Signal (software)


Signal's icon for iOS and Chrome on the left, Android on the right
Developer(s) Open Whisper Systems and contributors
Initial release July 2014 (2014-07)[1]
Stable release

Android 3.23.0 (November 28, 2016 (2016-11-28)[2]) [±]

iOS 2.6.6 (November 30, 2016 (2016-11-30)[3]) [±]
Preview release Chrome app 0.26.0 (November 17, 2016 (2016-11-17)[4]) [±]
Development status Active
Available in 31 languages
Type Encrypted voice calling and instant messaging

Signal is an encrypted instant messaging and voice calling application for Android and iOS. It uses the Internet to send one-to-one and group messages, which can include images and video messages, and make one-to-one voice calls. Signal uses standard cellular mobile numbers as identifiers and end-to-end encryption to secure all communications to other Signal users. The applications include mechanisms by which users can independently verify the identity of their messaging correspondents and the integrity of the data channel. In addition, a Chrome app that can link with a Signal client has been released.

Signal is developed by Open Whisper Systems. The clients are published as free and open-source software under the GPLv3 license. The server code is published under the AGPLv3 license.


Background (2010–2013)

Signal is the successor of an encrypted voice calling app called RedPhone and an encrypted texting program called TextSecure. The beta versions of RedPhone and TextSecure were first launched in May 2010 by Whisper Systems,[9] a startup company co-founded by security researcher Moxie Marlinspike and roboticist Stuart Anderson.[10][11] Whisper Systems also produced a firewall and tools for encrypting other forms of data.[10][12] 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. The financial terms of the deal were not disclosed by either company.[13] The acquisition was done "primarily so that Mr. Marlinspike could help the then-startup improve its security".[14] Shortly after the acquisition, Whisper Systems' RedPhone service was made unavailable.[15] 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 2011 Egyptian revolution.[16]

Twitter released TextSecure as free and open-source software under the GPLv3 license in December 2011.[10][17][18][19] RedPhone was also released under the same license in July 2012.[20] Marlinspike later left Twitter and founded Open Whisper Systems as a collaborative Open Source project for the continued development of TextSecure and RedPhone.[1][21]

Open Whisper Systems (2013–present)

A timeline of the development of Signal. a) Addition of encrypted group chat and instant messaging capabilities to TextSecure. b) End of encrypted SMS/MMS messaging in TextSecure, which prompted the creation of a fork. c) The Android version of Signal was launched after RedPhone was merged into TextSecure. d) Signal for iOS was launched as a RedPhone counterpart for iOS. e) Addition of encrypted group chat and instant messaging capabilities to the iOS version of Signal.

Open Whisper Systems' website was launched in January 2013.[21]

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.[22] Toward the end of July 2014, Open Whisper Systems announced plans to unify its RedPhone and TextSecure applications as Signal.[23] 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.[23] Signal was the first iOS app to enable easy, strongly encrypted voice calls for free.[1][24] TextSecure compatibility was added to the iOS application in March 2015.[25][26]

From its launch in May 2010[9] until March 2015, the Android version of Signal (then called TextSecure) included support for encrypted SMS/MMS messaging.[27] From version 2.7.0 onward, the Android application only supported sending and receiving encrypted messages via the data channel.[28] Reasons for this included security flaws of SMS/MMS and problems with the key exchange.[28] Open Whisper Systems' abandonment of SMS/MMS encryption prompted some users to create a fork named Silence (initially called SMSSecure[29]) that is meant solely for the exchange of encrypted SMS and MMS messages.[30][31]

In November 2015, the TextSecure and RedPhone applications on Android were merged to become Signal for Android.[32] A month later, Open Whisper Systems announced Signal Desktop, a Chrome app that can link with a Signal client.[33] At launch, the app could only be linked with the Android version of Signal.[34] On September 26, 2016, Open Whisper Systems announced that Signal Desktop could now be linked with the iOS version of Signal as well.[35]

On October 4, 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.[36][37][38] 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".[37][36] Along with the subpoena, OWS received a gag order requiring OWS not to tell anyone about the subpoena for one year.[36] OWS approached the ACLU, and they were able to lift part of the gag order after challenging it in court.[36] 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".[38]


Signal allows users to call other Signal users on iOS and Android. All calls are made over a Wi-Fi or data connection and with the exception of data fees are free of charge, including long distance and international.[24] Signal also allows users to send group, text, picture, and video messages over a Wi-Fi or data connection to other Signal users on iOS and on Android.

All communications to other Signal users are automatically end-to-end encrypted. The keys that are used to encrypt the user's communications are generated and stored at the endpoints (i.e. by users, not by servers).[39] Signal also has built-in mechanisms for verifying that no man-in-the-middle attack has occurred. For calls, Signal displays two words on the screen. If the words match on both ends of the call, the call is secure.[24][40] For messages, Signal users can compare key fingerprints (or scan QR codes) out-of-band.[41] The app employs a trust on first use mechanism in order to notify the user if a correspondent's key changes.[41]

On Android, users can 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.[22] Users can also opt into allowing Signal to fall back to unencrypted SMS/MMS when communicating with contacts who do not have Signal.[22]

The Android version of Signal allows the user to set a passphrase that encrypts the local message database and the user's encryption keys.[42] This does not encrypt the user's contact database or message timestamps.[42] The user can define a time period after which the application "forgets" the passphrase, providing an additional protection mechanism in case the phone is lost or stolen.[41] On iOS, the local message database is encrypted by the operating system if the user has a passphrase on their lock screen.

Signal also allows users to set timers to messages.[43] After a specified time interval, the messages will be deleted from both the sender's and the receivers' devices.[43] The time interval can be between five seconds and one week long,[43] and the timer begins for each recipient once they have read their copy of the message.[44] The developers have stressed that this is meant to be "a collaborative feature for conversations where all participants want to automate minimalist data hygiene, not for situations where your contact is your adversary".[43][44]


Signal requires that the user has a phone number for verification.[45] The number does not have to be the same as on the device's SIM card; it can also be a VoIP number[45] 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 to one device at a time.[45]

Signal also requires that the primary device is an Android or iOS based smartphone with an Internet connection. A Chrome app that can link with a Signal client is in development.[34]

Android specific

Signal's official Android client requires the proprietary Google Play Services because the app is dependent on Google's GCM push messaging framework.[46] As of March 2015, Signal's message delivery has been done by Open Whisper Systems themselves and the client relies on GCM only for a wakeup event.[47]

In May 2016, Signal's lead developer Moxie Marlinspike requested that LibreSignal, a fork that had removed the GCM dependency, not use the Signal servers or the Signal name.[48] As a result, on 24 May 2016 the project README stated the project was "abandoned".[49]


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.[50] 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 the majority of these users still believed they had succeeded while in reality they failed.[50]


Encryption protocols

Main articles: Signal Protocol and ZRTP

Signal instant messages are encrypted with the Signal Protocol (formerly known as the Axolotl protocol[51]). The protocol combines the Double Ratchet Algorithm, prekeys, and a 3-DH handshake.[52] It uses Curve25519, AES-256, and HMAC-SHA256 as primitives.[53] The protocol provides confidentiality, integrity, authentication, participant consistency, destination validation, forward secrecy, backward secrecy (aka future secrecy), causality preservation, message unlinkability, message repudiation, participation repudiation, and asynchronicity.[54] It does not provide anonymity preservation, and requires servers for the relaying of messages and storing of public key material.[54]

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.[54] 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.[54]

In October 2014, researchers from Ruhr University Bochum published an analysis of the Signal Protocol.[53] Among other findings, they presented an unknown key-share attack on the protocol, but in general, they found that it was secure.[55]

As of October 2016, the Signal Protocol has also been implemented into WhatsApp, Facebook Messenger, and Google Allo, encrypting the conversations of "more than a billion people worldwide".[56]

Signal voice calls are encrypted with SRTP and the ZRTP key-agreement protocol, which was developed by Phil Zimmermann.[1][57]


Signal relies on centralized servers that are maintained by Open Whisper Systems. In addition to routing Signal's messages and calls, the servers also facilitate the discovery of contacts who are also registered Signal users and the automatic exchange of users' public keys. Open Whisper Systems has set up dozens of servers to handle the encrypted calls in more than 10 countries around the world to minimize latency.[1]

Contact discovery

The servers store registered users' phone numbers, public key material and push tokens which are necessary for setting up calls and transmitting messages.[58] In order to determine which contacts are also Signal users, cryptographic hashes of the user's contact numbers are periodically transmitted to the server.[59] 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.[59] The hashed numbers are thereafter discarded from the server.[58] Moxie Marlinspike has written 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 "practical privacy preserving contact discovery remains an unsolved problem".[60][59]


All client-server communications are protected by TLS.[57][61] Once the server removes this layer of encryption, each message contains either the phone number of the sender or the receiver in plaintext.[60] This metadata could in theory allow the creation of "a detailed overview on when and with whom users communicated".[60] Signal's privacy policy states that these identifiers are only kept on the servers as long as necessary in order to place each call or transmit each message.[58] Open Whisper Systems have asserted that their servers do not keep logs about who called whom and when.[62] 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."[63]

The group messaging mechanism is designed so that the servers do not have access to the membership list, group title, or group icon.[28] 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.[64][65]


Signal's server architecture was federated between December 2013 and February 2016. In December 2013, it was announced that the messaging protocol that is used in Signal had successfully been integrated into the Android-based open-source operating system CyanogenMod.[66][67][68] Since CyanogenMod 11.0, the client logic was contained in a system app called WhisperPush. According to Open Whisper Systems, the Cyanogen team ran their own Signal messaging server for WhisperPush clients, which federated with Open Whisper Systems' Signal server, so that both clients could exchange messages with each-other.[68] The WhisperPush source code was available under the GPLv3 license.[69] In February 2016, the CyanogenMod team discontinued WhisperPush and recommended that its users switch to Signal.[70] In May 2016, Moxie Marlinspike wrote that federation with the CyanogenMod servers degraded the user experience and held back development, and that Open Whisper Systems' servers will probably not federate with other servers again.[48]


The complete source code of the Signal clients for Android, iOS and Google Chrome are available on GitHub under a free software license.[5][6][7] 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 Open Whisper Systems. In March 2016, Moxie Marlinspike wrote that, apart from some shared libraries that aren't compiled with the project build due to a lack of Gradle NDK support, Signal for Android is reproducible.[71]

The software that handles Signal's message routing is called Signal-Server and it is also open source.[8] In January 2013, Moxie Marlinspike wrote that the server-side component of Signal's voice calling feature (formerly RedPhone) is a TURN server that acts as a bridge for the data connections that are required to establish calls between mobile devices.[72][73][74] In November 2016, a web developer questioned why Open Whisper Systems had not open sourced the server-side component of Signal's voice calling feature.[75] Open Whisper Systems has tweeted that there are "plenty of [open-source software] options to choose from".[76] Open Whisper Systems does not officially provide support for people to host their own servers.[77]


Signal is officially distributed only through the Google Play store, Apple's App Store, and the Chrome Web Store. Applications distributed via Google Play are signed by the developer of the application, and the Android operating system checks that updates are signed with the same key, preventing others from distributing updates that the developer themselves did not sign.[78][79] The same applies to iOS applications that are distributed via Apple's App Store.[80]

Open Whisper Systems have declined requests to distribute the Android version of Signal as a separate APK package binary or through third-party distribution platforms. Signal's predecessor (TextSecure) was briefly included in the F-Droid software repository in 2012, but was removed at the developer's request because it was an unverified build and exceptionally out of date. Open Whisper Systems have subsequently said that they will not support their applications being distributed through F-Droid because it does not provide timely software updates, relies on a centralized trust model and necessitates allowing the installation of apps from unknown sources which harms Android's security for average users.[46] F-Droid subverts the other distribution platforms' decentralized trust model by often building and signing their own packages, meaning F-Droid administrators or anyone else with access could be compelled to push out malicious updates to any application.[78] Additionally, the keys for most applications in the main repository are stored on machines that are accessible from the Internet[81] (i.e. not separated by an air gap), which Marlinspike asserts is an unacceptable security risk.[82]


In October 2014, the Electronic Frontier Foundation (EFF) included Signal in their updated surveillance self-defense guide.[83] In November 2014, Signal received a perfect score on the EFF's secure messaging scorecard;[39] it received points for having communications encrypted in transit, having communications encrypted with keys the provider doesn't 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.[39] 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.[39]

On December 28, 2014, Der Spiegel published 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, and when used in conjunction with other privacy tools such as Cspace, Tor, Tails, and TrueCrypt was ranked as "catastrophic," leading to a "near-total loss/lack of insight to target communications, presence..."[84][85]

Former NSA contractor Edward Snowden has endorsed Signal on multiple occasions.[33] In his keynote speech at SXSW in March 2014, he praised Signal's predecessors (TextSecure and RedPhone) for their ease-of-use.[86] During an interview with The New Yorker in October 2014, he recommended using "anything from Moxie Marlinspike and Open Whisper Systems".[87] During a remote appearance at an event hosted by Ryerson University and Canadian Journalists for Free Expression in March 2015, Snowden said that Signal is "very good" and that he knew the security model.[88] Asked about encrypted messaging apps during a Reddit AMA in May 2015, he recommended Signal.[89][90] In November 2015, Snowden tweeted that he used Signal "every day".[32][91]

In September 2015, the American Civil Liberties Union called on officials at the U.S. Capitol to ensure that lawmakers and staff members have secure communications technology.[92] One of the applications that the ACLU recommended in their letter to the Senate Sergeant at Arms and to the House Sergeant at Arms was Signal, writing:

One of the most widely respected encrypted communication apps, Signal, from Open Whisper Systems, has received significant financial support from the U.S. government, has been audited by independent security experts, and is now widely used by computer security professionals, many of the top national security journalists, and public interest advocates. Indeed, members of the ACLU’s own legal department regularly use Signal to make encrypted telephone calls.[93]

Following the 2016 Democratic National Committee email leak, Vanity Fair reported that Marc Elias, the general counsel for Hillary Clinton's presidential campaign, had instructed DNC staffers to exclusively use Signal when saying anything "remotely contentious or disparaging" about Republican presidential nominee, Donald Trump.[94][95]

Developers and funding

Main article: Open Whisper Systems

Signal is developed by a nonprofit software group called Open Whisper Systems.[96] The group is funded by a combination of donations and grants, and all of its products are published as free and open-source software.

As of October 2016, the project has received an unknown amount of donations from individual sponsors via the Freedom of the Press Foundation.[97] Open Whisper Systems has received grants from the Knight Foundation,[98] the Shuttleworth Foundation,[99] and the Open Technology Fund,[100] a U.S. government funded program that has also supported other privacy projects like the anonymity software Tor and the encrypted instant messaging app Cryptocat.

See also


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  • 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 & Services (iiWAS2015). ACM International Conference Proceedings Series. ISBN 978-1-4503-3491-4. Retrieved 18 March 2016. 
  • Schröder, 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 1-891562-45-2. Retrieved 29 August 2016. 
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