Signal (software)
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| Signal (software) | |
|---|---|
| Name | Signal |
| Developer | Signal Foundation |
| Released | 2014 |
| Programming language | Java, Objective-C, C++ |
| Operating system | Android, iOS, Windows, macOS, Linux |
| License | GPLv3 (libsignal), AGPLv3 (server) |
Signal (software) Signal is a cross-platform encrypted messaging application developed by the Signal Foundation and Signal Messenger LLC. It provides end-to-end encrypted text, voice, video, and file transfers, emphasizing privacy and security for individual and group communication. Signal has been influential in debates involving surveillance, cryptography, and digital rights.
History
Signal traces its lineage to work by Open Whisper Systems, founded by Moxie Marlinspike and collaborators, and leverages protocols developed in research involving Whisper Systems. The project grew amid debates involving Edward Snowden, Julian Assange, and disclosures about NSA surveillance, prompting interest from activists like Glenn Greenwald and organizations such as the ACLU. Major milestones include adoption of the Signal protocol in apps by WhatsApp, Google, and Facebook Messenger, following cryptographic collaborations with groups including researchers at OpenSSL, MIT, Stanford University, and contributors from Listen Technologies. Funding and structural changes involved the establishment of the Signal Foundation with support from Brian Acton, co-founder of WhatsApp, and public endorsements from figures including Edward Snowden and Elon Musk. Legal and policy discussions around Signal intersected with cases before courts influenced by precedent from United States v. Jones and debates involving legislation such as the CALEA and proposals in the United Kingdom Investigatory Powers Act 2016.
Features and technology
Signal implements the Signal Protocol, originally called the Axolotl Ratchet, developed by Trevor Perrin and Moxie Marlinspike, building on concepts from OTR and work by cryptographers at Phil Zimmermann's projects. Core features include end-to-end encryption for messages, voice calls, and video calls, using cryptographic primitives like Curve25519, XSalsa20, Poly1305, and HKDF. The app supports disappearing messages, sealed sender, and safety numbers for contact verification; it integrates forward secrecy and post-compromise security inspired by research from David Chaum and practices discussed at conferences like CRYPTO and ACM CCS. Group messaging uses sender keys and asymmetric ratchets with server-assisted metadata minimization approaches examined by teams at University of California, Berkeley and University College London. Signal uses a federated model for some signaling but centralizes delivery services hosted on servers overseen by the Signal Foundation.
Security and privacy
Signal's architecture emphasizes minimal metadata retention, a design discussed in analyses by researchers from EFF and the Center for Internet and Society at Stanford Law School. The open-source codebases for client and server have been audited by security firms such as Cure53 and reviewed by academics from University of Waterloo and University of Oxford. Threat models addressed include state-level actors like the NSA, corporate surveillance enterprises represented by Cambridge Analytica, and criminal interception techniques highlighted in filings with bodies such as the FCC. Signal resists interception through features like local key storage and registration locking, analogous to protections advocated by Bruce Schneier and standards discussed at IETF working groups. Nonetheless, debates involve lawful access requests by authorities in jurisdictions including France, Germany, and Australia, reflecting tensions examined in hearings before legislatures like the United States Congress.
Platforms and availability
Signal is available for mobile operating systems such as Android and iOS, and desktop platforms including Microsoft Windows, macOS, and Linux. Distribution channels have included app stores like Google Play and Apple App Store, as well as direct packages distributed via repositories affiliated with organizations like Debian and Ubuntu. International adoption patterns show uptake in regions affected by events like the 2019–20 Hong Kong protests, the Russian invasion of Ukraine (2022), and the Arab Spring, with activist communities, journalists from outlets such as The Guardian and The New York Times, and nonprofit organizations including Amnesty International relying on Signal for secure communications.
Reception and impact
Signal has been praised by technologists including Bruce Schneier, journalists such as Kashmir Hill, and whistleblowers including Edward Snowden for its security posture and transparency. It has also been the subject of controversy in contexts involving content moderation by platforms like Facebook and regulatory scrutiny by authorities in countries such as India and Brazil. The app influenced commercial services—its protocol was adopted by WhatsApp, Google Allo (discontinued), and Facebook Messenger—and shaped policy debates before institutions like the Council of Europe and the European Commission. Awards and recognition include mentions in lists by Time (magazine) and citations in academic literature from IEEE and ACM conferences. Its role in protests and diplomatic communications has linked Signal to events like the 2020 United States presidential election and international crises where encrypted messaging affected reporting and activism.
Business model and governance
Signal is governed by the nonprofit Signal Foundation, co-founded by Brian Acton and Moxie Marlinspike, with operational work performed by Signal Messenger LLC. Funding sources include philanthropic grants, donations from individuals, and initial endowment by Brian Acton; governance structures emphasize nonprofit stewardship and community-driven development similar to other organizations like the Mozilla Foundation and Electronic Frontier Foundation. Transparency reports and governance disclosures have been presented to oversight bodies and civil society groups, drawing comparisons to models used by Tor Project and OpenSSL governance discussions. Policy decisions about moderation, feature prioritization, and legal compliance are informed by engagement with stakeholders including human rights groups like Human Rights Watch and standards bodies such as the IETF.