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Tor, short for The Onion Router, is free and open-source software for enabling anonymous communication. It directs
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, p ...
traffic through a free, worldwide, volunteer overlay network, consisting of more than seven thousand relays, to conceal a user's location and usage from anyone performing
network surveillance Computer and network surveillance is the monitoring of computer activity and data stored locally on a computer or data being transferred over computer networks such as the Internet. This monitoring is often carried out covertly and may be comple ...
or traffic analysis. Using Tor makes it more difficult to trace a user's Internet activity. Tor's intended use is to protect the personal privacy of its users, as well as their freedom and ability to communicate confidentially through
IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...
anonymity using Tor exit nodes.


History

The core principle of Tor, onion routing, was developed in the mid-1990s by United States Naval Research Laboratory 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, structure, space, models, and change. History On ...
Paul Syverson, and
computer scientist A computer scientist is a person who is trained in the academic study of computer science. Computer scientists typically work on the theoretical side of computation, as opposed to the hardware side on which computer engineers mainly focus (a ...
s Michael G. Reed and David Goldschlag, to protect American intelligence communications online. Onion routing is implemented by means of
encryption In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can d ...
in the application layer of the communication protocol stack, nested like the layers of an
onion An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), 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 on ...
. The alpha version of Tor, developed by Syverson and computer scientists
Roger Dingledine Roger Dingledine is an American computer scientist known for having co-founded the Tor Project. A student of mathematics, computer science, and electrical engineering, Dingledine is also known by the pseudonym arma. As of December 2016, he contin ...
and
Nick Mathewson Nick Mathewson is an American computer scientist and co-founder of The Tor Project. He, along with Roger Dingledine, began working on onion routing shortly after they graduated from Massachusetts Institute of Technology (MIT) in the early 2000s. ...
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 (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 Seattle-based 501(c)(3) research-education nonprofit organization founded by computer scientists Roger Dingledine, Nick Mathewson and five others. The Tor Project is primarily responsible for maintaining software f ...
, a
Massachusetts Massachusetts (Massachusett: ''Muhsachuweesut Massachusett_writing_systems.html" ;"title="nowiki/> məhswatʃəwiːsət.html" ;"title="Massachusett writing systems">məhswatʃəwiːsət">Massachusett writing systems">məhswatʃəwiːsət'' En ...
-based
501(c)(3) A 501(c)(3) organization is a United States corporation, 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 the 29 types of ...
research-education
nonprofit organization A nonprofit organization (NPO) or non-profit organisation, also known as a non-business entity, not-for-profit organization, or nonprofit institution, is a legal entity organized and operated for a collective, public or social benefit, in co ...
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 and
International Broadcasting Bureau The International Broadcasting Bureau (IBB) is the technical support outlet within the United States Agency for Global Media (USAGM) (former Broadcasting Board of Governors, BBG), which is a U.S. independent agency. The IBB supports the day-to- ...
, Internews,
Human Rights Watch Human Rights Watch (HRW) is an international non-governmental organization, headquartered in New York City, that conducts research and advocacy on human rights. The group pressures governments, policy makers, companies, and individual human ...
, the
University of Cambridge , mottoeng = Literal: From here, light and sacred draughts. Non literal: From this place, we gain enlightenment and precious knowledge. , established = , other_name = The Chancellor, Masters and Schola ...
,
Google Google LLC () is an American Multinational corporation, multinational technology company focusing on Search Engine, search engine technology, online advertising, cloud computing, software, computer software, quantum computing, e-commerce, ar ...
, and Netherlands-based Stichting NLnet. Over the course of its existence, various Tor weaknesses 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 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 United States Department of State (DOS), or State Department, is an executive department of the U.S. federal government responsible for the country's foreign policy and relations. Equivalent to the ministry of foreign affairs of other n ...
, the National Science Foundation, and – through the Broadcasting Board of Governors, which itself partially funded Tor until October 2012 – Radio Free Asia) and seek to subvert it. 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 is also 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 weekly newspaper printed in demitab format and published digitally. It focuses on current affairs, international business, politics, technology, and culture. Based in London, the newspaper is owned by The Eco ...
'', in relation to Bitcoin and Silk Road, as being "a dark corner of the web". It has been targeted by the American
National Security Agency The National Security Agency (NSA) is a national-level 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, collecti ...
and the British GCHQ signals intelligence agencies, albeit with marginal success, and more successfully by the British National Crime Agency 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 leaks of sensitive information,
copyright infringement Copyright infringement (at times referred to as piracy) is the use of works protected by copyright without permission for a usage where such permission is required, thereby infringing certain exclusive rights granted to the copyright holder, ...
, distribution of illegal sexual content, selling controlled substances, weapons, and stolen credit card numbers, money laundering, 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 ...
,
identity theft Identity theft occurs when someone uses another person's personal identifying information, like their name, identifying number, or credit card number, without their permission, to commit fraud or other crimes. The term ''identity theft'' was c ...
and the exchange of
counterfeit currency Counterfeit money is currency produced without the legal sanction of a state or government, usually in a deliberate attempt to imitate that currency and so as to deceive its recipient. Producing or using counterfeit money is a form of fraud or fo ...
; the
black market A black market, underground economy, or shadow economy is a clandestine market or series of transactions that has some aspect of illegality or is characterized by noncompliance with an institutional set of rules. If the rule defines the ...
utilizes the Tor infrastructure, at least in part, in conjunction with Bitcoin. It has also been used to brick
IoT The Internet of things (IoT) describes physical objects (or groups of such objects) with sensors, processing ability, software and other technologies that connect and exchange data with other devices and systems over the Internet or other comm ...
devices. In its complaint against
Ross William Ulbricht Ross William Ulbricht (born March 27, 1984) is an American serving life imprisonment for creating and operating the darknet market website Silk Road from 2011 until his arrest in 2013. The site operated as a hidden service on the Tor network an ...
of Silk Road, the US
Federal Bureau of Investigation The Federal Bureau of Investigation (FBI) is the domestic intelligence and security service of the United States and its principal federal law enforcement agency. Operating under the jurisdiction of the United States Department of Justice ...
acknowledged that Tor has "known legitimate uses". According to CNET, Tor's anonymity function is "endorsed by the Electronic Frontier Foundation (EFF) and other civil liberties groups as a method for
whistleblower A whistleblower (also written as whistle-blower or whistle blower) is a person, often an employee, who reveals information about activity within a private or public organization that is deemed illegal, immoral, illicit, unsafe or fraudulent. Whi ...
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'' 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 (also known as domestic abuse or family violence) is violence or other abuse that occurs in a domestic setting, such as in a marriage or cohabitation. ''Domestic violence'' is often used as a synonym for '' intimate partn ...
and the social workers 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 "on line" ...
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 1821 as ''The Manchester Guardian'', and changed its name in 1959. Along with its sister papers '' The Observer'' and '' The Guardian Weekly'', ''The Guardian'' is part of the ...
'', ''
The New Yorker ''The New Yorker'' is an American weekly magazine featuring journalism, commentary, criticism, essays, fiction, satire, cartoons, and poetry. Founded as a weekly in 1925, the magazine is published 47 times annually, with five of these issues ...
'', ProPublica and ''
The Intercept ''The Intercept'' is an American left-wing news website founded by Glenn Greenwald, Jeremy Scahill, Laura Poitras and funded by billionaire eBay co-founder Pierre Omidyar. Its current editor is Betsy Reed. The publication initially report ...
'' 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 A whistleblower (also written as whistle-blower or whistle blower) is a person, often an employee, who reveals information about activity within a private or public organization that is deemed illegal, immoral, illicit, unsafe or fraudulent. Whi ...
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, 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, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can d ...
in a multi-layered manner (hence the onion metaphor) to ensure
perfect forward secrecy In cryptography, forward secrecy (FS), also known as perfect forward secrecy (PFS), is a feature of specific key agreement protocols that gives assurances that session keys will not be compromised even if long-term secrets used in the session key ...
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 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 connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...
of the sender and the recipient are not ''both'' in
cleartext In cryptography, plaintext usually means unencrypted information pending input into cryptographic algorithms, usually encryption algorithms. This usually refers to data that is transmitted or stored unencrypted. Overview With the advent of co ...
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, line ...
(called the exit node), rather than the sender, is the originator of the communication.


Originating traffic

A Tor user's SOCKS-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 device used to access it. It is used to access the network services that are running on the host via the loopback network interface. Using the loopback interface bypasses ...
. Tor periodically creates virtual circuits through the Tor network through which it can
multiplex Multiplex may refer to: * Multiplex (automobile), a former American car make * Multiplex (comics), a DC comic book supervillain * Multiplex (company), a global contracting and development company * Multiplex (assay), a biological assay which measu ...
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 In cryptography, plaintext usually means unencrypted information pending input into cryptographic algorithms, usually encryption algorithms. This usually refers to data that is transmitted or stored unencrypted. Overview With the advent of co ...
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 protocols of the Internet protocol suite. It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, the entire suite is commonl ...
(TCP) stream level. Applications whose traffic is commonly anonymized using Tor include
Internet Relay Chat Internet Relay Chat (IRC) 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 an ...
(IRC),
instant messaging Instant messaging (IM) technology is a type of online chat allowing real-time text transmission over the Internet or another computer network. Messages are typically transmitted between two or more parties, when each user inputs text and tri ...
, and World Wide Web 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 Tor, short for The Onion Router, is free and open-source software for enabling anonymous communication. It directs Internet traffic through a free, worldwide, volunteer overlay network, consisting of more than seven thousand relays, to conc ...
. The Tor network understands these addresses by looking up their corresponding public keys and ''introduction points'' from a distributed hash table within the network. It can route data to and from onion services, even those hosted behind firewalls or network address translators (NAT), while preserving the anonymity of both parties. Tor is necessary to access these onion services. Onion services were first specified in 2003 and have been deployed on the Tor network since 2004. Other than the database that stores the onion service descriptors, Tor is decentralized by design; there is no direct readable list of all onion services, although a number of onion services catalog publicly known onion addresses. Because onion services route their traffic entirely through the Tor network, connection to an onion service is encrypted end-to-end and not subject to eavesdropping. There are, however, security issues involving Tor onion services. For example, services that are reachable through Tor onion services ''and'' the public Internet are susceptible to correlation attacks and thus not perfectly hidden. Other pitfalls include misconfigured services (e.g. identifying information included by default in web server error responses), uptime and downtime statistics, intersection attacks, and user error. The open source OnionScan program, written by independent security researcher
Sarah Jamie Lewis Sarah Jamie Lewis is an anonymity and privacy researcher with a special interest in the privacy protocols (or lack thereof) of sex toys. She has been cited in academic research regarding the security and ethics considerations associated with this t ...
, comprehensively examines onion services for numerous flaws and vulnerabilities. (Lewis has also pioneered the field of "Onion Dildonics",
sex toy A sex toy is an object or device that is primarily used to facilitate Human sexual activity, human sexual pleasure, such as a dildo, artificial vagina or Vibrator (sex toy), vibrator. Many popular sex toys are designed to resemble human genita ...
s which make use of Tor through the Ricochet protocol) Onion services can also be accessed from a standard web browser without client-side connection to the Tor network, using services like
Tor2web Tor2web (pronounced "Tor to Web") is a software project to allow Tor hidden services to be accessed from a standard browser without being connected to the Tor network. It was created by Aaron Swartz and Virgil Griffith. History Tor is a net ...
. Popular sources of
.onion .onion is a special-use top level domain name designating an anonymous onion service, which was formerly known as a "hidden service", reachable via the Tor network. Such addresses are not actual DNS names, and the .onion TLD is not in the I ...
links include
Pastebin A pastebin or text storage site is a type of online content-hosting service where users can store plain text (e.g. source code snippets for code review via Internet Relay Chat (IRC)). The first pastebin was the eponymous pastebin.com. Other s ...
,
Twitter Twitter is an online social media and social networking service owned and operated by American company Twitter, Inc., on which users post and interact with 280-character-long messages known as "tweets". Registered users can post, like, and ...
,
Reddit Reddit (; stylized in all lowercase as reddit) is an American social news aggregation, content rating, and discussion website. Registered users (commonly referred to as "Redditors") submit content to the site such as links, text posts, imag ...
, and other
Internet forum An Internet forum, or message board, is an online discussion site where people can hold conversations in the form of posted messages. They differ from chat rooms in that messages are often longer than one line of text, and are at least tempora ...
s.


Nyx status monitor

Nyx (formerly ARM) is a command-line status monitor written in Python for Tor. This functions much like top does for system usage, providing real time statistics for: * Resource usage (Bandwidth, CPU, and memory usage) * General relaying information (Nickname, fingerprint, flags, or/dir/controlports) * Event log with optional regex filtering and deduplication * Connections correlated against Tor's consensus data (IP address, connection types, relay details, etc.) * Torrc configuration file with syntax highlighting and validation Most of Nyx's attributes are configurable through an optional configuration file. It runs on any platform supported by curses including
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whi ...
,
macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and la ...
, and other
Unix-like A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
variants. The project began in the summer of 2009, and since 18 July 2010 it has been an official part of the Tor Project. It is
free software Free software or libre software is computer software distributed under terms that allow users to run the software for any purpose as well as to study, change, and distribute it and any adapted versions. Free software is a matter of liberty, n ...
, available under the
GNU General Public License The GNU General Public License (GNU GPL or simply GPL) is a series of widely used free software licenses that guarantee end users the four freedoms to run, study, share, and modify the software. The license was the first copyleft for general ...
.


Weaknesses

Like all current low-latency
anonymity network In computer networking, a proxy server is a server application that acts as an intermediary between a client requesting a resource and the server providing that resource. Instead of connecting directly to a server that can fulfill a request ...
s, Tor cannot and does not attempt to protect against 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, it cannot prevent traffic confirmation (also called ''end-to-end correlation''). A 2009 study revealed that Tor and the alternative network system JonDonym (Java Anon Proxy, JAP) are considered more resilient to website fingerprinting techniques than other tunneling protocols. The reason for this is that conventional single-hop VPN protocols do not need to reconstruct packet data nearly as much as a multi-hop service like Tor or JonDonym. Website fingerprinting yielded greater than 90% accuracy for identifying
HTTP The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...
packets on conventional VPN protocols versus Tor which yielded only 2.96% accuracy. However, some protocols like OpenSSH and
OpenVPN OpenVPN is a virtual private network (VPN) system that implements techniques to create secure point-to-point or site-to-site connections in routed or bridged configurations and remote access facilities. It implements both client and server app ...
required a large amount of data before HTTP packets were identified. Researchers from the
University of Michigan , mottoeng = "Arts, Knowledge, Truth" , former_names = Catholepistemiad, or University of Michigania (1817–1821) , budget = $10.3 billion (2021) , endowment = $17 billion (2021)As o ...
developed a network scanner allowing identification of 86% of live Tor "bridges" with a single scan.


Consensus blocking

Like many decentralized systems, Tor relies on a consensus mechanism to periodically update its current operating parameters, which for Tor are network parameters like which nodes are good/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 eight 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 network congestion attack, such as a
DDoS In computing, a denial-of-service attack (DoS attack) is a cyber-attack 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 conn ...
, can prevent the consensus nodes from communicating and thus prevent voting to update the consensus.


Eavesdropping


Autonomous system (AS) eavesdropping

If the same autonomous system (AS), responsible for routing packets at least partly, is present on both path segments from a client to entry relay and from exit relay to destination, such an AS can statistically correlate traffic on the entry and exit segments of the path (i.e. traffic confirmation) and potentially infer the destination with which the client communicated. In 2012, LASTor proposed a method to predict a set of potential ASes on these two segments and then avoid choosing this path during the path selection algorithm on the client side. In this paper, they also improve latency by choosing shorter geographical paths between a client and destination.


Exit node eavesdropping

In September 2007, Dan Egerstad, a Swedish security consultant, revealed he had intercepted usernames and passwords for email accounts by operating and monitoring Tor exit nodes. As Tor cannot encrypt the traffic between an exit node and the target server, any exit node is in a position to capture traffic passing through it that does not use end-to-end encryption such as Secure Sockets Layer (SSL) or Transport Layer Security (TLS). While this may not inherently breach the anonymity of the source, traffic intercepted in this way by malicious Tor exit nodes operators can expose information about the source in either or both of payload and protocol data. Furthermore, Egerstad is circumspect about the possible subversion of Tor by intelligence agencies: In October 2019, a Tor researcher revealed that since at least 2017, there were hundreds of highly suspicious entry, relay, and exit nodes, run by an unknown group, in an unprecedented scale. It was alleged that this number of servers could pose the risk of a sybil attack as it could map Tor users' routes inside the network, increasing risk of deanonymization. At some point there were about 900 nodes running and by November 2021 about 600 of them were purged. Although described as being a deanonymization attempt, the motives and the achievements of this possibly on-going event are still unknown.


Internal communication attack

In October 2011, a research team from ESIEA claimed to have discovered a way to compromise the Tor network by decrypting communication passing over it. The technique they describe requires creating a map of Tor network nodes, controlling one-third of them, and then acquiring their encryption
keys Key or The Key may refer to: Common meanings * Key (cryptography), a piece of information that controls the operation of a cryptography algorithm * Key (lock), device used to control access to places or facilities restricted by a lock * Key (m ...
and algorithm seeds. Then, using these known keys and seeds, they claim the ability to decrypt two encryption layers out of three. They claim to break the third key by a statistical attack. In order to redirect Tor traffic to the nodes they controlled, they used a denial-of-service attack. A response to this claim has been published on the official Tor Blog stating these rumors of Tor's compromise are greatly exaggerated.


Traffic-analysis attack

There are two methods of traffic-analysis attack, passive and active. In the passive traffic-analysis method, the attacker extracts features from the traffic of a specific flow on one side of the network and looks for those features on the other side of the network. In the active traffic-analysis method, the attacker alters the timings of the packets of a flow according to a specific pattern and looks for that pattern on the other side of the network; therefore, the attacker can link the flows in one side to the other side of the network and break the anonymity of it. It is shown that, although timing noise is added to the packets, there are active traffic analysis methods that are robust against such a noise. Steven Murdoch and George Danezis from
University of Cambridge , mottoeng = Literal: From here, light and sacred draughts. Non literal: From this place, we gain enlightenment and precious knowledge. , established = , other_name = The Chancellor, Masters and Schola ...
presented an article at the 2005
IEEE The Institute of Electrical and Electronics Engineers (IEEE) is a 501(c)(3) professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operati ...
Symposium In ancient Greece, the symposium ( grc-gre, συμπόσιον ''symposion'' or ''symposio'', from συμπίνειν ''sympinein'', "to drink together") was a part of a banquet that took place after the meal, when drinking for pleasure was acc ...
on security and privacy on traffic-analysis techniques that allow adversaries with only a partial view of the network to infer which nodes are being used to relay the anonymous streams. These techniques greatly reduce the anonymity provided by Tor. Murdoch and Danezis have also shown that otherwise unrelated streams can be linked back to the same initiator. This attack, however, fails to reveal the identity of the original user. Murdoch has been working with and has been funded by Tor since 2006.


Tor exit node block

Operators of Internet sites have the ability to prevent traffic from Tor exit nodes or to offer reduced functionality for Tor users. For example, it is not generally possible to edit
Wikipedia Wikipedia is a multilingual free online encyclopedia written and maintained by a community of volunteers, known as Wikipedians, through open collaboration and using a wiki-based editing system. Wikipedia is the largest and most-read refer ...
when using Tor or when using an IP address also used by a Tor exit node. The BBC blocks the IP addresses of all known Tor exit nodes from its
iPlayer BBC iPlayer (stylised as iPLAYER or BBC iPLAYER) is a video on demand service from the BBC. The service is available on a wide range of devices, including mobile phones and tablets, personal computers and smart televisions. iPlayer services del ...
service, although non-exit relays and bridges are not blocked.


Bad apple attack

In March 2011, researchers with the Rocquencourt French Institute for Research in Computer Science and Automation (''Institut national de recherche en informatique et en automatique'', INRIA), documented an attack that is capable of revealing the IP addresses of BitTorrent users on the Tor network. The "bad apple attack" exploits Tor's design and takes advantage of insecure application used to associate the simultaneous use of a secure application with the IP address of the Tor user in question. One method of attack depends on control of an exit node or hijacking tracker responses, while a secondary attack method is based in part on the statistical exploitation of distributed hash table tracking. According to the study: The results presented in the bad apple attack research paper are based on an attack launched against the Tor network by the authors of the study. The attack targeted six exit nodes, lasted for twenty-three days, and revealed a total of 10,000 IP addresses of active Tor users. This study is significant because it is the first documented attack designed to target P2P file-sharing applications on Tor. BitTorrent may generate as much as 40% of all traffic on Tor. Furthermore, the bad apple attack is effective against insecure use of any application over Tor, not just BitTorrent.


Some protocols exposing IP addresses

Researchers from the French Institute for Research in Computer Science and Automation (INRIA) showed that the Tor dissimulation technique in BitTorrent can be bypassed by attackers controlling a Tor exit node. The study was conducted by monitoring six exit nodes for a period of twenty-three days. Researches used three attack vectors: ;Inspection of BitTorrent control messages: Tracker announces and extension protocol handshakes may optionally contain a client
IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...
. Analysis of collected data revealed that 35% and 33% of messages, respectively, contained addresses of clients. ;Hijacking trackers' responses: Due to lack of encryption or authentication in communication between the tracker and peer, typical man-in-the-middle attacks allow attackers to determine peer IP addresses and even verify the distribution of content. Such attacks work when Tor is used only for tracker communication. ;Exploiting distributed hash tables (DHT): This attack exploits the fact that distributed hash table (DHT) connections through Tor are impossible, so an attacker is able to reveal a target's IP address by looking it up in the DHT even if the target uses Tor to connect to other peers. With these techniques, researchers were able to identify other streams initiated by users, whose IP addresses were revealed.


Sniper attack

Jansen ''et al.''., describes a
DDoS In computing, a denial-of-service attack (DoS attack) is a cyber-attack 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 conn ...
attack targeted at the Tor node software, as well as defenses against that attack and its variants. The attack works using a colluding client and server, and filling the queues of the exit node until the node runs out of memory, and hence can serve no other (genuine) clients. By attacking a significant proportion of the exit nodes this way, an attacker can degrade the network and increase the chance of targets using nodes controlled by the attacker.


Heartbleed bug

The Heartbleed OpenSSL bug disrupted the Tor network for several days in April 2014 while private keys were renewed. The Tor Project recommended Tor relay operators and onion service operators revoke and generate fresh keys after patching OpenSSL, but noted Tor relays use two sets of keys and Tor's multi-hop design minimizes the impact of exploiting a single relay. Five hundred eighty-six relays later found to be susceptible to the Heartbleed bug were taken offline as a precautionary measure.


Relay early traffic confirmation attack

On 30 July 2014, the Tor Project issued the security advisory "relay early traffic confirmation attack" in which the project discovered a group of relays that tried to de-anonymize onion service users and operators. In summary, the attacking onion service directory node changed the headers of cells being relayed tagging them as "relay" or "relay early" cells differently to encode additional information and sent them back to the requesting user/operator. If the user's/operator's guard/entry node was also part of the attacking relays, the attacking relays might be able to capture the IP address of the user/operator along with the onion service information that the user/operator was requesting. The attacking relays were stable enough to be designated as "suitable as hidden service directory" and "suitable as entry guard"; therefore, both the onion service users and the onion services might have used those relays as guards and hidden service directory nodes. The attacking nodes joined the network early in the year on 30 January and the project removed them on 4 July. Although the attack's beginning is unclear, the project implied that between February and July, IP addresses of onion service users and operators might have been exposed. The project mentioned the following mitigations besides removing the attacking relays from the network: * patched relay software to prevent relays from relaying cells with "relay early" headers that were not intended. * planned update for users' proxy software so that they could inspect if they received "relay early" cells from the relays (as they are not supposed to), along with the settings to connect to just one guard node instead of selecting randomly from 3 to reduce the probability of connecting to an attacking relay * recommended that onion services should consider changing their locations * reminded users and onion service operators that Tor could not prevent de-anonymization if the attacker controlled or could listen to both ends of the Tor circuit, like in this attack. In November 2014 there was speculation in the aftermath of
Operation Onymous Operation Onymous was an international law enforcement operation targeting darknet markets and other hidden services operating on the Tor network. Background Operation Onymous was formed as a joint law enforcement operation between the Federa ...
, resulting in 17 arrests internationally, that a Tor weakness had been exploited. A representative of Europol 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 source cited a "technical breakthrough" that allowed tracking physical locations of servers, and the initial number of infiltrated sites led to the exploit speculation. Andrew Lewman—a Tor Project representative—downplayed this possibility, suggesting that execution of more traditional police work was more likely. In November 2015 court documents on the matter addressed concerns about security research ethics and the right of not being unreasonably searched as guaranteed by the US Fourth Amendment. Moreover, the documents, along with expert opinions, may also show the connection between the network attack and the law enforcement operation including: * the search warrant for an administrator of Silkroad 2.0 indicated that from January 2014 until July, the FBI received information from a "university-based research institute" with the information being "reliable IP addresses for Tor and onion services such as SR2" that led to the identification of "at least another seventeen black markets on Tor" and "approximately 78 IP addresses that accessed a vendor
.onion .onion is a special-use top level domain name designating an anonymous onion service, which was formerly known as a "hidden service", reachable via the Tor network. Such addresses are not actual DNS names, and the .onion TLD is not in the I ...
address." One of these IP addresses led to the arrest of the administrator * the chronology and nature of the attack fitted well with the operation * a senior researcher of International Computer Science Institute, part of
University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant un ...
, said in an interview that the institute which worked with the FBI was "almost certainly" Carnegie Mellon University (CMU), and this concurred with the Tor Project's assessment and with an earlier analysis of Edward Felten, a computer security professor at
Princeton University Princeton University is a private research university in Princeton, New Jersey. Founded in 1746 in Elizabeth as the College of New Jersey, Princeton is the fourth-oldest institution of higher education in the United States and one of the ...
, about researchers from CMU's
CERT/CC The CERT Coordination Center (CERT/CC) is the coordination center of the computer emergency response team (CERT) for the Software Engineering Institute (SEI), a non-profit United States federally funded research and development center. The CERT ...
being involved In his analysis published on 31 July, besides raising ethical issues, Felten also questioned the fulfillment of CERT/CC's purposes which were to prevent attacks, inform the implementers of vulnerabilities, and eventually inform the public. Because in this case, CERT/CC's staff did the opposite which was to carry out a large-scale long-lasting attack, withhold vulnerability information from the implementers, and withhold the same information from the public. CERT/CC is a non-profit, computer security research organization publicly funded through the
US federal government The federal government of the United States (U.S. federal government or U.S. government) is the national government of the United States, a federal republic located primarily in North America, composed of 50 states, a city within a f ...
.


Mouse fingerprinting

In March 2016, a security researcher based in
Barcelona Barcelona ( , , ) is a city on the coast of northeastern Spain. It is the capital and largest city of the autonomous community of Catalonia, as well as the second most populous municipality of Spain. With a population of 1.6 million within c ...
demonstrated laboratory techniques using time measurement via
JavaScript JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, of ...
at the 1- millisecond level which could potentially identify and correlate a user's unique mouse movements, provided the user has visited the same "fingerprinting" website with both the Tor browser and a regular browser. This proof of concept exploits the "time measurement via JavaScript" issue, which had been an open ticket on the Tor Project for ten months.


Circuit fingerprinting attack

In 2015, the administrators of Agora, a
darknet market A darknet market is a commercial website on the dark web that operates via darknets such as Tor or I2P. They function primarily as black markets, selling or brokering transactions involving drugs, cyber-arms, weapons, counterfeit currency, sto ...
, announced they were taking the site offline in response to a recently discovered security vulnerability in Tor. They did not say what the vulnerability was, but Wired speculated it was the "Circuit Fingerprinting Attack" presented at the Usenix security conference.


Volume information

A study showed "anonymization solutions protect only partially against target selection that may lead to efficient surveillance" as they typically "do not hide the volume information necessary to do target selection".


Implementations

The main implementation of Tor is written primarily in C.


Tor Browser

The Tor Browser is a
web browser A web browser is application software 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 screen. Browsers are used o ...
capable of accessing the Tor network. It was created as the Tor Browser Bundle by
Steven J. Murdoch Steven James Murdoch is Professor of Security Engineering in the Computer Science Department, University College London. His research covers privacy-enhancing technology, Internet censorship, and anonymous communication, in particular Tor. He ...
and announced in January 2008. The Tor Browser consists of a modified Mozilla Firefox ESR web browser, the TorButton, TorLauncher, NoScript and the Tor proxy. Users can run the Tor Browser from
removable media Expandable storage is a form of computer storage that is designed to be inserted and removed from a system. Some forms of removable media, such as optical discs, require a reader to be installed in the computer, while others, such as USB flash dr ...
. It can operate under
Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...
,
macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and la ...
, Android and
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whi ...
. The default search engine 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 and
canvas fingerprinting Canvas fingerprinting is one of a number of browser fingerprinting techniques for tracking online users that allow websites to identify and track visitors using the HTML5 canvas element instead of browser cookies or other similar means. The techni ...
, and it also helps to prevent creation of a filter bubble. To allow download from places where accessing the Tor Project URL may be risky or blocked, a
GitHub GitHub, Inc. () is an Internet hosting service for software development and version control using Git. It provides the distributed version control of Git plus access control, bug tracking, software feature requests, task management, cont ...
repository is maintained with links for releases hosted in other domains.


Firefox/Tor browser attack

In 2011, the Dutch authority investigating
child pornography Child pornography (also called CP, child sexual abuse material, CSAM, child porn, or kiddie porn) is pornography that unlawfully exploits children for sexual stimulation. It may be produced with the direct involvement or sexual assault of a ...
discovered the IP address of a Tor onion service site called "Pedoboard" from an unprotected administrator's account and gave it to the FBI, who traced it to Aaron McGrath. After a year of surveillance, the FBI launched "
Operation Torpedo Operation Torpedo was a 2011 operation in which the Federal Bureau of Investigation (FBI) compromised three different hidden services hosting child pornography, which would then target anyone who happened to access them using a network investigativ ...
" which resulted in McGrath's arrest and allowed them to install their Network Investigative Technique (NIT) malware on the servers for retrieving information from the users of the three onion service sites that McGrath controlled. The technique, exploiting a Firefox/Tor browser's vulnerability that had been patched and targeting users that had not updated, had a Flash application pinging 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, with at least 18 other users including a former Acting HHS Cyber Security Director being sentenced in subsequent cases. In August 2013 it was discovered that the Firefox 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
Federal Bureau of Investigation The Federal Bureau of Investigation (FBI) is the domestic intelligence and security service of the United States and its principal federal law enforcement agency. Operating under the jurisdiction of the United States Department of Justice ...
(FBI) operation targeting
Freedom Hosting :''This defunct hosting site is not related to Freedom Hosting Pty Ltd, which is operating in Australia.'' Freedom Hosting is a defunct Tor specialist web hosting service that was established in 2008. At its height in August 2013, it was the la ...
's owner, Eric Eoin Marques, who was arrested on a provisional extradition 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 Child pornography (also called CP, child sexual abuse material, CSAM, child porn, or kiddie porn) is pornography that unlawfully exploits children for sexual stimulation. It may be produced with the direct involvement or sexual assault of a ...
, as well as aiding and abetting advertising of child pornography. The warrant alleged that Marques was "the largest facilitator of child porn on the planet". The FBI acknowledged the attack in a 12 September 2013 court filing in
Dublin Dublin (; , or ) is the capital and largest city of Ireland. On a bay at the mouth of the River Liffey, it is in the province of Leinster, bordered on the south by the Dublin Mountains, a part of the Wicklow Mountains range. At the 2016 ...
; further technical details from a training presentation leaked by Edward Snowden revealed the code name for the exploit as "EgotisticalGiraffe". In 2022, Kaspersky researchers found that when looking up 'Tor Browser' in Chinese on Youtube, one of the URL links provided under the top-ranked Chinese language video actually pointed to a malware disguised as Tor Browser. Once installed, it saves browsing history and form data that genuine Tor forgot by default, and even downloads malicious components to computers with IP addresses in China. Kaspersky researchers noted that the malware was not stealing data to sell for profit, but was designed to identify users.


Tor Messenger

On 29 October 2015, the Tor Project released Tor Messenger Beta, an instant messaging program based on
Instantbird Instantbird is a discontinued cross-platform instant messaging client based on Mozilla's XULRunner and the open-source library ''libpurple'' used in Pidgin. Instantbird is free software available under the GNU General Public License. Over 250 a ...
with Tor and OTR built in and used by default. Like Pidgin 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 bases on
CopperheadOS CopperheadOS is a mobile operating system for smartphones, based on the Android mobile platform. It adds privacy and security features to the official releases of the Android Open Source Project by Google. CopperheadOS is developed by Copperhead ...
. 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 (formerly Azureus) BitTorrent client,
Bitmessage Bitmessage is a decentralized, encrypted, peer-to-peer, trustless communications protocol that can be used by one person to send encrypted messages to another person, or to multiple subscribers. Bitmessage was conceived by software developer J ...
anonymous messaging system, and
TorChat TorChat was a peer-to-peer anonymous instant messenger that used Tor onion services as its underlying network. It provided cryptographically secure text messaging and file transfers. The characteristics of Tor's onion services ensure that all t ...
instant messenger include Tor support. 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 ChatSecure is a messaging application for iOS which allows OTR and OMEMO encryption for the XMPP protocol. ChatSecure is free and open source software available under the GPL-3.0-or-later license. ChatSecure has been used by international ind ...
instant messaging client,
Orbot Orbot is a free software Proxy server project to provide anonymity on the Internet for users of the Android operating system. It acts as an instance of the Tor network on such devices and allows traffic routing from a device's web browser, em ...
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 An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), 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 ...
is open-source, privacy-enhancing web browser for iOS, which uses Tor. It is available in the iOS App Store, and source code is available on
GitHub GitHub, Inc. () is an Internet hosting service for software development and version control using Git. It provides the distributed version control of Git plus access control, bug tracking, software feature requests, task management, cont ...
. Brave added support for Tor in its desktop browser's private-browsing mode. Users can switch to Tor-enabled browsing by clicking on the hamburger menu on the top right corner of the browser. File:Orbot-logo.svg, Orbot logo File:Onion Browser on iPad.png, Onion Browser on
iPad The iPad is a brand of iOS and iPadOS-based tablet computers that are developed by Apple Inc., Apple Inc. The iPad was conceived before the related iPhone but the iPhone was developed and released first. Speculation about the development, ...
File:Onion Browser 2.8.1 on iPhone.png, Onion Browser 2.8.1 on iPhone


Security-focused operating systems

Several security-focused operating systems make or made extensive use of Tor. These include
Hardened Linux From Scratch ''Linux From Scratch'' (LFS) is a type of a Linux installation and the name of a book written by Gerard Beekmans, and as of May 2021, mainly maintained by Bruce Dubbs. The book gives readers instructions on how to build a Linux system from so ...
, Incognito, Liberté Linux,
Qubes OS Qubes OS is a security-focused desktop Linux distribution that aims to provide security through isolation. Virtualization is performed by Xen, and user environments can be based on (with official support) Fedora or Debian, or (with communit ...
, Subgraph, Tails, Tor-ramdisk, and
Whonix Whonix (, ) is a Kicksecure–based security hardened Linux distribution. Its main goals are to provide strong privacy and anonymity on the Internet. The operating system consists of two virtual machines, a "Workstation" and a Tor "Gateway", ru ...
.


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'' 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 right to freedom of expression has been recog ...
, 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, 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, with the
U.S. State Department The United States Department of State (DOS), or State Department, is an executive department of the U.S. federal government responsible for the country's foreign policy and relations. Equivalent to the ministry of foreign affairs of other n ...
, the Broadcasting Board of Governors, and the
National Science Foundation The National Science Foundation (NSF) is an independent agency of the United States government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National ...
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 A ...
, the U.S. Naval Research Laboratory, and the Government of Sweden. 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 including
Human Rights Watch Human Rights Watch (HRW) is an international non-governmental organization, headquartered in New York City, that conducts research and advocacy on human rights. The group pressures governments, policy makers, companies, and individual human ...
, and private sponsors including
Reddit Reddit (; stylized in all lowercase as reddit) is an American social news aggregation, content rating, and discussion website. Registered users (commonly referred to as "Redditors") submit content to the site such as links, text posts, imag ...
and
Google Google LLC () is an American Multinational corporation, multinational technology company focusing on Search Engine, search engine technology, online advertising, cloud computing, software, computer software, quantum computing, e-commerce, ar ...
. Dingledine said that the
United States Department of Defense The United States Department of Defense (DoD, USDOD or DOD) is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government directly related to national sec ...
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 :''This defunct hosting site is not related to Freedom Hosting Pty Ltd, which is operating in Australia.'' Freedom Hosting is a defunct Tor specialist web hosting service that was established in 2008. At its height in August 2013, it was the la ...
and online marketplace Silk Road. 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 ''Der Spiegel'' (, lit. ''"The Mirror"'') is a German weekly news magazine published in Hamburg. With a weekly circulation of 695,100 copies, it was the largest such publication in Europe in 2011. 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,
RedPhone Open Whisper Systems (abbreviated OWS) was a software development group that was founded by Moxie Marlinspike in 2013. The group picked up the open source development of TextSecure and RedPhone, and was later responsible for starting the deve ...
, Tails, and
TrueCrypt TrueCrypt is a discontinued source-available freeware utility used for on-the-fly encryption (OTFE). It can create a virtual encrypted disk within a file, or encrypt a partition or the whole storage device (pre-boot authentication). On 28 M ...
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, to support the free software movement, with the organization's preference for software being distributed under copyleft ( ...
'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, and also called Persia, is a country located in Western Asia. It is bordered by Iraq and Turkey to the west, by Azerbaijan and Armenia to the northwest, by the Caspian Sea and Turkmeni ...
and more recently
Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a List of transcontinental countries, transcontinental country spanning the North Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via a land bridg ...
." 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'' 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 1 April 1983) is an American independent journalist, computer security researcher, artist, and hacker. He studied at the Eindhoven University of Technology and was a core member of the Tor project, a free software networ ...
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 used Tor to send information about PRISM to ''
The Washington Post ''The Washington Post'' (also known as the ''Post'' and, informally, ''WaPo'') is an American daily newspaper published in Washington, D.C. It is the most widely circulated newspaper within the Washington metropolitan area and has a large n ...
'' and ''
The Guardian ''The Guardian'' is a British daily newspaper. It was founded in 1821 as ''The Manchester Guardian'', and changed its name in 1959. Along with its sister papers '' The Observer'' and '' The Guardian Weekly'', ''The Guardian'' is part of the ...
''.


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 American Cable Systems and Comcast Holdings),Before the AT&T merger in 2001, the parent company was Comcast Holdings Corporation. Comcast Holdings Corporation now refers to a subsidiary of Comcast Corpora ...
had been discouraging customers from using the Tor Browser,
Comcast Comcast Corporation (formerly known as American Cable Systems and Comcast Holdings),Before the AT&T merger in 2001, the parent company was Comcast Holdings Corporation. Comcast Holdings Corporation now refers to a subsidiary of Comcast Corpora ...
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, commonly known as the Office of the High Commissioner for Human Rights (OHCHR) or the United Nations Human Rights Office, is a department of the Secretariat of the United Nati ...
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, 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 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 lawyer and politician serving as a U.S. representative from California. A member of the Democratic Party, Lofgren is in her 13th term in Congress, having been first elected in ...
(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 Healthcare, an Indian BPO s ...
(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 Valencia ( va, València) is the capital of the autonomous community of Valencia and the third-most populated municipality in Spain, with 791,413 inhabitants. It is also the capital of the province of the same name. The wider urban area al ...
, Spain, implemented in the first months of 2016. In August 2015, an IBM 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, 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 (of the Pentagon Papers), Cory Doctorow (of Boing Boing), Edward Snowden, 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, 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. 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 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 legally represented lovecruft. On 2 December 2016, ''
The New Yorker ''The New Yorker'' is an American weekly magazine featuring journalism, commentary, criticism, essays, fiction, satire, cartoons, and poetry. Founded as a weekly in 1925, the magazine is published 47 times annually, with five of these issues ...
'' reported on burgeoning digital privacy and security workshops in the
San Francisco Bay Area The San Francisco Bay Area, often referred to as simply the Bay Area, is a populous region surrounding the San Francisco, San Pablo, and Suisun Bay estuaries in Northern California. The Bay Area is defined by the Association of Bay Area G ...
, 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, ...
Noisebridge Noisebridge is an anarchistic maker and hackerspace located in San Francisco, inspired by European hackerspaces Metalab and c-base in Berlin. It describes itself as "a space for sharing, creation, collaboration, research, development, mentori ...
, in the wake of the
2016 United States presidential election The 2016 United States presidential election was the 58th quadrennial presidential election, held on Tuesday, November 8, 2016. The Republican ticket of businessman Donald Trump and Indiana governor Mike Pence defeated the Democratic ticke ...
; 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) was fundamental to the operation of the darkweb marketplace
AlphaBay AlphaBay is a darknet market operating both as an onion service on the Tor network and as an I2P node on I2P. After it was shut down in July 2017 following law enforcement action in the United States, Canada, and Thailand as part of Operation B ...
, 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 if friendly actions can be observed by enemy intelligence, determines if information obtained by adversaries could be interpreted to be useful to them, a ...
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 Left-wing politics, left-wing Democratic Socialists of America#Tendencies within the DSA, multi-tendency Socialism, socialist and Labour movement, labor-oriented political organization. Its roots ...
recommended intermittent Tor usage. 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 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 torservers.net is an independent network of non-profit organisations that provide nodes to the Tor anonymity network. The network started in June 2010 and currently transfers up to 7.4GB/s (~59.2Gb/s) of exit node traffic as of May 2022. Torserve ...
, 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 convention. Tor came out strongly against the raid against 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 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), for example * Simplification of algebraic expressions ...
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 Chinese Communist Party (CCP), officially the Communist Party of China (CPC), is the founding and sole ruling party of the People's Republic of China (PRC). Under the leadership of Mao Zedong, the CCP emerged victorious in the Chinese Ci ...
.


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 during the Russian invasion of Ukraine, the BBC 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. Iran implemented rolling internet blackouts during the Mahsa Amini protests, and Tor and Snowflake 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 versions of websites, not to
torrent Torrent or torrents may refer to: * A fast flowing stream Animals * Torrent duck, a species of the family Anatidae * Torrent fish * Torrent frog, various unrelated frogs * Torrent robin, a bird species * Torrent salamander, a family of sa ...
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, diplomat, and former lawyer who served as the 67th United States Secretary of State for President Barack Obama from 2009 to 2013, as a United States sen ...
'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 Nick Mathewson is an American computer scientist and co-founder of The Tor Project. He, along with Roger Dingledine, began working on onion routing shortly after they graduated from Massachusetts Institute of Technology (MIT) in the early 2000s. ...
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.


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 (default) – at this security level, all browser features are enabled. #* This level provides the most usable experience, and the lowest level of security. # Safer – at this security level, the following changes apply: #* JavaScript is disabled on non-HTTPS sites. #* On sites where JavaScript is enabled, performance optimizations are disabled. Scripts on some sites may run slower. #* Some mechanisms of displaying math equations are disabled. #* Audio and video (HTML5 media), and WebGL are click-to-play. # Safest – at this security level, these additional changes apply: #* JavaScript is disabled by default on all sites. #* Some fonts, icons, math symbols, and images are disabled. #* Audio and video (HTML5 media), and WebGL are click-to-play.


See also

*
.onion .onion is a special-use top level domain name designating an anonymous onion service, which was formerly known as a "hidden service", reachable via the Tor network. Such addresses are not actual DNS names, and the .onion TLD is not in the I ...
*
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 Private browsing is a privacy feature in some web browsers. When operating in such a mode, the browser creates a temporary session that is isolated from the browser's main session and user data. Browsing history is not saved, and local data as ...
* Briar: messaging app on Tor network * Crypto-anarchism *
Darknet A dark net or darknet is an overlay network within the Internet that can only be accessed with specific software, configurations, or authorization, and often uses a unique customized communication protocol. Two typical darknet types are social ne ...
*
Dark web The dark web is the World Wide Web content that exists on ''darknets'': overlay networks that use the Internet but require specific software, configurations, or authorization to access. Through the dark web, private computer networks can comm ...
*
Deep web The deep web, invisible web, or hidden web are parts of the World Wide Web whose contents are not indexed by standard web search-engine programs. This is in contrast to the "surface web", which is accessible to anyone using the Internet. Co ...
*
Freedom of information Freedom of information is freedom of a person or people to publish and consume information. Access to information is the ability for an individual to seek, receive and impart information effectively. This sometimes includes "scientific, indigen ...
* Freenet * GNUnet *
I2P The Invisible Internet Project (I2P) is an anonymous network layer (implemented as a mix network) that allows for censorship-resistant, peer-to-peer communication. Anonymous connections are achieved by encrypting the user's traffic (by using ...
* Internet censorship * Internet censorship circumvention * Internet privacy *
Privoxy Privoxy is a free non- caching web proxy with filtering capabilities for enhancing privacy, manipulating cookies and modifying web page data and HTTP headers before the page is rendered by the browser. Privoxy is a "privacy enhancing proxy", fi ...
* Proxy server * Psiphon *
Tor2web Tor2web (pronounced "Tor to Web") is a software project to allow Tor hidden services to be accessed from a standard browser without being connected to the Tor network. It was created by Aaron Swartz and Virgil Griffith. History Tor is a net ...
*
Tor Phone Tor, TOR or ToR may refer to: Places * Tor, Pallars, a village in Spain * Tor, former name of Sloviansk, Ukraine, a city * Mount Tor, Tasmania, Australia, an extinct volcano * Tor Bay, Devon, England * Tor River, Western New Guinea, Indonesia ...
*
torservers.net torservers.net is an independent network of non-profit organisations that provide nodes to the Tor anonymity network. The network started in June 2010 and currently transfers up to 7.4GB/s (~59.2Gb/s) of exit node traffic as of May 2022. Torserve ...


Citations


General and cited references

* * * * *


External links

*
Anonymity Bibliography

Old website



Animated introduction

Tor: Hidden Services and Deanonymisation
presentation at the 31st Chaos Computer Conference
TorFlow
a dynamic visualization of data flowing over the Tor network
Tor onion services: more useful than you think
in a 2016 presentation at the 32nd Annual Chaos Communication Congress
A core Tor developer lectures at the
Radboud University Nijmegen in The Netherlands on anonymity systems in 2016
A technical presentation given at the
University of Waterloo in Canada: Tor's Circuit-Layer Cryptography: Attacks, Hacks, and Improvements
A Presentation at the March 2017 BSides Vancouver Conference on security practices on Tor's hidden services given by
Sarah Jamie Lewis Sarah Jamie Lewis is an anonymity and privacy researcher with a special interest in the privacy protocols (or lack thereof) of sex toys. She has been cited in academic research regarding the security and ethics considerations associated with this t ...
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