal messaging app Politico EU 20 February 2020 Archived from the original on 20 February 2020 Retrieved 20 February 2020 Molla Rani 3 June 2020 From Citizen to Signal the most popular apps right now reflect America s protests Vox Archived from the original on 7 June 2020 Retrieved 7 June 2020 Lee Timothy B 8 July 2020 Hong Kong downloads of Signal surge as residents fear crackdown Ars Technica Archived from the original on 11 July 2020 Retrieved 12 July 2020 How to share documents and news tips with Washington Post journalists The Washington Post Archived from the original on 4 June 2020 Retrieved 17 January 2021 Hoyland Luke Fenn Chris Contact the Guardian securely The Guardian Archived from the original on 7 January 2021 Retrieved 17 January 2021 Tips The New York Times 14 December 2016 ISSN 0362 4331 Archived from the original on 9 June 2020 Retrieved 17 January 2021 WSJ com Secure Drop The Wall Street Journal Archived from the original on 24 July 2020 Retrieved 17 January 2021 Marczak Bill Scott Railton John Berdan Kristin Razzak Bahr Abdul Deibert Ron 15 July 2021 Hooking Candiru Another Mercenary Spyware Vendor Comes into Focus The Citizen Lab Archived from the original on 29 May 2023 Retrieved 20 July 2021 PM lodges report over hacked Telegram Signal accounts The Star Archived from the original on 9 August 2022 Retrieved 9 August 2022 Greenberg Andy 6 April 2021 Signal Adds a Payments Feature With a Privacy Focused Cryptocurrency Wired Archived from the original on 8 April 2021 Retrieved 12 January 2022 Signal Adds Cryptocurrency Support Schneier on Security 7 April 2021 Archived from the original on 8 April 2021 Retrieved 8 April 2021 Cox Joseph 19 December 2016 Signal Claims Egypt Is Blocking Access to Encrypted Messaging App Motherboard Vice Media LLC Archived from the original on 29 June 2017 Retrieved 17 March 2024 a b Marlinspike Moxie 21 December 2016 Doodles stickers and censorship circumvention for Signal Android Open Whisper Systems Archived from the original on 28 December 2016 Retrieved 20 July 2017 Greenberg Andy 21 December 2016 Encryption App Signal Fights Censorship with a Clever Workaround Wired Archived from the original on 11 July 2017 Retrieved 20 July 2017 SignalServiceNetworkAccess kt GitHub Signal Foundation Archived from the original on 17 May 2022 Retrieved 17 May 2022 a b c Frenkel Sheera 2 January 2018 Iranian Authorities Block Access to Social Media Tools The New York Times Archived from the original on 16 January 2018 Retrieved 15 January 2018 a b Domain Fronting for Iran 7311 GitHub 1 January 2018 Retrieved 17 March 2024 Brandom Russell 2 January 2018 Iran blocks encrypted messaging apps amid nationwide protests The Verge Archived from the original on 22 March 2018 Retrieved 23 March 2018 Marlinspike Moxie 1 May 2018 A letter from Amazon signal org Open Whisper Systems Archived from the original on 3 January 2019 Retrieved 10 January 2019 Gallagher Sean 2 May 2018 Amazon blocks domain fronting threatens to shut down Signal s account Ars Technica Archived from the original on 24 January 2019 Retrieved 23 January 2019 Parrelli Greyson 4 April 2019 Attempt to resolve connectivity problems for some users GitHub Signal Messenger LLC Archived from the original on 17 January 2021 Retrieved 2 May 2019 حذف سیگنال از فروشگاه های نرم افزار آنلاین در ایران BBC News فارسی in Persian 15 January 2021 Archived from the original on 17 January 2021 Retrieved 17 January 2021 پیام رسان سیگنال به دستور کمیته فیلترینگ از فروشگاه های آنلاین در ایران حذف شد رادیو فردا in Persian Radio Farda Archived from the original on 13 December 2023 Retrieved 17 January 2021 Jones Rhett 27 January 2021 Signal Blocked by Iran as Encrypted Messaging App s Popularity Explodes Gizmodo Archived from the original on 28 January 2021 Retrieved 28 January 2021 Rita Liao 15 March 2021 Rising encrypted app Signal is down in China TechCrunch Verizon Media Archived from the original on 16 March 2021 Retrieved 16 March 2021 Nierenberg Amelia 11 June 2020 Signal Downloads Are Way Up Since the Protests Began The New York Times Archived from the original on 25 June 2020 Retrieved 12 May 2023 Nebehay Stephanie 17 March 2021 U N team seeks evidence linking Myanmar military leaders to crimes Reuters Archived from the original on 25 March 2021 Retrieved 24 March 2021 a b c Newton Casey 26 January 2021 The battle inside Signal platformer news Archived from the original on 13 December 2023 Retrieved 17 March 2024 Cohn Gordon et al 2016 Tripathi Rahul 11 April 2016 Dangerous Signal This encrypted app is helping ISIS members in India to communicate The Times of India Archived from the original on 30 September 2020 Retrieved 13 January 2021 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 Frenkel Sheera 11 January 2021 Fringe Groups Splinter Online After Facebook and Twitter Bans The New York Times Archived from the original on 12 January 2021 Retrieved 13 January 2021 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 Glaser April 11 August 2018 How White Supremacists Planned Their Rally in D C Slate Archived from the original on 24 December 2020 Retrieved 13 January 2021 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 Daly Kye 11 January 2021 The online far right is moving underground Axios Archived from the original on 12 January 2021 Retrieved 12 January 2021 Telegram and Signal are far more stable and secure and could prove more enduring homes and recruitment stations for far right groups Glaser April 9 October 2018 White Supremacists Still Have a Safe Space Online Slate Archived from the original on 6 January 2021 Retrieved 12 January 2021 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 Secure messaging app Signal has been effectively criminalized in Turkey Nordic Monitor nordicmonitor com 8 July 2021 Archived from the original on 25 December 2023 Retrieved 25 December 2023 Turkey s new media law is bad news but don t report it Brookings Archived from the original on 25 December 2023 Retrieved 25 December 2023 Bibliography editCohn Gordon Katriel Cremers Cas Dowling Benjamin Garratt Luke Stebila Douglas 25 October 2016 A Formal Security Analysis of the Signal Messaging Protocol PDF Cryptology ePrint Archive International Association for Cryptologic Research IACR Archived PDF from the original on 22 February 2017 Retrieved 11 December 2016 Frosch Tilman Mainka Christian Bader Christoph Bergsma Florian Schwenk Jorg Holz Thorsten March 2016 How Secure is TextSecure PDF 2016 IEEE European Symposium on Security and Privacy EuroS amp P 2016 IEEE European Symposium on Security and Privacy EuroS amp P Saarbrucken Germany IEEE pp 457 472 CiteSeerX 10 1 1 689 6003 doi 10 1109 EuroSP 2016 41 ISBN 978 1 5090 1752 2 Archived PDF from the original on 28 February 2019 Retrieved 8 February 2022 Rottermanner Christoph Kieseberg Peter Huber Markus Schmiedecker Martin Schrittwieser Sebastian December 2015 Privacy and Data Protection in Smartphone Messengers PDF Proceedings of the 17th International Conference on Information Integration and Web based Applications amp Services iiWAS2015 ACM International Conference Proceedings Series ISBN 978 1 4503 3491 4 Archived PDF from the original on 27 March 2016 Retrieved 17 March 2024 Schroder Svenja Huber Markus Wind David Rottermanner Christoph 18 July 2016 When Signal hits the Fan On the Usability and Security of State of the Art Secure Mobile Messaging PDF Proceedings of the 1st European Workshop on Usable Security EuroUSEC 16 Darmstadt Germany Internet Society ISOC ISBN 978 1 891562 45 7 Archived from the original PDF on 28 August 2016 Retrieved 29 August 2016 Unger Nik Dechand Sergej Bonneau Joseph Fahl Sascha Perl Henning Goldberg Ian Avrum Smith Matthew 2015 SoK Secure Messaging PDF 2015 IEEE Symposium on Security and Privacy Proceedings of the 2015 IEEE Symposium on Security and Privacy IEEE Computer Society s Technical Committee on Security and Privacy pp 232 249 doi 10 1109 SP 2015 22 ISBN 978 1 4673 6949 7 Archived PDF from the original on 4 March 2016 Retrieved 17 March 2024 External links edit nbsp Wikimedia Commons has media related to Signal software Official website nbsp Retrieved from https en wikipedia org w index php title Signal messaging app amp oldid 1216634542, wikipedia, wiki, book, books, library,

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