Tor network
   HOME

TheInfoList



OR:

Tor is a free
overlay network An overlay network is a logical computer network that is protocol layering, layered on top of a physical network. The concept of overlay networking is distinct from the traditional model of OSI model, OSI layered networks, and almost always assum ...
for enabling anonymous communication. It is built on
free and open-source software Free and open-source software (FOSS) is software available under a license that grants users the right to use, modify, and distribute the software modified or not to everyone free of charge. FOSS is an inclusive umbrella term encompassing free ...
run by over seven thousand volunteer-operated relays worldwide, as well as by millions of users who route their
Internet traffic Internet traffic is the flow of data within the entire Internet, or in certain network links of its constituent networks. Common traffic measurements are total volume, in units of multiples of the byte, or as transmission rates in bytes per cert ...
via random paths through these relays. Using Tor makes it more difficult to trace a user's
Internet The Internet (or internet) is the Global network, global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a internetworking, network of networks ...
activity by preventing any single point on the Internet (other than the user's device) from being able to view both where traffic originated from and where it is ultimately going to at the same time. This conceals a user's location and usage from anyone performing network surveillance or
traffic analysis Traffic analysis is the process of intercepting and examining messages in order to deduce information from patterns in communication. It can be performed even when the messages are encrypted. In general, the greater the number of messages observ ...
from any such point, protecting the user's freedom and ability to communicate confidentially.


History

The core principle of Tor, known as
onion routing Onion routing is a technique for anonymous communication over a computer network. In an onion network, messages are encapsulated in layers of encryption, analogous to the layers of an onion. The encrypted data is transmitted through a series o ...
, was developed in the mid-1990s by
United States Naval Research Laboratory The United States Naval Research Laboratory (NRL) is the corporate research laboratory for the United States Navy and the United States Marine Corps. Located in Washington, DC, it was founded in 1923 and conducts basic scientific research, appl ...
employees,
mathematician A mathematician is someone who uses an extensive knowledge of mathematics in their work, typically to solve mathematical problems. Mathematicians are concerned with numbers, data, quantity, mathematical structure, structure, space, Mathematica ...
Paul Syverson Paul Syverson is a computer scientist best known for inventing onion routing, a feature of the Tor anonymity network. In 2012, ''Foreign Policy Foreign policy, also known as external policy, is the set of strategies and actions a State ( ...
, and
computer scientist A computer scientist is a scientist who specializes in the academic study of computer science. Computer scientists typically work on the theoretical side of computation. Although computer scientists can also focus their work and research on ...
s Michael G. Reed and David Goldschlag, to protect American intelligence communications online. Onion routing is implemented by means of
encryption In Cryptography law, cryptography, encryption (more specifically, Code, encoding) is the process of transforming information in a way that, ideally, only authorized parties can decode. This process converts the original representation of the inf ...
in the
application layer An application layer is an abstraction layer that specifies the shared communication protocols and interface methods used by hosts in a communications network. An ''application layer'' abstraction is specified in both the Internet Protocol Su ...
of the
communication protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any variation of a physical quantity. The protocol defines the rules, syntax, semantics (computer science), sem ...
stack, nested like the layers of an
onion An onion (''Allium cepa'' , from Latin ), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus '' Allium''. The shallot is a botanical variety of the onion which was classifie ...
. The alpha version of Tor, developed by Syverson and computer scientists Roger Dingledine and Nick Mathewson and then called The Onion Routing project (which was later given the acronym "Tor"), was launched on 20 September 2002. The first public release occurred a year later. In 2004, the Naval Research Laboratory released the code for Tor under a free license, and the
Electronic Frontier Foundation The Electronic Frontier Foundation (EFF) is an American international non-profit digital rights group based in San Francisco, California. It was founded in 1990 to promote Internet civil liberties. It provides funds for legal defense in court, ...
(EFF) began funding Dingledine and Mathewson to continue its development. In 2006, Dingledine, Mathewson, and five others founded
The Tor Project The Tor Project, Inc. is a 501(c)(3) research-education nonprofit organization based in Winchester, Massachusetts. It is founded by computer scientists Roger Dingledine, Nick Mathewson, and five others. The Tor Project is primarily responsibl ...
, a
Massachusetts Massachusetts ( ; ), officially the Commonwealth of Massachusetts, is a U.S. state, state in the New England region of the Northeastern United States. It borders the Atlantic Ocean and the Gulf of Maine to its east, Connecticut and Rhode ...
-based
501(c)(3) A 501(c)(3) organization is a United States corporation, Trust (business), trust, unincorporated association or other type of organization exempt from federal income tax under section 501(c)(3) of Title 26 of the United States Code. It is one of ...
research-education
nonprofit organization A nonprofit organization (NPO), also known as a nonbusiness entity, nonprofit institution, not-for-profit organization, or simply a nonprofit, is a non-governmental (private) legal entity organized and operated for a collective, public, or so ...
responsible for maintaining Tor. The EFF acted as The Tor Project's fiscal sponsor in its early years, and early financial supporters included the U.S.
Bureau of Democracy, Human Rights, and Labor The Bureau of Democracy, Human Rights and Labor Affairs (DRL) is a bureau within the United States Department of State. The bureau is under the purview of the Under Secretary of State for Civilian Security, Democracy, and Human Rights. History ...
and International Broadcasting Bureau, Internews,
Human Rights Watch Human Rights Watch (HRW) is an international non-governmental organization that conducts research and advocacy on human rights. Headquartered in New York City, the group investigates and reports on issues including War crime, war crimes, crim ...
, the
University of Cambridge The University of Cambridge is a Public university, public collegiate university, collegiate research university in Cambridge, England. Founded in 1209, the University of Cambridge is the List of oldest universities in continuous operation, wo ...
,
Google Google LLC (, ) is an American multinational corporation and technology company focusing on online advertising, search engine technology, cloud computing, computer software, quantum computing, e-commerce, consumer electronics, and artificial ...
, and Netherlands-based Stichting NLnet. Over the course of its existence, various Tor vulnerabilities have been discovered and occasionally exploited. Attacks against Tor are an active area of academic research that is welcomed by The Tor Project itself.


Usage

Tor enables its users to surf the Internet, chat and send instant messages anonymously, and is used by a wide variety of people for both licit and illicit purposes. Tor has, for example, been used by criminal enterprises,
hacktivism Hacktivism (or hactivism; a portmanteau of ''hack'' and ''activism''), is the use of computer-based techniques such as hacking as a form of civil disobedience to promote a political agenda or social change. A form of Internet activism with roots ...
groups, and law enforcement agencies at cross purposes, sometimes simultaneously; likewise, agencies within the U.S. government variously fund Tor (the U.S. State Department, the National Science Foundation, and – through the Broadcasting Board of Governors, which itself partially funded Tor until October 2012 –
Radio Free Asia Radio Free Asia (RFA) is a news service that publishes online news, information, commentary and broadcasts radio programs for its audiences in Asia. The service, which provides editorially independent reporting, has the stated mission of pro ...
) and seek to subvert it. Tor was one of a dozen circumvention tools evaluated by a
Freedom House Freedom House is a nonprofit organization based in Washington, D.C. It is best known for political advocacy surrounding issues of democracy, Freedom (political), political freedom, and human rights. Freedom House was founded in October 1941, wi ...
-funded report based on user experience from China in 2010, which include Ultrasurf, Hotspot Shield, and Freegate. Tor is not meant to completely solve the issue of anonymity on the web. Tor is not designed to completely erase tracking but instead to reduce the likelihood for sites to trace actions and data back to the user. Tor can also be used for illegal activities. These can include privacy protection or censorship circumvention, as well as distribution of child abuse content, drug sales, or malware distribution. Tor has been described by ''
The Economist ''The Economist'' is a British newspaper published weekly in printed magazine format and daily on Electronic publishing, digital platforms. It publishes stories on topics that include economics, business, geopolitics, technology and culture. M ...
'', in relation to
Bitcoin Bitcoin (abbreviation: BTC; Currency symbol, sign: ₿) is the first Decentralized application, decentralized cryptocurrency. Based on a free-market ideology, bitcoin was invented in 2008 when an unknown entity published a white paper under ...
and
Silk Road The Silk Road was a network of Asian trade routes active from the second century BCE until the mid-15th century. Spanning over , it played a central role in facilitating economic, cultural, political, and religious interactions between the ...
, as being "a dark corner of the web". It has been targeted by the American
National Security Agency The National Security Agency (NSA) is an intelligence agency of the United States Department of Defense, under the authority of the director of national intelligence (DNI). The NSA is responsible for global monitoring, collection, and proces ...
and the British
GCHQ Government Communications Headquarters (GCHQ) is an intelligence and security organisation responsible for providing signals intelligence (SIGINT) and information assurance (IA) to the government and armed forces of the United Kingdom. Primar ...
signals intelligence Signals intelligence (SIGINT) is the act and field of intelligence-gathering by interception of ''signals'', whether communications between people (communications intelligence—abbreviated to COMINT) or from electronic signals not directly u ...
agencies, albeit with marginal success, and more successfully by the British
National Crime Agency The National Crime Agency (NCA) is a Law enforcement agency#natlea, national law enforcement agency in the United Kingdom. It is the UK's lead agency against organised crime; Human trafficking, human, Arms trafficking, weapon and Illegal drug t ...
in its Operation Notarise. At the same time, GCHQ has been using a tool named "Shadowcat" for "end-to-end encrypted access to VPS over SSH using the Tor network". Tor can be used for anonymous defamation, unauthorized
news leak A news leak is the unsanctioned release of confidential information to news media. It can also be the premature publication of information by a news outlet, of information that it has agreed not to release before a specified time, in violation of a ...
s of sensitive information,
copyright infringement Copyright infringement (at times referred to as piracy) is the use of Copyright#Scope, works protected by copyright without permission for a usage where such permission is required, thereby infringing certain exclusive rights granted to the c ...
, distribution of illegal sexual content, selling controlled substances, weapons, and stolen credit card numbers,
money laundering Money laundering is the process of illegally concealing the origin of money obtained from illicit activities (often known as dirty money) such as drug trafficking, sex work, terrorism, corruption, and embezzlement, and converting the funds i ...
, bank fraud,
credit card fraud Credit card fraud is an inclusive term for fraud committed using a payment card, such as a credit card or debit card. The purpose may be to obtain goods or services or to make payment to another account, which is controlled by a criminal. The P ...
,
identity theft Identity theft, identity piracy or identity infringement occurs when someone uses another's personal identifying information, like their name, identifying number, or credit card number, without their permission, to commit fraud or other crimes. ...
and the exchange of counterfeit currency; the
black market A black market is a Secrecy, clandestine Market (economics), market or series of transactions that has some aspect of illegality, or is not compliant with an institutional set of rules. If the rule defines the set of goods and services who ...
utilizes the Tor infrastructure, at least in part, in conjunction with Bitcoin. It has also been used to brick IoT devices. In its complaint against Ross William Ulbricht of
Silk Road The Silk Road was a network of Asian trade routes active from the second century BCE until the mid-15th century. Spanning over , it played a central role in facilitating economic, cultural, political, and religious interactions between the ...
, the US
Federal Bureau of Investigation The Federal Bureau of Investigation (FBI) is the domestic Intelligence agency, intelligence and Security agency, security service of the United States and Federal law enforcement in the United States, its principal federal law enforcement ag ...
acknowledged that Tor has "known legitimate uses". According to CNET, Tor's anonymity function is "endorsed by the
Electronic Frontier Foundation The Electronic Frontier Foundation (EFF) is an American international non-profit digital rights group based in San Francisco, California. It was founded in 1990 to promote Internet civil liberties. It provides funds for legal defense in court, ...
(EFF) and other civil liberties groups as a method for
whistleblower Whistleblowing (also whistle-blowing or whistle blowing) is the activity of a person, often an employee, revealing information about activity within a private or public organization that is deemed illegal, immoral, illicit, unsafe, unethical or ...
s and human rights workers to communicate with journalists". EFF's Surveillance Self-Defense guide includes a description of where Tor fits in a larger strategy for protecting privacy and anonymity. In 2014, the EFF's Eva Galperin told ''
Businessweek ''Bloomberg Businessweek'', previously known as ''BusinessWeek'' (and before that ''Business Week'' and ''The Business Week''), is an American monthly business magazine published 12 times a year. The magazine debuted in New York City in Septembe ...
'' that "Tor's biggest problem is press. No one hears about that time someone wasn't stalked by their abuser. They hear how somebody got away with downloading child porn." The Tor Project states that Tor users include "normal people" who wish to keep their Internet activities private from websites and advertisers, people concerned about cyber-spying, and users who are evading censorship such as activists, journalists, and military professionals. In November 2013, Tor had about four million users. According to the ''Wall Street Journal'', in 2012 about 14% of Tor's traffic connected from the United States, with people in "Internet-censoring countries" as its second-largest user base. Tor is increasingly used by victims of
domestic violence Domestic violence is violence that occurs in a domestic setting, such as in a marriage Marriage, also called matrimony or wedlock, is a culturally and often legally recognised union between people called spouses. It establishes r ...
and the
social worker Social work is an academic discipline and practice-based profession concerned with meeting the basic needs of individuals, families, groups, communities, and society as a whole to enhance their individual and collective well-being. Social wo ...
s and agencies that assist them, even though shelter workers may or may not have had professional training on cyber-security matters. Properly deployed, however, it precludes digital stalking, which has increased due to the prevalence of digital media in contemporary
online In computer technology and telecommunications, online indicates a state of connectivity, and offline indicates a disconnected state. In modern terminology, this usually refers to an Internet connection, but (especially when expressed as "on lin ...
life. Along with SecureDrop, Tor is used by news organizations such as ''
The Guardian ''The Guardian'' is a British daily newspaper. It was founded in Manchester in 1821 as ''The Manchester Guardian'' and changed its name in 1959, followed by a move to London. Along with its sister paper, ''The Guardian Weekly'', ''The Guardi ...
'', ''
The New Yorker ''The New Yorker'' is an American magazine featuring journalism, commentary, criticism, essays, fiction, satire, cartoons, and poetry. It was founded on February 21, 1925, by Harold Ross and his wife Jane Grant, a reporter for ''The New York T ...
'',
ProPublica ProPublica (), legally Pro Publica, Inc., is a nonprofit investigative journalism organization based in New York City. ProPublica's investigations are conducted by its staff of full-time reporters, and the resulting stories are distributed to ne ...
and '' The Intercept'' to protect the privacy of whistleblowers. In March 2015, the Parliamentary Office of Science and Technology released a briefing which stated that "There is widespread agreement that banning online anonymity systems altogether is not seen as an acceptable policy option in the U.K." and that "Even if it were, there would be technical challenges." The report further noted that Tor "plays only a minor role in the online viewing and distribution of indecent images of children" (due in part to its inherent latency); its usage by the Internet Watch Foundation, the utility of its onion services for
whistleblower Whistleblowing (also whistle-blowing or whistle blowing) is the activity of a person, often an employee, revealing information about activity within a private or public organization that is deemed illegal, immoral, illicit, unsafe, unethical or ...
s, and its circumvention of the Great Firewall of China were touted. Tor's executive director, Andrew Lewman, also said in August 2014 that agents of the NSA and the GCHQ have anonymously provided Tor with bug reports. The Tor Project's FAQ offers supporting reasons for the EFF's endorsement:


Operation

Tor aims to conceal its users' identities and their online activity from surveillance and traffic analysis by separating identification and routing. It is an implementation of
onion routing Onion routing is a technique for anonymous communication over a computer network. In an onion network, messages are encapsulated in layers of encryption, analogous to the layers of an onion. The encrypted data is transmitted through a series o ...
, which encrypts and then randomly bounces communications through a network of relays run by volunteers around the globe. These onion routers employ
encryption In Cryptography law, cryptography, encryption (more specifically, Code, encoding) is the process of transforming information in a way that, ideally, only authorized parties can decode. This process converts the original representation of the inf ...
in a multi-layered manner (hence the onion metaphor) to ensure perfect forward secrecy between relays, thereby providing users with anonymity in a network location. That anonymity extends to the hosting of censorship-resistant content by Tor's anonymous onion service feature. Furthermore, by keeping some of the entry relays (bridge relays) secret, users can evade
Internet censorship Internet censorship is the legal control or suppression of what can be accessed, published, or viewed on the Internet. Censorship is most often applied to specific internet domains (such as ''Wikipedia.org'', for example) but exceptionally may ...
that relies upon blocking public Tor relays. Because the
IP address An Internet Protocol address (IP address) is a numerical label such as that is assigned to a device connected to a computer network that uses the Internet Protocol for communication. IP addresses serve two main functions: network interface i ...
of the sender and the recipient are not ''both'' in cleartext at any hop along the way, anyone eavesdropping at any point along the communication channel cannot directly identify both ends. Furthermore, to the recipient, it appears that the last Tor
node In general, a node is a localized swelling (a "knot") or a point of intersection (a vertex). Node may refer to: In mathematics * Vertex (graph theory), a vertex in a mathematical graph *Vertex (geometry), a point where two or more curves, lines ...
(called the exit node), rather than the sender, is the originator of the communication.


Originating traffic

A Tor user's
SOCKS A sock is a piece of clothing worn on the feet and often covering the ankle or some part of the Calf (leg), calf. Some types of shoes or boots are typically worn over socks. In ancient times, socks were made from leather or matted animal hair. ...
-aware applications can be configured to direct their network traffic through a Tor instance's SOCKS interface, which is listening on TCP port 9050 (for standalone Tor) or 9150 (for Tor Browser bundle) at
localhost In computer networking, localhost is a hostname that refers to the current computer used to access it. The name ''localhost'' is reserved for loopback purposes. It is used to access the network services that are running on the host via t ...
. Tor periodically creates virtual circuits through the Tor network through which it can
multiplex Multiplex may refer to: Science and technology * Multiplex communication, combining many signals into one transmission circuit or channel ** Multiplex (television), a group of digital television or radio channels that are combined for broadcast * ...
and onion-route that traffic to its destination. Once inside a Tor network, the traffic is sent from router to router along the circuit, ultimately reaching an exit node at which point the cleartext packet is available and is forwarded on to its original destination. Viewed from the destination, the traffic appears to originate at the Tor exit node. Tor's application independence sets it apart from most other anonymity networks: it works at the
Transmission Control Protocol The Transmission Control Protocol (TCP) is one of the main communications protocol, protocols of the Internet protocol suite. It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, th ...
(TCP) stream level. Applications whose traffic is commonly anonymized using Tor include
Internet Relay Chat IRC (Internet Relay Chat) is a text-based chat system for instant messaging. IRC is designed for Many-to-many, group communication in discussion forums, called ''#Channels, channels'', but also allows one-on-one communication via instant mess ...
(IRC),
instant messaging Instant messaging (IM) technology is a type of synchronous computer-mediated communication involving the immediate ( real-time) transmission of messages between two or more parties over the Internet or another computer network. Originally involv ...
, and
World Wide Web The World Wide Web (WWW or simply the Web) is an information system that enables Content (media), content sharing over the Internet through user-friendly ways meant to appeal to users beyond Information technology, IT specialists and hobbyis ...
browsing.


Onion services

Tor can also provide anonymity to websites and other servers. Servers configured to receive inbound connections only through Tor are called onion services (formerly, hidden services). Rather than revealing a server's IP address (and thus its network location), an onion service is accessed through its onion address, usually via the Tor Browser or some other software designed to use Tor. The Tor network understands these addresses by looking up their corresponding
public key Public-key cryptography, or asymmetric cryptography, is the field of cryptographic systems that use pairs of related keys. Each key pair consists of a public key and a corresponding private key. Key pairs are generated with cryptographic alg ...
s and ''introduction points'' from a
distributed hash table A distributed hash table (DHT) is a Distributed computing, distributed system that provides a lookup service similar to a hash table. Key–value pairs are stored in a DHT, and any participating node (networking), node can efficiently retrieve the ...
within the network. It can route data to and from onion services, even those hosted behind firewalls or
network address translator Network address translation (NAT) is a method of mapping an IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device. The technique was initia ...
s (NAT), while preserving the anonymity of both parties. Tor is necessary to access these onion services. Because the connection never leaves the Tor network, and is handled by the Tor application on both ends, the connection is always end-to-end encrypted. Onion services were first specified in 2003 and have been deployed on the Tor network since 2004. They are unlisted by design, and can only be discovered on the network if the onion address is already known, though a number of sites and services do catalog publicly known onion addresses. Popular sources of .onion links include Pastebin,
Twitter Twitter, officially known as X since 2023, is an American microblogging and social networking service. It is one of the world's largest social media platforms and one of the most-visited websites. Users can share short text messages, image ...
,
Reddit Reddit ( ) is an American Proprietary software, proprietary social news news aggregator, aggregation and Internet forum, forum Social media, social media platform. Registered users (commonly referred to as "redditors") submit content to the ...
, other Internet forums, and tailored search engines. While onion services are often discussed in terms of websites, they can be used for any TCP service, and are commonly used for increased security or easier routing to non-web services, such as
secure shell The Secure Shell Protocol (SSH Protocol) is a cryptographic network protocol for operating network services securely over an unsecured network. Its most notable applications are remote login and command-line execution. SSH was designed for ...
remote login, chat services such as
IRC IRC (Internet Relay Chat) is a text-based chat system for instant messaging. IRC is designed for group communication in discussion forums, called '' channels'', but also allows one-on-one communication via private messages as well as chat ...
and
XMPP Extensible Messaging and Presence Protocol (abbreviation XMPP, originally named Jabber) is an Open standard, open communication protocol designed for instant messaging (IM), presence information, and contact list maintenance. Based on XML (Ext ...
, or
file sharing File sharing is the practice of distributing or providing access to digital media, such as computer programs, multimedia (audio, images and video), documents or electronic books. Common methods of storage, transmission and dispersion include ...
. They have also become a popular means of establishing
peer-to-peer Peer-to-peer (P2P) computing or networking is a distributed application architecture that partitions tasks or workloads between peers. Peers are equally privileged, equipotent participants in the network, forming a peer-to-peer network of Node ...
connections in messaging and file sharing applications. Web-based onion services can be accessed from a standard web browser without client-side connection to the Tor network using services like Tor2web, which remove client anonymity.


Attacks and limitations

Like all software with an
attack surface The attack surface of a software environment is the sum of the different points (for " attack vectors") where an unauthorized user (the "attacker") can try to enter data to, extract data, control a device or critical software in an environment. Ke ...
, Tor's protections have limitations, and Tor's implementation or design have been vulnerable to attacks at various points throughout its history. While most of these limitations and attacks are minor, either being fixed without incident or proving inconsequential, others are more notable.


End-to-end traffic correlation

Tor is designed to provide relatively high performance network anonymity against an attacker with a single vantage point on the connection (e.g., control over one of the three relays, the destination server, or the user's
internet service provider An Internet service provider (ISP) is an organization that provides a myriad of services related to accessing, using, managing, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, no ...
). Like all current low-latency anonymity networks, Tor cannot and does not attempt to protect against an attacker performing simultaneous monitoring of traffic at the boundaries of the Tor network—i.e., the traffic entering and exiting the network. While Tor does provide protection against
traffic analysis Traffic analysis is the process of intercepting and examining messages in order to deduce information from patterns in communication. It can be performed even when the messages are encrypted. In general, the greater the number of messages observ ...
, it cannot prevent traffic confirmation via end-to-end correlation. There are no documented cases of this limitation being used at scale; as of the 2013 Snowden leaks, law enforcement agencies such as the NSA were unable to perform dragnet surveillance on Tor itself, and relied on attacking other software used in conjunction with Tor, such as
vulnerabilities Vulnerability refers to "the quality or state of being exposed to the possibility of being attacked or harmed, either physically or emotionally." The understanding of social and environmental vulnerability, as a methodological approach, involves ...
in
web browsers A web browser, often shortened to browser, is an application for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's scree ...
. However, targeted attacks have been able to make use of traffic confirmation on individual Tor users, via police surveillance or investigations confirming that a particular person already under suspicion was sending Tor traffic at the exact times the connections in question occurred. The relay early traffic confirmation attack also relied on traffic confirmation as part of its mechanism, though on requests for onion service descriptors, rather than traffic to the destination server.


Consensus attacks

Like many decentralized systems, Tor relies on a consensus mechanism to periodically update its current operating parameters. For Tor, these include network parameters like which nodes are good and bad relays, exits, guards, and how much traffic each can handle. Tor's architecture for deciding the consensus relies on a small number of directory authority nodes voting on current network parameters. Currently, there are nine directory authority nodes, and their health is publicly monitored. The IP addresses of the authority nodes are hard coded into each Tor client. The authority nodes vote every hour to update the consensus, and clients download the most recent consensus on startup. A compromise of the majority of the directory authorities could alter the consensus in a way that is beneficial to an attacker. Alternatively, a network congestion attack, such as a
DDoS In computing, a denial-of-service attack (DoS attack) is a cyberattack in which the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host co ...
, could theoretically prevent the consensus nodes from communicating, and thus prevent voting to update the consensus (though such an attack would be visible).


Server-side restrictions

Tor makes no attempt to conceal the IP addresses of exit relays, or hide from a destination server the fact that a user is connecting via Tor. Operators of Internet sites therefore have the ability to prevent traffic from Tor exit nodes or to offer reduced functionality for Tor users. For example,
Wikipedia Wikipedia is a free content, free Online content, online encyclopedia that is written and maintained by a community of volunteers, known as Wikipedians, through open collaboration and the wiki software MediaWiki. Founded by Jimmy Wales and La ...
generally forbids all editing when using Tor or when using an IP address also used by a Tor exit node, and the
BBC The British Broadcasting Corporation (BBC) is a British public service broadcaster headquartered at Broadcasting House in London, England. Originally established in 1922 as the British Broadcasting Company, it evolved into its current sta ...
blocks the IP addresses of all known Tor exit nodes from its iPlayer service. Apart from intentional restrictions of Tor traffic, Tor use can trigger defense mechanisms on websites intended to block traffic from IP addresses observed to generate malicious or abnormal traffic. Because traffic from all Tor users is shared by a comparatively small number of exit relays, tools can misidentify distinct sessions as originating from the same user, and attribute the actions of a malicious user to a non-malicious user, or observe an unusually large volume of traffic for one IP address. Conversely, a site may observe a single session connecting from different exit relays, with different
Internet geolocation In computing, Internet geolocation is software capable of deducing the geographic position of a device connected to the Internet. For example, the device's IP address can be used to determine the country, city, or ZIP code, determining its geograph ...
s, and assume the connection is malicious, or trigger geo-blocking. When these defense mechanisms are triggered, it can result in the site blocking access, or presenting
captcha Completely Automated Public Turing Test to tell Computers and Humans Apart (CAPTCHA) ( ) is a type of challenge–response authentication, challenge–response turing test used in computing to determine whether the user is human in order to de ...
s to the user.


Relay early traffic confirmation attack

In July 2014, the Tor Project issued a security advisory for a "relay early traffic confirmation" attack, disclosing the discovery of a group of relays attempting to de-anonymize onion service users and operators. A set of onion service directory nodes (i.e., the Tor relays responsible for providing information about onion services) were found to be modifying traffic of requests. The modifications made it so the requesting client's guard relay, if controlled by the same adversary as the onion service directory node, could easily confirm that the traffic was from the same request. This would allow the adversary to simultaneously know the onion service involved in the request, and the IP address of the client requesting it (where the requesting client could be a visitor or owner of the onion service). The attacking nodes joined the network on 30 January, using a Sybil attack to comprise 6.4% of guard relay capacity, and were removed on 4 July. In addition to removing the attacking relays, the Tor application was patched to prevent the specific traffic modifications that made the attack possible. In November 2014, there was speculation in the aftermath of Operation Onymous, resulting in 17 arrests internationally, that a Tor weakness had been exploited. A representative of
Europol Europol, officially the European Union Agency for Law Enforcement Cooperation, is the law enforcement agency of the European Union (EU). Established in 1998, it is based in The Hague, Netherlands, and serves as the central hub for coordinating c ...
was secretive about the method used, saying: "This is something we want to keep for ourselves. The way we do this, we can't share with the whole world, because we want to do it again and again and again." A
BBC The British Broadcasting Corporation (BBC) is a British public service broadcaster headquartered at Broadcasting House in London, England. Originally established in 1922 as the British Broadcasting Company, it evolved into its current sta ...
source cited a "technical breakthrough" that allowed tracking physical locations of servers, and the initial number of infiltrated sites led to the exploit speculation. A Tor Project representative downplayed this possibility, suggesting that execution of more traditional police work was more likely. In November 2015, court documents suggested a connection between the attack and arrests, and raised concerns about security research ethics. The documents revealed that the
FBI The Federal Bureau of Investigation (FBI) is the domestic Intelligence agency, intelligence and Security agency, security service of the United States and Federal law enforcement in the United States, its principal federal law enforcement ag ...
obtained IP addresses of onion services and their visitors from a "university-based research institute", leading to arrests. Reporting from
Motherboard A motherboard, also called a mainboard, a system board, a logic board, and informally a mobo (see #Nomenclature, "Nomenclature" section), is the main printed circuit board (PCB) in general-purpose computers and other expandable systems. It ho ...
found that the timing and nature of the relay early traffic confirmation attack matched the description in the court documents. Multiple experts, including a senior researcher with the ICSI of
UC Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California), is a public land-grant research university in Berkeley, California, United States. Founded in 1868 and named after the Anglo-Irish philosopher George Berkele ...
, Edward Felten of
Princeton University Princeton University is a private university, private Ivy League research university in Princeton, New Jersey, United States. Founded in 1746 in Elizabeth, New Jersey, Elizabeth as the College of New Jersey, Princeton is the List of Colonial ...
, and
the Tor Project The Tor Project, Inc. is a 501(c)(3) research-education nonprofit organization based in Winchester, Massachusetts. It is founded by computer scientists Roger Dingledine, Nick Mathewson, and five others. The Tor Project is primarily responsibl ...
agreed that the CERT Coordination Center of
Carnegie Mellon University Carnegie Mellon University (CMU) is a private research university in Pittsburgh, Pennsylvania, United States. The institution was established in 1900 by Andrew Carnegie as the Carnegie Technical Schools. In 1912, it became the Carnegie Institu ...
was the institute in question. Concerns raised included the role of an academic institution in policing, sensitive research involving non-consenting users, the non-targeted nature of the attack, and the lack of disclosure about the incident.


Vulnerable applications

Many attacks targeted at Tor users result from flaws in applications used with Tor, either in the application itself, or in how it operates in combination with Tor. E.g., researchers with
Inria The National Institute for Research in Digital Science and Technology (Inria) () is a French national research institution focusing on computer science and applied mathematics. It was created under the name French Institute for Research in Comp ...
in 2011 performed an attack on
BitTorrent BitTorrent is a Protocol (computing), communication protocol for peer-to-peer file sharing (P2P), which enables users to distribute data and electronic files over the Internet in a Decentralised system, decentralized manner. The protocol is d ...
users by attacking clients that established connections both using and not using Tor, then associating other connections shared by the same Tor circuit.


Fingerprinting

When using Tor, applications may still provide data tied to a device, such as information about screen resolution, installed fonts, language configuration, or supported graphics functionality, reducing the set of users a connection could possibly originate from, or uniquely identifying them. This information is known as the
device fingerprint A device fingerprint or machine fingerprint is information collected about the software and hardware of a remote computing device for the purpose of identification. The information is usually assimilated into a brief identifier using a fingerprint ...
, or browser fingerprint in the case of web browsers. Applications implemented with Tor in mind, such as Tor Browser, can be designed to minimize the amount of information leaked by the application and reduce its fingerprint.


Eavesdropping

Tor cannot encrypt the traffic between an exit relay and the destination server. If an application does not add an additional layer of
end-to-end encryption End-to-end encryption (E2EE) is a method of implementing a secure communication system where only communicating users can participate. No one else, including the system provider, telecom providers, Internet providers or malicious actors, can ...
between the client and the server, such as
Transport Layer Security Transport Layer Security (TLS) is a cryptographic protocol designed to provide communications security over a computer network, such as the Internet. The protocol is widely used in applications such as email, instant messaging, and voice over ...
(TLS, used in
HTTPS Hypertext Transfer Protocol Secure (HTTPS) is an extension of the Hypertext Transfer Protocol (HTTP). It uses encryption for secure communication over a computer network, and is widely used on the Internet. In HTTPS, the communication protoc ...
) or the
Secure Shell The Secure Shell Protocol (SSH Protocol) is a cryptographic network protocol for operating network services securely over an unsecured network. Its most notable applications are remote login and command-line execution. SSH was designed for ...
(SSH) protocol, this allows the exit relay to capture and modify traffic. Attacks from malicious exit relays have recorded usernames and passwords, and modified
Bitcoin Bitcoin (abbreviation: BTC; Currency symbol, sign: ₿) is the first Decentralized application, decentralized cryptocurrency. Based on a free-market ideology, bitcoin was invented in 2008 when an unknown entity published a white paper under ...
addresses to redirect transactions. Some of these attacks involved actively removing the HTTPS protections that would have otherwise been used. To attempt to prevent this, Tor Browser has since made it so only connections via onion services or HTTPS are allowed by default.


Firefox/Tor browser attacks

In 2011, the Dutch authority investigating
child pornography Child pornography (also abbreviated as CP, also called child porn or kiddie porn, and child sexual abuse material, known by the acronym CSAM (underscoring that children can not be deemed willing participants under law)), is Eroticism, erotic ma ...
discovered the IP address of a Tor onion service site from an unprotected administrator's account and gave it to the
FBI The Federal Bureau of Investigation (FBI) is the domestic Intelligence agency, intelligence and Security agency, security service of the United States and Federal law enforcement in the United States, its principal federal law enforcement ag ...
, who traced it to Aaron McGrath. After a year of surveillance, the FBI launched " Operation Torpedo" which resulted in McGrath's arrest and allowed them to install their
Network Investigative Technique Network Investigative Technique (NIT) is a form of malware (or hacking) employed by the FBI since at least 2002. It is a drive-by download computer program designed to provide access to a computer. Controversies Its usage has raised both Fourth ...
(NIT) malware on the servers for retrieving information from the users of the three onion service sites that McGrath controlled. The technique exploited a vulnerability in Firefox/Tor Browser that had already been patched, and therefore targeted users that had not updated. A Flash application sent a user's IP address directly back to an FBI server, and resulted in revealing at least 25 US users as well as numerous users from other countries. McGrath was sentenced to 20 years in prison in early 2014, while at least 18 others (including a former Acting HHS Cyber Security Director) were sentenced in subsequent cases. In August 2013, it was discovered that the
Firefox Mozilla Firefox, or simply Firefox, is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. It uses the Gecko rendering engine to display web pages, which implements curr ...
browsers in many older versions of the Tor Browser Bundle were vulnerable to a JavaScript-deployed
shellcode In hacking, a shellcode is a small piece of code used as the payload in the exploitation of a software vulnerability. It is called "shellcode" because it typically starts a command shell from which the attacker can control the compromised ma ...
attack, as NoScript was not enabled by default. Attackers used this vulnerability to extract users' MAC and IP addresses and Windows computer names. News reports linked this to a FBI operation targeting Freedom Hosting's owner, Eric Eoin Marques, who was arrested on a provisional
extradition In an extradition, one Jurisdiction (area), jurisdiction delivers a person Suspect, accused or Conviction, convicted of committing a crime in another jurisdiction, into the custody of the other's law enforcement. It is a cooperative law enforc ...
warrant issued by a United States' court on 29 July. The FBI extradited Marques from Ireland to the state of Maryland on 4 charges: distributing; conspiring to distribute; and advertising child pornography, as well as aiding and abetting advertising of child pornography. The FBI acknowledged the attack in a 12 September 2013 court filing in
Dublin Dublin is the capital and largest city of Republic of Ireland, Ireland. Situated on Dublin Bay at the mouth of the River Liffey, it is in the Provinces of Ireland, province of Leinster, and is bordered on the south by the Dublin Mountains, pa ...
; further technical details from a training presentation leaked by
Edward Snowden Edward Joseph Snowden (born June 21, 1983) is a former National Security Agency (NSA) intelligence contractor and whistleblower who leaked classified documents revealing the existence of global surveillance programs. Born in 1983 in Elizabeth ...
revealed the code name for the exploit as "EgotisticalGiraffe". In 2022, Kaspersky researchers found that when looking up "Tor Browser" in Chinese on
YouTube YouTube is an American social media and online video sharing platform owned by Google. YouTube was founded on February 14, 2005, by Steve Chen, Chad Hurley, and Jawed Karim who were three former employees of PayPal. Headquartered in ...
, one of the
URL A uniform resource locator (URL), colloquially known as an address on the Web, is a reference to a resource that specifies its location on a computer network and a mechanism for retrieving it. A URL is a specific type of Uniform Resource Identi ...
s provided under the top-ranked Chinese-language video actually pointed to malware disguised as Tor Browser. Once installed, it saved browsing history and form data that genuine Tor forgot by default, and downloaded malicious components if the device's IP addresses was in China. Kaspersky researchers noted that the malware was not stealing data to sell for profit, but was designed to identify users.


Onion service configuration

Like client applications that use Tor, servers relying on onion services for protection can introduce their own weaknesses. Servers that are reachable through Tor onion services and the public Internet can be subject to correlation attacks, and all onion services are susceptible to misconfigured services (e.g., identifying information included by default in web server error responses), leaking uptime and downtime statistics, intersection attacks, or various user errors. The OnionScan program, written by independent security researcher Sarah Jamie Lewis, comprehensively examines onion services for such flaws and vulnerabilities.


Software

The main implementation of Tor is written primarily in C. Starting in 2020, the Tor Project began development of a full rewrite of the C Tor codebase in
Rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH) ...
. The project, named Arti, was publicly announced in July 2021.


Tor Browser

The Tor Browser is a
web browser A web browser, often shortened to browser, is an application for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's scr ...
capable of accessing the Tor network. It was created as the Tor Browser Bundle by Steven J. Murdoch and announced in January 2008. The Tor Browser consists of a modified Mozilla
Firefox Mozilla Firefox, or simply Firefox, is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. It uses the Gecko rendering engine to display web pages, which implements curr ...
ESR web browser, the TorButton, TorLauncher, NoScript and the Tor proxy. Users can run the Tor Browser from
removable media In computing, a removable media is a data storage media that is designed to be readily inserted and removed from a system. Most early removable media, such as floppy disks and optical discs, require a dedicated read/write device (i.e. a drive) ...
. It can operate under
Microsoft Windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
,
macOS macOS, previously OS X and originally Mac OS X, is a Unix, Unix-based operating system developed and marketed by Apple Inc., Apple since 2001. It is the current operating system for Apple's Mac (computer), Mac computers. With ...
, Android and
Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...
. The default
search engine A search engine is a software system that provides hyperlinks to web pages, and other relevant information on World Wide Web, the Web in response to a user's web query, query. The user enters a query in a web browser or a mobile app, and the sea ...
is DuckDuckGo (until version 4.5, Startpage.com was its default). The Tor Browser automatically starts Tor background processes and routes traffic through the Tor network. Upon termination of a session the browser deletes privacy-sensitive data such as HTTP cookies and the browsing history. This is effective in reducing
web tracking Web tracking is the practice by which operators of websites and third parties collect, store and share information about visitors' activities on the World Wide Web. Analysis of a user's behaviour may be used to provide content that enables the op ...
and canvas fingerprinting, and it also helps to prevent creation of a
filter bubble A filter bubble or ideological frame is a state of intellectual isolationTechnopediaDefinition – What does Filter Bubble mean?, Retrieved October 10, 2017, "....A filter bubble is the intellectual isolation, that can occur when websites make ...
. To allow download from places where accessing the Tor Project URL may be risky or blocked, a
GitHub GitHub () is a Proprietary software, proprietary developer platform that allows developers to create, store, manage, and share their code. It uses Git to provide distributed version control and GitHub itself provides access control, bug trackin ...
repository is maintained with links for releases hosted in other domains.


Tor Messenger

On 29 October 2015, the Tor Project released Tor Messenger Beta, an instant messaging program based on Instantbird with Tor and OTR built in and used by default. Like
Pidgin A pidgin , or pidgin language, is a grammatically simplified form of contact language that develops between two or more groups of people that do not have a language in common: typically, its vocabulary and grammar are limited and often drawn f ...
and Adium, Tor Messenger supports multiple different instant messaging protocols; however, it accomplishes this without relying on ''libpurple'', implementing all chat protocols in the memory-safe language JavaScript instead. According to Lucian Armasu of Toms Hardware, in April 2018, the Tor Project shut down the Tor Messenger project for three reasons: the developers of "Instabird" discontinued support for their own software, limited resources and known metadata problems. The Tor Messenger developers explained that overcoming any vulnerabilities discovered in the future would be impossible due to the project relying on outdated software dependencies.


Tor Phone

In 2016, Tor developer Mike Perry announced a prototype tor-enabled smartphone based on CopperheadOS. It was meant as a direction for Tor on mobile. The project was called 'Mission Improbable'. Copperhead's then lead developer Daniel Micay welcomed the prototype.


Third-party applications

The
Vuze Vuze (previously Azureus) is a BitTorrent client used to transfer files via the BitTorrent protocol. Vuze is written in Java, and uses the Azureus Engine. In addition to downloading data linked to .torrent files, Azureus allows users to view, p ...
(formerly Azureus)
BitTorrent BitTorrent is a Protocol (computing), communication protocol for peer-to-peer file sharing (P2P), which enables users to distribute data and electronic files over the Internet in a Decentralised system, decentralized manner. The protocol is d ...
client, Bitmessage anonymous messaging system, and TorChat instant messenger include Tor support. The Briar messenger routes all messaging via Tor by default. OnionShare allows users to share files using Tor. The Guardian Project is actively developing a free and open-source suite of applications and firmware for the Android operating system to improve the security of mobile communications. The applications include the ChatSecure instant messaging client, Orbot Tor implementation (also available for iOS), Orweb (discontinued) privacy-enhanced mobile browser, Orfox, the mobile counterpart of the Tor Browser, ProxyMob Firefox add-on, and ObscuraCam. Onion Browser is open-source, privacy-enhancing web browser for
iOS Ios, Io or Nio (, ; ; locally Nios, Νιός) is a Greek island in the Cyclades group in the Aegean Sea. Ios is a hilly island with cliffs down to the sea on most sides. It is situated halfway between Naxos and Santorini. It is about long an ...
, which uses Tor. It is available in the iOS
App Store An app store, also called an app marketplace or app catalog, is a type of digital distribution platform for computer software called applications, often in a mobile context. Apps provide a specific set of functions which, by definition, do not i ...
, and source code is available on
GitHub GitHub () is a Proprietary software, proprietary developer platform that allows developers to create, store, manage, and share their code. It uses Git to provide distributed version control and GitHub itself provides access control, bug trackin ...
. Brave added support for Tor in its desktop browser's private-browsing mode. File:Orbot-logo.svg, Orbot logo File:Orbot, running under Samsung Internet.png, Orbot 16.6.3, running under Samsung Internet File:Onion Browser on iPad.png, Onion Browser on
iPad The iPad is a brand of tablet computers developed and marketed by Apple Inc., Apple that run the company's mobile operating systems iOS and later iPadOS. The IPad (1st generation), first-generation iPad was introduced on January 27, 2010. ...


Security-focused operating systems

In September of 2024, it was announced that Tails, a
security-focused operating system This is a list of operating systems specifically focused on computer security, security. Similar concepts include security-evaluated operating systems that have achieved certification from an code audit, auditing organization, and trusted operati ...
, had become part of the Tor Project. Other security-focused operating systems that make or made extensive use of Tor include Hardened Linux From Scratch, Incognito, Liberté Linux, Qubes OS, Subgraph, Parrot OS, Tor-ramdisk, and Whonix.


Reception, impact, and legislation

Tor has been praised for providing privacy and anonymity to vulnerable Internet users such as political activists fearing surveillance and arrest, ordinary web users seeking to circumvent censorship, and people who have been threatened with violence or abuse by stalkers. The U.S. National Security Agency (NSA) has called Tor "the king of high-secure, low-latency Internet anonymity", and ''
BusinessWeek ''Bloomberg Businessweek'', previously known as ''BusinessWeek'' (and before that ''Business Week'' and ''The Business Week''), is an American monthly business magazine published 12 times a year. The magazine debuted in New York City in Septembe ...
'' magazine has described it as "perhaps the most effective means of defeating the online surveillance efforts of intelligence agencies around the world". Other media have described Tor as "a sophisticated privacy tool", "easy to use" and "so secure that even the world's most sophisticated electronic spies haven't figured out how to crack it". Advocates for Tor say it supports
freedom of expression Freedom of speech is a principle that supports the freedom of an individual or a community to articulate their opinions and ideas without fear of retaliation, censorship, or legal sanction. The rights, right to freedom of expression has been r ...
, including in countries where the Internet is censored, by protecting the privacy and anonymity of users. The mathematical underpinnings of Tor lead it to be characterized as acting "like a piece of
infrastructure Infrastructure is the set of facilities and systems that serve a country, city, or other area, and encompasses the services and facilities necessary for its economy, households and firms to function. Infrastructure is composed of public and pri ...
, and governments naturally fall into paying for infrastructure they want to use". The project was originally developed on behalf of the U.S. intelligence community and continues to receive U.S. government funding, and has been criticized as "more resembl nga spook project than a tool designed by a culture that values accountability or transparency". , 80% of The Tor Project's $2M annual budget came from the
United States government The Federal Government of the United States of America (U.S. federal government or U.S. government) is the Federation#Federal governments, national government of the United States. The U.S. federal government is composed of three distinct ...
, with the U.S. State Department, the Broadcasting Board of Governors, and the
National Science Foundation The U.S. National Science Foundation (NSF) is an Independent agencies of the United States government#Examples of independent agencies, independent agency of the Federal government of the United States, United States federal government that su ...
as major contributors, aiming "to aid democracy advocates in authoritarian states". Other public sources of funding include
DARPA The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the Adva ...
, the U.S. Naval Research Laboratory, and the
Government of Sweden The Government of the Kingdom of Sweden () is the Cabinet (government), national cabinet of Sweden, and the country's Executive (government), executive authority. The Government consists of the Prime Minister of Sweden, Prime Minister and their ...
. Some have proposed that the government values Tor's commitment to free speech, and uses the darknet to gather intelligence. Tor also receives funding from
NGOs A non-governmental organization (NGO) is an independent, typically nonprofit organization that operates outside government control, though it may get a significant percentage of its funding from government or corporate sources. NGOs often focus ...
including
Human Rights Watch Human Rights Watch (HRW) is an international non-governmental organization that conducts research and advocacy on human rights. Headquartered in New York City, the group investigates and reports on issues including War crime, war crimes, crim ...
, and private sponsors including
Reddit Reddit ( ) is an American Proprietary software, proprietary social news news aggregator, aggregation and Internet forum, forum Social media, social media platform. Registered users (commonly referred to as "redditors") submit content to the ...
and
Google Google LLC (, ) is an American multinational corporation and technology company focusing on online advertising, search engine technology, cloud computing, computer software, quantum computing, e-commerce, consumer electronics, and artificial ...
. Dingledine said that the
United States Department of Defense The United States Department of Defense (DoD, USDOD, or DOD) is an United States federal executive departments, executive department of the federal government of the United States, U.S. federal government charged with coordinating and superv ...
funds are more similar to a research grant than a procurement contract. Tor executive director Andrew Lewman said that even though it accepts funds from the U.S. federal government, the Tor service did not collaborate with the NSA to reveal identities of users. Critics say that Tor is not as secure as it claims, pointing to U.S. law enforcement's investigations and shutdowns of Tor-using sites such as web-hosting company Freedom Hosting and online marketplace
Silk Road The Silk Road was a network of Asian trade routes active from the second century BCE until the mid-15th century. Spanning over , it played a central role in facilitating economic, cultural, political, and religious interactions between the ...
. In October 2013, after analyzing documents leaked by Edward Snowden, ''The Guardian'' reported that the NSA had repeatedly tried to crack Tor and had failed to break its core security, although it had had some success attacking the computers of individual Tor users. ''The Guardian'' also published a 2012 NSA classified slide deck, entitled "Tor Stinks", which said: "We will never be able to de-anonymize all Tor users all the time", but "with manual analysis we can de-anonymize a very small fraction of Tor users". When Tor users are arrested, it is typically due to human error, not to the core technology being hacked or cracked. On 7 November 2014, for example, a joint operation by the FBI, ICE Homeland Security investigations and European Law enforcement agencies led to 17 arrests and the seizure of 27 sites containing 400 pages. A late 2014 report by ''
Der Spiegel (, , stylized in all caps) is a German weekly news magazine published in Hamburg. With a weekly circulation of about 724,000 copies in 2022, it is one of the largest such publications in Europe. It was founded in 1947 by John Seymour Chaloner ...
'' using a new cache of Snowden leaks revealed, however, that the NSA deemed Tor on its own as a "major threat" to its mission, and when used in conjunction with other privacy tools such as OTR, Cspace,
ZRTP ZRTP (composed of Z and Real-time Transport Protocol) is a cryptographic key-agreement protocol to negotiate the keys for encryption between two end points in a Voice over IP (VoIP) phone telephony call based on the Real-time Transport Protocol ...
, RedPhone, Tails, and
TrueCrypt TrueCrypt is a discontinued source-available freeware utility software, utility used for on-the-fly encryption (OTFE). It can create a virtual encrypted disk within a file, encrypt a Disk partitioning, partition, or encrypt the whole Data storag ...
was ranked as "catastrophic," leading to a "near-total loss/lack of insight to target communications, presence..."


2011

In March 2011, The Tor Project received the
Free Software Foundation The Free Software Foundation (FSF) is a 501(c)(3) non-profit organization founded by Richard Stallman on October 4, 1985. The organisation supports the free software movement, with the organization's preference for software being distributed ...
's 2010 Award for Projects of Social Benefit. The citation read, "Using free software, Tor has enabled roughly 36 million people around the world to experience freedom of access and expression on the Internet while keeping them in control of their privacy and anonymity. Its network has proved pivotal in dissident movements in both
Iran Iran, officially the Islamic Republic of Iran (IRI) and also known as Persia, is a country in West Asia. It borders Iraq to the west, Turkey, Azerbaijan, and Armenia to the northwest, the Caspian Sea to the north, Turkmenistan to the nort ...
and more recently
Egypt Egypt ( , ), officially the Arab Republic of Egypt, is a country spanning the Northeast Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via the Sinai Peninsula. It is bordered by the Mediterranean Sea to northe ...
." Iran tried to block Tor at least twice in 2011. One attempt simply blocked all servers with 2-hour-expiry security certificates; it was successful for less than 24 hours.


2012

In 2012, ''
Foreign Policy Foreign policy, also known as external policy, is the set of strategies and actions a State (polity), state employs in its interactions with other states, unions, and international entities. It encompasses a wide range of objectives, includ ...
'' magazine named Dingledine, Mathewson, and Syverson among its Top 100 Global Thinkers "for making the web safe for whistleblowers".


2013

In 2013,
Jacob Appelbaum Jacob Appelbaum (born April 1, 1983) is an American independent journalist, computer security researcher, artist, Hacking (innovation), hacker and teacher. Appelbaum, who earned his PhD from the Eindhoven University of Technology, first became not ...
described Tor as a "part of an ecosystem of software that helps people regain and reclaim their autonomy. It helps to enable people to have agency of all kinds; it helps others to help each other and it helps you to help yourself. It runs, it is open and it is supported by a large community spread across all walks of life." In June 2013, whistleblower
Edward Snowden Edward Joseph Snowden (born June 21, 1983) is a former National Security Agency (NSA) intelligence contractor and whistleblower who leaked classified documents revealing the existence of global surveillance programs. Born in 1983 in Elizabeth ...
used Tor to send information about PRISM to ''
The Washington Post ''The Washington Post'', locally known as ''The'' ''Post'' and, informally, ''WaPo'' or ''WP'', is an American daily newspaper published in Washington, D.C., the national capital. It is the most widely circulated newspaper in the Washington m ...
'' and ''
The Guardian ''The Guardian'' is a British daily newspaper. It was founded in Manchester in 1821 as ''The Manchester Guardian'' and changed its name in 1959, followed by a move to London. Along with its sister paper, ''The Guardian Weekly'', ''The Guardi ...
''.


2014

In 2014, the Russian government offered a $111,000 contract to "study the possibility of obtaining technical information about users and users' equipment on the Tor anonymous network". In September 2014, in response to reports that
Comcast Comcast Corporation, formerly known as Comcast Holdings,Before the AT&T Broadband, AT&T merger in 2001, the parent company was Comcast Holdings Corporation. Comcast Holdings Corporation now refers to a subsidiary of Comcast Corporation, not th ...
had been discouraging customers from using the Tor Browser,
Comcast Comcast Corporation, formerly known as Comcast Holdings,Before the AT&T Broadband, AT&T merger in 2001, the parent company was Comcast Holdings Corporation. Comcast Holdings Corporation now refers to a subsidiary of Comcast Corporation, not th ...
issued a public statement that "We have no policy against Tor, or any other browser or software." In October 2014, The Tor Project hired the public relations firm Thomson Communications to improve its public image (particularly regarding the terms "Dark Net" and "hidden services," which are widely viewed as being problematic) and to educate journalists about the technical aspects of Tor. Turkey blocked downloads of Tor Browser from the Tor Project.


2015

In June 2015, the special rapporteur from the United Nations'
Office of the High Commissioner for Human Rights The Office of the United Nations High Commissioner for Human Rights (OHCHR) is a department of the United Nations Secretariat that works to promote and protect human rights that are guaranteed under international law and stipulated in the Univers ...
specifically mentioned Tor in the context of the debate in the U.S. about allowing so-called backdoors in encryption programs for law enforcement purposes in an interview for ''The Washington Post''. In July 2015, the Tor Project announced an alliance with the Library Freedom Project to establish exit nodes in public libraries. The pilot program, which established a middle relay running on the excess bandwidth afforded by the Kilton Library in
Lebanon, New Hampshire Lebanon ( ) is the only city in Grafton County, New Hampshire, United States. The population was 14,282 at the 2020 United States census, 2020 census, up from 13,151 at the 2010 United States census, 2010 census. Lebanon is in western New Hampshi ...
, making it the first library in the U.S. to host a Tor node, was briefly put on hold when the local city manager and deputy sheriff voiced concerns over the cost of defending search warrants for information passed through the Tor exit node. Although the
Department of Homeland Security The United States Department of Homeland Security (DHS) is the U.S. federal executive department responsible for public security, roughly comparable to the interior, home, or public security ministries in other countries. Its missions invol ...
(DHS) had alerted New Hampshire authorities to the fact that Tor is sometimes used by criminals, the Lebanon Deputy Police Chief and the Deputy City Manager averred that no pressure to strong-arm the library was applied, and the service was re-established on 15 September 2015. U.S. Rep.
Zoe Lofgren Susan Ellen "Zoe" Lofgren ( ; born December 21, 1947) is an American politician and lawyer serving as a United States House of Representatives, U.S. representative from California. A member of the Democratic Party (United States), Democratic Part ...
(D-Calif) released a letter on 10 December 2015, in which she asked the DHS to clarify its procedures, stating that "While the Kilton Public Library's board ultimately voted to restore their Tor relay, I am no less disturbed by the possibility that DHS employees are pressuring or persuading public and private entities to discontinue or degrade services that protect the privacy and anonymity of U.S. citizens." In a 2016 interview, Kilton Library IT Manager Chuck McAndrew stressed the importance of getting libraries involved with Tor: "Librarians have always cared deeply about protecting privacy, intellectual freedom, and
access to information Access may refer to: Companies and organizations * ACCESS (Australia), an Australian youth network * Access (credit card), a former credit card in the United Kingdom * Access Co., a Japanese software company * Access International Advisors, a h ...
(the freedom to read). Surveillance has a very well-documented chilling effect on intellectual freedom. It is the job of librarians to remove barriers to information." The second library to host a Tor node was the Las Naves Public Library in
Valencia, Spain Valencia ( , ), formally València (), is the capital of the Province of Valencia, province and Autonomous communities of Spain, autonomous community of Valencian Community, the same name in Spain. It is located on the banks of the Turia (r ...
, implemented in the first months of 2016. In August 2015, an
IBM International Business Machines Corporation (using the trademark IBM), nicknamed Big Blue, is an American Multinational corporation, multinational technology company headquartered in Armonk, New York, and present in over 175 countries. It is ...
security research group, called "X-Force", put out a quarterly report that advised companies to block Tor on security grounds, citing a "steady increase" in attacks from Tor exit nodes as well as botnet traffic. In September 2015, Luke Millanta created OnionView (now defunct), a web service that plots the location of active Tor relay nodes onto an interactive map of the world. The project's purpose was to detail the network's size and escalating growth rate. In December 2015,
Daniel Ellsberg Daniel Ellsberg (April 7, 1931June 16, 2023) was an American political activist, economist, and United States military analyst. While employed by the RAND Corporation, he precipitated a national political controversy in 1971 when he released th ...
(of the
Pentagon Papers The ''Pentagon Papers'', officially titled ''Report of the Office of the Secretary of Defense Vietnam Task Force'', is a United States Department of Defense history of the United States in the Vietnam War, United States' political and militar ...
),
Cory Doctorow Cory Efram Doctorow (; born 17 July 1971) is a Canadian-British blogger, journalist, and science fiction author who served as co-editor of the blog ''Boing Boing''. He is an activist in favour of liberalising copyright laws and a proponent of th ...
(of
Boing Boing ''Boing Boing'' is a website, first established as a zine in 1988, later becoming a group blog. Common topics and themes include technology, futurism, science fiction, gadgets, intellectual property, Disney, and left-wing politics. It twice wo ...
),
Edward Snowden Edward Joseph Snowden (born June 21, 1983) is a former National Security Agency (NSA) intelligence contractor and whistleblower who leaked classified documents revealing the existence of global surveillance programs. Born in 1983 in Elizabeth ...
, and artist-activist Molly Crabapple, amongst others, announced their support of Tor.


2016

In March 2016, New Hampshire state representative Keith Ammon introduced a bill allowing public libraries to run privacy software. The bill specifically referenced Tor. The text was crafted with extensive input from
Alison Macrina Alison Macrina is a librarian, internet activist, founder and executive director of the Library Freedom Project. Biography Macrina grew up in Collingswood, New Jersey. She was an undergraduate at Temple University. She received a Master of Libra ...
, the director of the Library Freedom Project. The bill was passed by the House 268–62. Also in March 2016, the first Tor node, specifically a middle relay, was established at a library in Canada, the Graduate Resource Centre (GRC) in the Faculty of Information and Media Studies (FIMS) at the
University of Western Ontario The University of Western Ontario (UWO; branded as Western University) is a Public university, public research university in London, Ontario, Canada. The main campus is located on of land, surrounded by residential neighbourhoods and the Thame ...
. Given that the running of a Tor exit node is an unsettled area of Canadian law, and that in general institutions are more capable than individuals to cope with legal pressures, Alison Macrina of the Library Freedom Project has opined that in some ways she would like to see intelligence agencies and law enforcement attempt to intervene in the event that an exit node were established. On 16 May 2016,
CNN Cable News Network (CNN) is a multinational news organization operating, most notably, a website and a TV channel headquartered in Atlanta. Founded in 1980 by American media proprietor Ted Turner and Reese Schonfeld as a 24-hour cable ne ...
reported on the case of core Tor developer "isis agora lovecruft", who had fled to Germany under the threat of a subpoena by the FBI during the Thanksgiving break of the previous year. The
Electronic Frontier Foundation The Electronic Frontier Foundation (EFF) is an American international non-profit digital rights group based in San Francisco, California. It was founded in 1990 to promote Internet civil liberties. It provides funds for legal defense in court, ...
legally represented lovecruft. On 2 December 2016, ''
The New Yorker ''The New Yorker'' is an American magazine featuring journalism, commentary, criticism, essays, fiction, satire, cartoons, and poetry. It was founded on February 21, 1925, by Harold Ross and his wife Jane Grant, a reporter for ''The New York T ...
'' reported on burgeoning
digital privacy Digital privacy is often used in contexts that promote advocacy on behalf of individual and consumer privacy rights in e-services and is typically used in opposition to the business practices of many e-marketers, businesses, and companies to coll ...
and security workshops in the
San Francisco Bay Area The San Francisco Bay Area, commonly known as the Bay Area, is a List of regions of California, region of California surrounding and including San Francisco Bay, and anchored by the cities of Oakland, San Francisco, and San Jose, California, S ...
, particularly at the
hackerspace A hackerspace (also referred to as a hacklab, hackspace, or makerspace) is a community-operated, often "not for profit" (501(c)(3) in the United States), workspace where people with common interests, such as computers, machining, technology, sci ...
Noisebridge Noisebridge is an anarchism, anarchistic maker and hackerspace located in San Francisco. It is inspired by the European hackerspaces Metalab in Vienna and c-base in Berlin. Noisebridge describes itself as "''a space for sharing, creation, coll ...
, in the wake of the
2016 United States presidential election United States presidential election, Presidential elections were held in the United States on November 8, 2016. The Republican Party (United States), Republican ticket of businessman Donald Trump and Indiana Governor, Indiana governor Mike P ...
; downloading the Tor browser was mentioned. Also, in December 2016, Turkey has blocked the usage of Tor, together with ten of the most used VPN services in Turkey, which were popular ways of accessing banned social media sites and services. Tor (and
Bitcoin Bitcoin (abbreviation: BTC; Currency symbol, sign: ₿) is the first Decentralized application, decentralized cryptocurrency. Based on a free-market ideology, bitcoin was invented in 2008 when an unknown entity published a white paper under ...
) was fundamental to the operation of the dark web marketplace AlphaBay, which was taken down in an international law enforcement operation in July 2017. Despite federal claims that Tor would not shield a user, however, elementary
operational security Operations security (OPSEC) is a process that identifies critical information to determine whether friendly actions can be observed by enemy intelligence, determines if information obtained by adversaries could be interpreted to be useful to th ...
errors outside of the ambit of the Tor network led to the site's downfall.


2017

In June 2017 the
Democratic Socialists of America The Democratic Socialists of America (DSA) is a political organization in the United States and the country's largest Socialism, socialist organization. Sitting on the Left-wing politics, left-wing to Far-left politics, far-left of the politic ...
recommended intermittent Tor usage for politically active organizations and individuals as a defensive mitigation against
information security Information security is the practice of protecting information by mitigating information risks. It is part of information risk management. It typically involves preventing or reducing the probability of unauthorized or inappropriate access to data ...
threats. And in August 2017, according to reportage, cybersecurity firms which specialize in monitoring and researching the dark web (which relies on Tor as its infrastructure) on behalf of banks and retailers routinely share their findings with the
FBI The Federal Bureau of Investigation (FBI) is the domestic Intelligence agency, intelligence and Security agency, security service of the United States and Federal law enforcement in the United States, its principal federal law enforcement ag ...
and with other law enforcement agencies "when possible and necessary" regarding illegal content. The Russian-speaking underground offering a crime-as-a-service model is regarded as being particularly robust.


2018

In June 2018, Venezuela blocked access to the Tor network. The block affected both direct connections to the network and connections being made via bridge relays. On 20 June 2018, Bavarian police raided the homes of the board members of the non-profit Zwiebelfreunde, a member of torservers.net, which handles the European financial transactions of riseup.net in connection with a blog post there which apparently promised violence against the upcoming
Alternative for Germany Alternative for Germany (, AfD, ) is a Far-right politics in Germany (1945–present), far-right,Far-right: * * * * * * * * * * * * * * Right-wing populism, right-wing populist and National conservatism, national-conservative p ...
convention. Tor came out strongly against the raid on its support organization, which provides legal and financial aid for the setting up and maintenance of high-speed relays and exit nodes. According to torservers.net, on 23 August 2018 the German court at Landgericht München ruled that the raid and seizures were illegal. The hardware and documentation seized had been kept under seal, and purportedly were neither analyzed nor evaluated by the Bavarian police. Since October 2018, Chinese online communities within Tor have begun to dwindle due to increased efforts to stop them by the Chinese government.


2019

In November 2019,
Edward Snowden Edward Joseph Snowden (born June 21, 1983) is a former National Security Agency (NSA) intelligence contractor and whistleblower who leaked classified documents revealing the existence of global surveillance programs. Born in 1983 in Elizabeth ...
called for a full, unabridged
simplified Chinese Simplification, Simplify, or Simplified may refer to: Mathematics Simplification is the process of replacing a mathematical expression by an equivalent one that is simpler (usually shorter), according to a well-founded ordering. Examples include: ...
translation of his autobiography, '' Permanent Record'', as the Chinese publisher had violated their agreement by expurgating all mentions of Tor and other matters deemed politically sensitive by the
Chinese Communist Party The Communist Party of China (CPC), also translated into English as Chinese Communist Party (CCP), is the founding and One-party state, sole ruling party of the People's Republic of China (PRC). Founded in 1921, the CCP emerged victorious in the ...
.


2021

On 8 December 2021, the Russian government agency Roskomnadzor announced it has banned Tor and six VPN services for failing to abide by the Russian Internet blacklist. Russian ISPs unsuccessfully attempted to block Tor's main website as well as several bridges beginning on 1 December 2021. The Tor Project has appealed to Russian courts over this ban.


2022

In response to
Internet censorship Internet censorship is the legal control or suppression of what can be accessed, published, or viewed on the Internet. Censorship is most often applied to specific internet domains (such as ''Wikipedia.org'', for example) but exceptionally may ...
during the
Russian invasion of Ukraine On 24 February 2022, , starting the largest and deadliest war in Europe since World War II, in a major escalation of the Russo-Ukrainian War, conflict between the two countries which began in 2014. The fighting has caused hundreds of thou ...
, the
BBC The British Broadcasting Corporation (BBC) is a British public service broadcaster headquartered at Broadcasting House in London, England. Originally established in 1922 as the British Broadcasting Company, it evolved into its current sta ...
and VOA have directed Russian audiences to Tor. The Russian government increased efforts to block access to Tor through technical and political means, while the network reported an increase in traffic from Russia, and increased Russian use of its anti-censorship Snowflake tool. Russian courts temporarily lifted the blockade on Tor's website (but not connections to relays) on May 24, 2022 due to Russian law requiring that the Tor Project be involved in the case. However, the blockade was reinstated on July 21, 2022. Iran implemented rolling internet blackouts during the Mahsa Amini protests, and Tor and
Snowflake A snowflake is a single ice crystal that is large enough to fall through the Earth's atmosphere as snow.Knight, C.; Knight, N. (1973). Snow crystals. Scientific American, vol. 228, no. 1, pp. 100–107.Hobbs, P.V. 1974. Ice Physics. Oxford: C ...
were used to circumvent them. China, with its highly centralized control of its internet, had effectively blocked Tor.


Improved security

Tor responded to earlier vulnerabilities listed above by patching them and improving security. In one way or another, human (user) errors can lead to detection. The Tor Project website provides the best practices (instructions) on how to properly use the Tor browser. When improperly used, Tor is not secure. For example, Tor warns its users that not all traffic is protected; only the traffic routed through the Tor browser is protected. Users are also warned to use
HTTPS Hypertext Transfer Protocol Secure (HTTPS) is an extension of the Hypertext Transfer Protocol (HTTP). It uses encryption for secure communication over a computer network, and is widely used on the Internet. In HTTPS, the communication protoc ...
versions of websites, not to torrent with Tor, not to enable browser plugins, not to open documents downloaded through Tor while online, and to use safe bridges. Users are also warned that they cannot provide their name or other revealing information in web forums over Tor and stay anonymous at the same time. Despite intelligence agencies' claims that 80% of Tor users would be de-anonymized within 6 months in the year 2013, that has still not happened. In fact, as late as September 2016, the FBI could not locate, de-anonymize and identify the Tor user who hacked into the email account of a staffer on
Hillary Clinton Hillary Diane Rodham Clinton ( Rodham; born October 26, 1947) is an American politician, lawyer and diplomat. She was the 67th United States secretary of state in the administration of Barack Obama from 2009 to 2013, a U.S. senator represent ...
's email server. The best tactic of law enforcement agencies to de-anonymize users appears to remain with Tor-relay adversaries running poisoned nodes, as well as counting on the users themselves using the Tor browser improperly. For example, downloading a video through the Tor browser and then opening the same file on an unprotected hard drive while online can make the users' real IP addresses available to authorities.


Odds of detection

When properly used, odds of being de-anonymized through Tor are said to be extremely low. Tor project's co-founder Nick Mathewson explained that the problem of "Tor-relay adversaries" running poisoned nodes means that a theoretical adversary of this kind is not the network's greatest threat: Tor does not provide protection against end-to-end timing attacks: if an attacker can watch the traffic coming out of the target computer, and also the traffic arriving at the target's chosen destination (e.g. a server hosting a .onion site), that attacker can use statistical analysis to discover that they are part of the same circuit. A similar attack has been used by German authorities to track down users related to Boystown.


Levels of security

Depending on individual user needs, Tor browser offers three levels of security located under the Security Level (the small gray shield at the top-right of the screen) icon > Advanced Security Settings. In addition to encrypting the data, including constantly changing an IP address through a virtual circuit comprising successive, randomly selected Tor relays, several other layers of security are at a user's disposal:


Standard

At this level, all features from the Tor Browser and other websites are enabled.


Safer

This level eliminates website features that are often pernicious to the user. This may cause some sites to lose functionality. JavaScript is disabled on all non-HTTPS sites; some fonts and mathematical symbols are disabled. Also, audio and video (HTML5 media) are click-to-play.


Safest

This level only allows website features required for static sites and basic services. These changes affect images, media, and scripts. Javascript is disabled by default on all sites; some fonts, icons, math symbols, and images are disabled; audio and video (HTML5 media) are click-to-play.


Introduction of proof-of-work defense for Onion services

In 2023, Tor unveiled a new defense mechanism to safeguard its onion services against denial of service (DoS) attacks. With the release of Tor 0.4.8, this proof-of-work (PoW) defense promises to prioritize legitimate network traffic while deterring malicious attacks.


Notes


See also

*
Anonymous P2P An anonymous P2P communication system is a peer-to-peer distributed application in which the nodes, which are used to share resources, or participants are anonymous or pseudonymous. Anonymity of participants is usually achieved by special routin ...
*
Anonymous web browsing Anonymous web browsing may refer to one of the following: * Private browsing * Dark web *Tor (network) Tor is a free overlay network for enabling anonymous communication. It is built on free and open-source software run by over seven thous ...
* Crypto-anarchism * Darknet * Deep web * Freedom of information *
Freenet Hyphanet (until mid-2023: Freenet) is a peer-to-peer platform for censorship-resistant, Anonymity application, anonymous communication. It uses a decentralized distributed data store to keep and deliver information, and has a suite of free soft ...
* GNUnet * I2P * Internet censorship circumvention *
Internet privacy Internet privacy involves the right or mandate of personal privacy concerning the storage, re-purposing, provision to third parties, and display of information pertaining to oneself via the Internet. Internet privacy is a subset of data privacy. P ...
*
Signal A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology. In ...
*
Privacy software Privacy software, also called privacy platform, is software built to protect the privacy of its users. The software typically works in conjunction with Internet usage to control or limit the amount of information made available to third parties. ...
* Privoxy * Proxy server * Psiphon * Tor Phone


Citations


General and cited references

* * * * *


External links

* {{Navboxes, list= {{Cryptographic software {{Privacy {{Routing software {{Web browsers, desktop 2002 software Application layer protocols Computer networking Cross-platform free software Cross-platform software Dark web File sharing Free network-related software Free routing software Free software programmed in C Free software programmed in Rust Free web browsers Hash-based data structures Internet privacy software Internet properties established in 2002 Internet protocols Internet security Internet Standards Key-based routing Onion routing Overlay networks Proxy servers Software using the BSD license