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47-633: The algorithm provides forward secrecy for messages, and implicit renegotiation of forward keys; properties for which the protocol is named. The Double Ratchet Algorithm was developed by Trevor Perrin and Moxie Marlinspike ( Open Whisper Systems ) in 2013 and introduced as part of the Signal Protocol in February 2014. The Double Ratchet Algorithm's design is based on the DH ratchet that was introduced by Off-the-Record Messaging (OTR) and combines it with

94-510: A Chrome app that could link with a Signal client. At launch, the app could only be linked with the Android version of Signal. On 26 September 2016, Open Whisper Systems announced that Signal Desktop could now be linked with the iOS version of Signal as well. On 31 October 2017, Open Whisper Systems announced that the Chrome app was deprecated . At the same time, they announced the release of

141-473: A man-in-the-middle attack . An implementation can also choose to employ a trust on first use mechanism in order to notify users if a correspondent's key changes. The Signal Protocol does not prevent a company from retaining information about when and with whom users communicate. There can therefore be differences in how messaging service providers choose to handle this information. Signal's privacy policy states that recipients' identifiers are only kept on

188-623: A "sealed sender" feature into Signal, which reduces the amount of metadata that the Signal servers have access to by concealing the sender's identifier. The sender's identity is conveyed to the recipient in each message, but is encrypted with a key that the server does not have. This is done automatically if the sender is in the recipient's contacts or has access to their Signal Profile. Users can also enable an option to receive "sealed sender" messages from non-contacts and people who do not have access to their Signal Profile. A contemporaneous wiretap of

235-633: A Double Ratchet Algorithm implementation. The developers of Wire have said that their app uses a custom implementation of the Double Ratchet Algorithm. Messaging Layer Security , an IETF proposal, uses Asynchronous ratcheting trees to efficiently improve upon security guarantees over Signal's Double Ratchet . Signal Messenger maintains a reference implementation of the Signal Protocol library written in Rust under

282-607: A collaborative open source project for the continued development of TextSecure and RedPhone. Marlinspike launched Open Whisper Systems' website in January 2013. In February 2014, Open Whisper Systems introduced the second version of their TextSecure Protocol (now Signal Protocol ), which added end-to-end encrypted group chat and instant messaging capabilities to TextSecure. Toward the end of July 2014, Open Whisper Systems announced plans to unify its RedPhone and TextSecure applications as Signal . These announcements coincided with

329-420: A common secret from a DH ratchet. At the same time it tries to use every opportunity to provide the remote peer with a new public DH value and advance the DH ratchet whenever a new DH value from the remote peer arrives. As soon as a new common secret is established, a new hash ratchet gets initialized. As cryptographic primitives, the Double Ratchet Algorithm uses The following is a list of applications that use

376-430: A firewall and tools for encrypting other forms of data. 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. The acquisition was done "primarily so that Mr. Marlinspike could help the then-startup improve its security". Shortly after the acquisition, Whisper Systems' RedPhone service was made unavailable. Some criticized

423-479: A new feature, WhatsApp Status, which uses the Signal Protocol to secure its contents. In October 2016, WhatsApp's parent company Facebook also deployed an optional mode called Secret Conversations in Facebook Messenger which provides end-to-end encryption using an implementation of the Signal Protocol. In September 2015, G Data Software launched a new messaging app called Secure Chat which used

470-425: A partnership with WhatsApp to provide end-to-end encryption by incorporating the Signal Protocol into each WhatsApp client platform. Open Whisper Systems said that they had already incorporated the protocol into the latest WhatsApp client for Android and that support for other clients, group/media messages, and key verification would be coming soon after. WhatsApp confirmed the partnership to reporters, but there

517-614: A partnership with WhatsApp to provide end-to-end encryption by incorporating the Signal Protocol into each WhatsApp client platform. Open Whisper Systems said that they had already incorporated the protocol into the latest WhatsApp client for Android and that support for other clients, group/media messages, and key verification would be coming soon after. On April 5, 2016, WhatsApp and Open Whisper Systems announced that they had finished adding end-to-end encryption to "every form of communication" on WhatsApp, and that users could now verify each other's keys. In February 2017, WhatsApp announced

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564-731: A remote appearance at an event hosted by Ryerson University and Canadian Journalists for Free Expression , in March 2015. Asked about encrypted messaging apps during a Reddit AMA in May 2015, he recommended "Signal for iOS, Redphone/TextSecure for Android". In November 2015, Snowden tweeted that he used Signal "every day". In October 2014, the Electronic Frontier Foundation (EFF) included TextSecure, RedPhone, and Signal in their updated Surveillance Self-Defense (SSD) guide. In November 2014, all three received top scores on

611-542: A standalone desktop client for certain Windows, MacOS and Linux distributions. On 4 October 2016, the American Civil Liberties Union (ACLU) and Open Whisper Systems published a series of documents revealing that OWS had received a subpoena requiring them to provide information associated with two phone numbers for a federal grand jury investigation in the first half of 2016. Only one of

658-580: A symmetric-key ratchet modeled after the Silent Circle Instant Messaging Protocol (SCIMP). The ratchet was initially named after the critically endangered aquatic salamander axolotl , which has extraordinary self-healing capabilities. In March 2016, the developers renamed the Axolotl Ratchet as the Double Ratchet Algorithm to better differentiate between the ratchet and the full protocol, because some had used

705-406: Is said to encrypt the conversations of "more than a billion people worldwide" or Google who provides end-to-end encryption by default to all RCS -based conversations between users of their Google Messages app for one-to-one conversations. Facebook Messenger also say they offer the protocol for optional Secret Conversations, as does Skype for its Private Conversations. The protocol combines

752-564: The Double Ratchet Algorithm , prekeys, and a triple Elliptic-curve Diffie–Hellman (3-DH) handshake, and uses Curve25519 , AES-256 , and HMAC-SHA256 as primitives . The development of the Signal Protocol was started by Trevor Perrin and Moxie Marlinspike (Open Whisper Systems) in 2013. The first version of the protocol, TextSecure v1, was based on Off-the-record messaging (OTR). On 24 February 2014, Open Whisper Systems introduced TextSecure v2, which migrated to

799-855: The Shuttleworth Foundation , the Knight Foundation , and the Open Technology Fund . Signal Messenger was initially funded by donations through the Freedom of the Press Foundation , which acted as Signal Messenger's fiscal sponsor while the Signal Foundation's non-profit status was pending. The Signal Foundation is officially tax-exempt as of February 2019. In January 2021, the tech billionaire Elon Musk tweeted his support for

846-473: The TextSecure Protocol ) is a non- federated cryptographic protocol that provides end-to-end encryption for voice and instant messaging conversations. The protocol was developed by Open Whisper Systems in 2013 and was introduced in the open-source TextSecure app, which later became Signal . Several closed-source applications have implemented the protocol, such as WhatsApp , which

893-492: The Axolotl Ratchet. The design of the Axolotl Ratchet is based on the ephemeral key exchange that was introduced by OTR and combines it with a symmetric-key ratchet modeled after the Silent Circle Instant Messaging Protocol (SCIMP). It brought about support for asynchronous communication ("offline messages") as its major new feature, as well as better resilience with distorted order of messages and simpler support for conversations with multiple participants. The Axolotl Ratchet

940-496: The Double Ratchet Algorithm or a custom implementation of it: Open Whisper Systems Open Whisper Systems (abbreviated OWS ) was a software development group that was founded by Moxie Marlinspike in 2013. The group picked up the open source development of TextSecure and RedPhone, and was later responsible for starting the development of the Signal Protocol and the Signal messaging app. In 2018, Signal Messenger

987-762: The EFF's Secure Messaging Scorecard, along with Cryptocat , Silent Phone , and Silent Text . They received points for having communications encrypted in transit, having communications encrypted with keys the providers don't have access to ( end-to-end encryption ), making it possible for users to independently verify their correspondent's identities, having past communications secure if the keys are stolen ( forward secrecy ), having their code open to independent review ( open source ), having their security designs well-documented, and having recent independent security audits. On 28 December 2014, Der Spiegel published slides from an internal NSA presentation dating to June 2012 in which

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1034-680: The NSA deemed 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..." Over its five-year existence from 2013 to 2018, the Open Whisper Systems group managed multiple projects, which included: Some of these projects were discontinued or merged into other projects: Signal Protocol The Signal Protocol (formerly known as

1081-492: The Signal Protocol for end-to-end encryption. In October 2016, Facebook deployed an optional mode called "secret conversations" in Facebook Messenger mobile apps which provides end-to-end encryption using an implementation of the Signal Protocol. In November 2015, the TextSecure and RedPhone applications on Android were merged to become Signal for Android. A month later, Open Whisper Systems announced Signal Desktop,

1128-648: The Signal Protocol. G Data discontinued the service in May 2018. In September 2016, Google launched a new messaging app called Allo , which featured an optional Incognito Mode that used the Signal Protocol for end-to-end encryption. In March 2019, Google discontinued Allo in favor of their Google Messages app on Android. In November 2020, Google announced that they would be using the Signal Protocol to provide end-to-end encryption by default to all RCS -based conversations between users of their Google Messages app, starting with one-to-one conversations. In January 2018, Open Whisper Systems and Microsoft announced

1175-548: The Signal app with two words "Use Signal", showing his favor for the app as an alternative to WhatsApp. Musk doubled down stating he had financially supported Signal in the past and that he will continue to do so. In addition to other platform mass migrations, Signal saw a large influx of new users and user donations. Former NSA contractor Edward Snowden endorsed Open Whisper Systems applications, including during an interview with The New Yorker in October 2014, and during

1222-436: The Signal servers as long as necessary in order to transmit each message. In June 2016, Moxie Marlinspike told The Intercept : "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." In October 2018, Signal Messenger announced that they had implemented

1269-455: The addition of Signal Protocol support to an optional Skype mode called Private Conversations. The Signal Protocol has had an influence on other cryptographic protocols. In May 2016, Viber said that their encryption protocol is a custom implementation that "uses the same concepts" as the Signal Protocol. Forsta's developers have said that their app uses a custom implementation of the Signal Protocol. The Double Ratchet Algorithm that

1316-501: The deniability properties. An example of this is the Signal Protocol, which combines the Double Ratchet Algorithm, prekeys, and a 3-DH handshake. 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. It does not provide anonymity preservation, and requires servers for

1363-521: The following keys. It is said to detect reordering, deletion, and replay of sent messages, and improve forward secrecy properties against passive eavesdropping in comparison to OTR messaging. Combined with public key infrastructure for the retention of pregenerated one-time keys (prekeys), it allows for the initialization of messaging sessions without the presence of the remote peer ( asynchronous communication ). The usage of triple Diffie–Hellman key exchange (3-DH) as initial key exchange method improves

1410-570: The gag order after challenging it in court. OWS said it was the first time they had received a subpoena, and that they were committed to treat "any future requests the same way". On February 21, 2018, Moxie Marlinspike and WhatsApp co-founder Brian Acton announced the formation of the Signal Foundation , a 501(c)(3) non-profit organization whose mission is "to support, accelerate, and broaden Signal’s mission of making private communication accessible and ubiquitous." The foundation

1457-539: The initial release of Signal as a RedPhone counterpart for iOS . The developers said that their next steps would be to provide TextSecure instant messaging capabilities for iOS, unify the RedPhone and TextSecure applications on Android, and launch a web client. Signal was the first iOS app to enable easy, strongly encrypted voice calls for free. TextSecure compatibility was added to the iOS application in March 2015. On 18 November 2014, Open Whisper Systems announced

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1504-467: The name Axolotl when referring to the Signal Protocol. The Double Ratchet Algorithm features properties that have been commonly available in end-to-end encryption systems for a long time: encryption of contents on the entire way of transport as well as authentication of the remote peer and protection against manipulation of messages. As a hybrid of DH and KDF ratchets, it combines several desired features of both principles. From OTR messaging it takes

1551-486: The properties of forward secrecy and automatically reestablishing secrecy in case of compromise of a session key, forward secrecy with a compromise of the secret persistent main key, and plausible deniability for the authorship of messages. Additionally, it enables session key renewal without interaction with the remote peer by using secondary KDF ratchets. An additional key-derivation step is taken to enable retaining session keys for out-of-order messages without endangering

1598-500: The protocol, concluding that the protocol was cryptographically sound. Another audit of the protocol was published in 2017. The protocol provides confidentiality, integrity, authentication , participant consistency, destination validation, forward secrecy , post-compromise security (aka future secrecy), causality preservation, message unlinkability, message repudiation , participation repudiation, and asynchronicity. It does not provide anonymity preservation and requires servers for

1645-458: The ratchet and the full protocol because some had used the name Axolotl when referring to the full protocol. As of October 2016 , the Signal Protocol is based on TextSecure v3, but with additional cryptographic changes. In October 2016, researchers from the UK's University of Oxford , Australia's Queensland University of Technology , and Canada's McMaster University published a formal analysis of

1692-699: The relaying of messages and storing of public key material. 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 . 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. For authentication, users can manually compare public key fingerprints through an outside channel. This makes it possible for users to verify each other's identities and avoid

1739-417: The relaying of messages and storing of public key material. A client attempts to renew session key material interactively with the remote peer using a Diffie-Hellman (DH) ratchet. If this is impossible, the clients renew the session key independently using a hash ratchet. With every message, a client advances one of two hash ratchets—one for sending and one for receiving. These two hash ratchets get seeded with

1786-691: 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 . Twitter released TextSecure as free and open-source software under the GPLv3 license in December 2011. RedPhone was also released under the same license in July 2012. Marlinspike later left Twitter and founded Open Whisper Systems as

1833-450: The two phone numbers was registered on Signal, and because of how the service is designed, OWS was only able to provide "the time the user’s account had been created and the last time it had connected to the service". Along with the subpoena, OWS received a gag order requiring OWS not to tell anyone about the subpoena for one year. OWS approached the ACLU, and they were able to lift part of

1880-412: The user's device and/or the Signal servers may still reveal that the device's IP address accessed a Signal server to send or receive messages at certain times. Open Whisper Systems first introduced the protocol in application TextSecure . They later merged an encrypted voice call application named RedPhone into TextSecure and renamed it Signal . In November 2014, Open Whisper Systems announced

1927-495: The wire protocol. In October 2014, researchers from Ruhr University Bochum published an analysis of TextSecure v3. Among other findings, they presented an unknown key-share attack on the protocol, but in general, they found that it was secure. In March 2016, the developers renamed the protocol as the Signal Protocol. They also renamed the Axolotl Ratchet as the Double Ratchet algorithm to better differentiate between

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1974-699: Was incorporated as an LLC by Moxie Marlinspike and Brian Acton and then rolled under the independent 501c3 non-profit Signal Technology Foundation . Today, the Signal app is developed by Signal Messenger LLC, which is funded by the Signal Technology Foundation. Security researcher Moxie Marlinspike and roboticist Stuart Anderson co-founded a startup company called Whisper Systems in 2010. The company produced proprietary enterprise mobile security software. Among these were an encrypted texting program called TextSecure and an encrypted voice calling app called RedPhone. They also developed

2021-560: Was introduced as part of the Signal Protocol has also been adopted by other protocols. OMEMO is an XMPP Extension Protocol (XEP) that was introduced in the Conversations messaging app and approved by the XMPP Standards Foundation (XSF) in December 2016 as XEP-0384. Matrix is an open communications protocol that includes Olm, a library that provides optional end-to-end encryption on a room-by-room basis via

2068-420: Was named after the critically endangered aquatic salamander Axolotl , which has extraordinary self-healing capabilities. The developers refer to the algorithm as self-healing because it automatically disables an attacker from accessing the cleartext of later messages after having compromised a session key . The third version of the protocol, TextSecure v3, made some changes to the cryptographic primitives and

2115-477: Was no announcement or documentation about the encryption feature on the official website, and further requests for comment were declined. On 5 April 2016, WhatsApp and Open Whisper Systems announced that they had finished adding end-to-end encryption to "every form of communication" on WhatsApp, and that users could now verify each other's keys. In September 2016, Google launched a new messaging app called Allo , which features an optional "incognito mode" that uses

2162-466: Was pending. The Signal Foundation became officially tax-exempt in February 2019. In May 2014, Moxie Marlinspike said that "Open Whisper Systems is a project rather than a company, and the project's objective is not financial profit." News media outlets later described Open Whisper Systems as a "non-profit software group" while the project was not registered as a non-profit organization . Between 2013 and 2016, Open Whisper Systems received grants from

2209-463: Was started with an initial $ 50 million in funding from Acton, who had left WhatsApp's parent company Facebook in September 2017. According to the announcement, Acton is the foundation's executive chairman and Marlinspike continued as CEO of Signal Messenger. The Freedom of the Press Foundation agreed to continue accepting donations on behalf of Signal while the Signal Foundation's non-profit status

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