Signal (software)

Signal
	File:Signal app logo.svg Logo of Signal
Company type	Non-profit
Industry	Instant messaging, Voice over IP
Founded	February 21, 2018; 8 years ago
Founder	Moxie Marlinspike, Brian Acton
Headquarters	United States
Area served	Worldwide
Key people	Brian Acton (executive chairman), Meredith Whittaker (President of Signal Messaging LLC), Moxie Marlinspike (former CEO)
Products	Encrypted instant messaging, voice, and video calling services
Owner	Signal Technology Foundation
Parent	Signal Technology Foundation
Website	signal.org

Adapted from the English Wikipedia article Signal (software), available under CC BY-SA 4.0Updated 9 August 2025

Signal is a secure, open-source communication platform that provides encrypted instant messaging, voice, and video calling services. It enables users to exchange texts, voice recordings, photos, videos, and various file types within private or group conversations.

Created by the non-profit Signal Foundation and its affiliate Signal Messenger LLC, Signal operates on a centralized model and supports multiple operating systems. The mobile applications, desktop client, and server software are all distributed under the AGPL-3.0-only license. While the Android app typically depends on proprietary Google Play Services, it can function independently of them. Signal is accessible on Android, iOS, Windows, macOS, and Linux platforms. Registering a desktop client requires linking it to an Android or iOS device.

User enrollment in Signal is conducted through mobile phone numbers; however, in March 2024, the platform introduced customizable usernames to improve user privacy by allowing phone number concealment. After discontinuing SMS support on Android in 2023, all communications are protected using end-to-end encryption. The application also offers tools for users to authenticate contact identities and verify data integrity.

Established in February 2018 with an initial $50 million investment from WhatsApp co-founder Brian Acton, the Signal Foundation manages the project’s development. By January 2025, Signal had garnered roughly 70 million active monthly users and surpassed 220 million total downloads.

2010–2013: Origins

Signal traces its roots to two separate encrypted communication apps: RedPhone for secure voice calls and TextSecure for encrypted messaging. Both entered beta testing in May 2010 under Whisper Systems, a startup co-founded by security expert Moxie Marlinspike and roboticist Stuart Anderson. Whisper Systems also produced proprietary mobile security solutions for Android enterprises, including firewalls and encryption utilities.

In November 2011, Twitter acquired Whisper Systems under undisclosed terms to leverage Marlinspike’s security expertise for its platform. Following this acquisition, RedPhone was discontinued—a move that faced criticism due to its role in aiding users under oppressive regimes during events such as Egypt’s 2011 revolution.

Twitter released TextSecure as free and open-source software licensed under GPLv3 in December 2011; RedPhone followed as open source in July 2012. Subsequently, Marlinspike departed Twitter to form Open Whisper Systems, an open-source initiative focused on advancing TextSecure and RedPhone development.

2013–2018: Open Whisper Systems

Open Whisper Systems launched its website in January 2013. The following year in February 2014 marked the release of TextSecure Protocol version two (now known as the Signal Protocol), which introduced end-to-end encrypted group chats alongside instant messaging capabilities. By July 2014, plans were announced to consolidate RedPhone and TextSecure into a single app named Signal, coinciding with Signal’s debut on iOS as a counterpart to RedPhone. The goal was to unify Android apps, introduce TextSecure messaging on iOS, and develop a web client. Signal became the first iOS application to offer free end-to-end encrypted voice calls. Support for TextSecure messaging was added to iOS in March 2015.

Between its initial launch in May 2010 and March 2015, Signal’s Android version (then called TextSecure) supported encrypted SMS/MMS messaging. Starting with version 2.7.0, it transitioned exclusively to encrypted data channel messages due to inherent security vulnerabilities with SMS/MMS and issues related to key exchange protocols. This change led to the creation of Silence, a fork dedicated solely to encrypted SMS/MMS communication.

In November 2015, Open Whisper Systems merged TextSecure and RedPhone into the unified Signal app for Android. One month later saw the release of Signal Desktop as a Chrome application linked with the Android client; by September 2016 it also supported pairing with iOS devices. In October 2017, the Chrome app was retired in favor of a standalone desktop client built with Electron for Windows, macOS, and Linux environments.

On October 4, 2016, Open Whisper Systems alongside the ACLU revealed a subpoena demanding user data tied to two phone numbers during a federal grand jury investigation. Due to Signal’s privacy-focused design, only minimal metadata—specifically account creation date and last connection time—was disclosed. An initial gag order prevented public disclosure of this subpoena but was partially lifted after legal challenges. This represented OWS’s first subpoena response and established their commitment to similarly handle any future requests.

In March 2017, Signal replaced its calling infrastructure from RedPhone with WebRTC, enabling video calling features on mobile applications.

Since 2018: Signal Technology Foundation

On February 21, 2018, Moxie Marlinspike and WhatsApp co-founder Brian Acton established the Signal Technology Foundation, a 501(c)(3) nonprofit dedicated to advancing Signal’s goal of providing accessible private communication. After departing Facebook in September 2017, Acton contributed $50 million to fund the foundation and took on the role of executive chairman, while Marlinspike continued to serve as CEO of Signal Messenger. As of that time, Signal relied exclusively on contributions for its operations.

From late 2019 through early 2020, Signal rolled out new capabilities including support for the iPad, media that can be viewed only once, stickers, message reactions, and announced forthcoming improvements to group chats and encrypted contact management.

During the George Floyd protests, Signal saw a notable increase in adoption within the United States as activists prioritized secure communication amid fears of police monitoring. The app’s downloads increased fivefold in early June 2020, which led Signal to introduce a face-blurring feature designed to protect protesters’ anonymity.

On January 7, 2021, registrations on Signal surged following changes to WhatsApp’s privacy policy and endorsements from figures like Elon Musk and Edward Snowden. This uptick coincided with the January 6 United States Capitol attack and was mirrored internationally, including in the United Arab Emirates. Within two days, over 100,000 new installations occurred, with Google Play downloads jumping from 10 million to more than 50 million between January 12 and 14. The sudden influx temporarily strained Signal’s infrastructure, causing outages until service was fully restored on January 16.

On January 10, 2022, Marlinspike resigned as CEO but maintained a position on the board; Brian Acton stepped in as interim CEO while a replacement was sought.

In August 2022, Signal revealed that a security breach at third-party vendor Twilio compromised data belonging to roughly 1,900 users, including phone numbers and SMS verification codes. This breach allowed unauthorized access to some accounts.

In September 2022, AI specialist and prominent critic of major technology companies Meredith Whittaker was appointed as the first President of Signal Messaging LLC, marking an expansion in strategic leadership.

Usage

Signal’s user base originated in May 2010 with its predecessor TextSecure, developed by Whisper Systems. By December 2020, monthly active users reached approximately 20 million and grew to over 40 million by January 2022. As of February 2025, Signal reported more than 7 million monthly active users in the United States. According to John Ratcliffe, by 2025 Signal is pre-installed on most CIA devices and included in standard onboarding training programs.

Developers and funding

Funding for Signal and its predecessors under Open Whisper Systems came from consulting work, donations, and grants. The Freedom of the Press Foundation acted as its fiscal sponsor. Between 2013 and 2016, financial support included grants from the Knight Foundation, the Shuttleworth Foundation, and nearly $3 million from the US government–supported Open Technology Fund.

Currently developed by Signal Messenger LLC—founded in 2018 by Moxie Marlinspike and Brian Acton—the company is fully owned by the nonprofit Signal Technology Foundation. Acton provided an initial $50 million loan to advance Signal’s mission of enabling private communication. All software products are free and open-source.

In November 2023, Meredith Whittaker estimated that Signal’s annual operating expenses would rise to $50 million by 2025, up from approximately $40 million at present.

Features

Signal facilitates one-on-one and group voice and video communications, supporting up to forty participants on iOS, Android, and desktop systems. These calls operate over both wired and wireless internet connections. The application enables the exchange of text messages, files, voice recordings, images, stickers, GIFs, and video clips, alongside group chat functionality.

All interactions are protected by end-to-end encryption, with cryptographic keys created and stored locally on user devices rather than on central servers. Contact verification is performed by comparing key fingerprints or scanning QR codes. A trust-on-first-use model notifies users if a correspondent’s encryption key changes unexpectedly.

Up until 2023, Android users had the option to designate Signal as their default SMS/MMS client, allowing unencrypted messages to be sent alongside encrypted ones; this capability was discontinued due to security considerations. The app can be secured using device PINs, passcodes, or biometric authentication, with adjustable screen lock timing to further safeguard message access.

Signal includes features such as message scheduling and disappearing messages that automatically erase from all devices after a chosen duration ranging from five seconds to one week. These are intended to support collaborative privacy rather than adversarial use cases. Users can customize the app’s icon and name for personalization or concealment purposes. Messages support formatting options like spoilers and italics, and contacts can be added through QR code scanning.

By default, messages are excluded from unencrypted cloud backups. Additionally, Signal offers automatic face-blurring in photos to enhance user anonymity.

The platform incorporates a cryptocurrency wallet enabling in-app payments via MobileCoin, which has been available worldwide since November 2021 except in specific jurisdictions.

In February 2024, Signal launched a beta feature introducing usernames, allowing communication without disclosing phone numbers—significantly improving privacy for both personal and business interactions.

Limitations

To register with Signal, users must provide a phone number for verification purposes; this approach eliminates the need for separate usernames or passwords and facilitates contact discovery. The phone number may be from a VoIP service or landline provided the user can receive the verification code and activate the app on a different device. Each phone number is restricted to registration on a single mobile device at any given time, and the initial setup requires an iOS or Android device.

The requirement to link accounts with phone numbers—common among applications like WhatsApp and KakaoTalk—has attracted criticism from privacy-focused users hesitant to share their personal numbers. A frequently employed solution is using an alternative secondary number. The highly requested option to replace phone numbers with public, changeable usernames was introduced in Signal’s beta release in February 2024.

Using phone numbers as identifiers also introduces security vulnerabilities such as account takeover via phone number hijacking. An incident exploiting Signal’s SMS service provider took place in August 2022. To address this threat, Signal provides a Registration Lock feature that acts as two-factor authentication by requiring a PIN before registering the phone number on any new device.

Usability

In July 2016, the Internet Society conducted a user study evaluating how effectively Signal users could identify man-in-the-middle attacks. Results revealed that 21 out of 28 participants failed to accurately compare public key fingerprints for identity verification and often mistakenly believed they had succeeded. In response, four months later Signal enhanced its user interface to make identity verification more straightforward.

Encryption protocols

Signal utilizes the Signal Protocol (previously known as the TextSecure Protocol) for its messaging system, which integrates the Double Ratchet Algorithm, prekeys, and an Extended Triple Diffie–Hellman (X3DH) handshake. The protocol incorporates cryptographic primitives such as Curve25519, AES-256, and HMAC-SHA256. It guarantees confidentiality, integrity, authentication, participant consistency, destination verification, forward and backward secrecy, message unlinkability, repudiation, and asynchronous communication. Despite these features, it does not offer anonymity and depends on servers to relay messages and store public keys.

For end-to-end encrypted group conversations, the Signal Protocol combines pairwise double ratchet mechanisms with multicast encryption. This approach provides speaker consistency, robustness against message loss or out-of-order delivery, equal computational and trust responsibilities among participants, subgroup messaging capabilities, and adaptable membership management.

Security evaluations conducted by Ruhr University Bochum in 2014 and a joint study by institutions from the UK, Australia, and Canada in 2016 affirmed the protocol’s cryptographic robustness while noting minor theoretical weaknesses. A 2017 investigation identified a theoretical vulnerability related to secret group identifiers in the group protocol but concluded that exploiting this flaw without detection would be challenging.

By August 2018, the Signal Protocol was integrated into platforms such as WhatsApp, Facebook Messenger, Skype, and Google Allo, facilitating end-to-end encryption for more than a billion users. On these services, however, encryption is not enabled by default but offered as an optional feature.

Until March 2017, Signal’s voice communication employed SRTP alongside the ZRTP key-agreement mechanism. Subsequently, Signal adopted a WebRTC-based framework supporting video calls and replaced ZRTP with the Signal Protocol channel for authentication purposes.

Authentication

User identity verification in Signal is performed by allowing participants to compare key fingerprints or scan QR codes through out-of-band methods. The system employs a trust on first use model that notifies users if a correspondent's key undergoes any change.

Local storage

Messages are decrypted on the device and saved locally within an encrypted SQLite database protected by SQLCipher. The cryptographic key is likewise stored locally and can be accessed when the device is unlocked. In December 2020, Cellebrite introduced a tool capable of retrieving this key to "decrypt the Signal app." While media outlets suggested Cellebrite could "break into Signal" or "crack its encryption," experts and Signal representatives clarified that this technology merely extracts data from devices physically possessed by law enforcement when unlocked; it does not circumvent encryption.

Comparable extraction utilities exist for iOS devices and Signal Desktop clients. In April 2021, Signal’s founder Moxie Marlinspike disclosed input validation flaws in Cellebrite’s UFED and Physical Analyzer software and accused Cellebrite of unauthorized redistribution of Apple software libraries.

Servers

Signal operates using infrastructure provided by major cloud services including Amazon Web Services, Google Compute Engine, and Microsoft Azure worldwide. These servers facilitate message routing, enable contact discovery among registered users, and handle automatic exchange of users’ public keys. By default, voice and video calls on Signal employ peer-to-peer connections; however, when calls involve contacts not listed in the recipient’s address book, traffic is routed through servers to mask users’ IP addresses.

Contact discovery

Signal’s servers retain users' registered phone numbers, public keys, and push notification tokens necessary for establishing calls and sending messages. To determine which contacts are Signal users, the application periodically transmits cryptographic hashes of the user’s contact list to the server. The server then compares these hashes against the SHA-256 hashes of registered accounts and informs the client of any matches before deleting the submitted hashed data. Despite this mechanism, founder Moxie Marlinspike acknowledged in 2014 that achieving fully privacy-preserving contact discovery remains difficult because phone numbers have a limited input range. In 2017, Signal revealed efforts to develop a technique allowing client applications to identify Signal users within address books without exposing contact information to the server, thereby improving user privacy while preserving functionality.

Metadata

All communications between clients and servers are protected using TLS. According to Signal’s developers, their servers do not retain logs detailing who contacted whom or when interactions occurred. In June 2016, Marlinspike clarified that the only metadata stored consists of the last connection time for each user, recorded with day-level granularity.

The group messaging architecture is designed so that servers cannot access information about group membership, names, or icons. Group creation and administration take place entirely on the client side, with messages sent individually between participants as in one-on-one conversations.

Federation

From December 2013 until February 2016, Signal employed a federated server model. In December 2013, Signal’s protocol was incorporated into the Android-based open-source operating system CyanogenMod, implemented as a system application named WhisperPush. CyanogenMod operated its own Signal messaging server federated with the primary server, enabling communication across clients. The WhisperPush source code was released under the GPLv3 license.

In February 2016, CyanogenMod discontinued WhisperPush and encouraged users to transition to the official Signal app. By May 2016, Marlinspike acknowledged that federation with CyanogenMod servers had negatively impacted user experience and slowed development progress, indicating that future federation was unlikely.

Also in May 2016, Marlinspike requested that the third-party client LibreSignal stop using both the Signal service and name. Consequently, LibreSignal was discontinued later that month, with its features integrated into Signal’s official application.

Licensing

The entire source code for Signal clients across Android, iOS, and desktop platforms is publicly accessible on GitHub under a free software license. This openness enables developers and advanced users to audit, compile, and compare the applications themselves. In March 2016, Moxie Marlinspike noted that Signal for Android supports reproducible builds except for certain shared libraries due to limitations in Gradle NDK. Instructions for generating reproducible builds are provided in the GitHub repository. While most of Signal's server-side software is open source, its anti-spam module remains proprietary to prevent misuse.

Security

In October 2014, the Electronic Frontier Foundation (EFF) included Signal in its guide for surveillance self-defense. By November 2014, Signal achieved a perfect rating on the EFF’s secure messaging scorecard for features such as end-to-end encryption, forward secrecy, open-source codebase, documented security protocols, and recent independent audits. Other applications with comparable scores included ChatSecure + Orbot, Pidgin (with OTR), Silent Phone, and Telegram’s “secret chats.”

NSA whistleblower Edward Snowden has repeatedly endorsed Signal and its predecessors for their user-friendly design. Leaked NSA documents from 2014 identified Signal’s encrypted voice calling as a significant obstacle to surveillance operations. Following the 2016 Democratic National Committee email breach, Hillary Clinton’s campaign legal team advised DNC staff to rely exclusively on Signal for sensitive communications.

In March 2017, the U.S. Senate authorized use of Signal for secure messaging by senators and their staff members. On 27 September 2019, a Google Project Zero security researcher disclosed an Android vulnerability in Signal that enabled silent eavesdropping through muted calls; this issue was patched on the same day.

In February 2020, the European Commission recommended Signal for internal staff communications. After the George Floyd protests in 2020, U.S. downloads surged to 121,000 within ten days. By July 2020, amid concerns over Hong Kong's national security law, Signal became the most downloaded app there.

As of January 2021, prominent news organizations such as The Washington Post, The Guardian, The New York Times, and The Wall Street Journal utilize Signal as a secure platform for anonymous tip submissions.

Commercial spyware firms Candiru and FinFisher claim capabilities to extract messages from devices running Signal using spyware techniques. Certain forks of Signal address this risk by encrypting data at rest.

On 9 August 2022, Malaysian Prime Minister Ismail Sabri Yaakob reported his Signal account had been compromised and used for impersonation; details surrounding this breach have not been publicly disclosed.

In-app payments

In April 2021, Signal launched a cryptocurrency wallet feature that allowed users to send and receive funds using MobileCoin. This new capability faced criticism from security analyst Bruce Schneier, who cautioned that it might increase the app’s size and invite regulatory attention. Initially, the wallet was accessible only in certain regions but was made available worldwide by November 2021.

Blocking

Signal was blocked in Egypt in December 2016, which led developers to adopt domain fronting—a method that helps users circumvent censorship by disguising traffic as other internet services. As of May 2022, domain fronting is activated by default in countries including Egypt, UAE, Oman, Qatar, Iran, Cuba, Pakistan, and Uzbekistan.

By January 2018, Iran had also blocked Signal. The app’s domain fronting depended on Google App Engine (GAE), but Iran’s limited access to GAE due to U.S. sanctions hindered its effectiveness. Early in 2018, Google ended global support for domain fronting; Signal briefly switched to Amazon CloudFront, but AWS similarly restricted this practice. As a result, Signal explored other solutions and returned to Google services in April 2019.

In January 2021, Iran removed Signal from app marketplaces and blocked the service entirely. China followed suit by blocking Signal in March 2021 and later removing it from its App Store in April 2024.

On August 9, 2024, Russia banned Signal, citing breaches of anti-terrorism legislation. Around the same period, Venezuela also blocked the app amid political turmoil following the 2024 presidential election.

Use by activists

During the George Floyd protests in 2020, demonstrators utilized Signal for coordination, relying on its end-to-end encryption to ensure secure communication.

In March 2021, the United Nations advised residents of Myanmar to use Signal and Proton Mail as tools for documenting human rights abuses during the 2021 coup.

Controversial use

Signal’s terms of service explicitly forbid illegal activities. In January 2021, its leadership reaffirmed a "hands-off approach to moderation," emphasizing that employees cannot access user messages and that the foundation does not intend to operate as a media platform.

In 2016, Indian authorities apprehended members of an alleged ISIS-linked cell who communicated via Signal.

Right-wing extremist militias and white nationalist groups have employed Signal for organizing events such as the 2018 Unite the Right II rally. Following bans on social media platforms, some fringe groups migrated to encrypted messaging apps like Signal and Telegram due to their enhanced security and reliability.

Turkey has criminalized general use of Signal over concerns regarding terrorist financing; however, critics argue this represents misuse of counterterrorism laws aimed at suppressing opposition.

Use by Trump administration

In March 2025, reports revealed that high-ranking officials from the Trump administration—including vice president JD Vance, secretary of state Marco Rubio, and defense secretary Pete Hegseth—used Signal for discussing sensitive topics such as military operations in Yemen. The existence of this chat was disclosed by The Atlantic editor-in-chief Jeffrey Goldberg, who was inadvertently added to the group. Brian Hughes from the National Security Council later verified Goldberg’s account as accurate.

Bibliography

External links