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Tor (anonymity network)

Developer(s) The Tor Project, Inc
Initial release 20 September 2002 (2002-09-20)[1]
Stable release[2] (20 October 2014 (2014-10-20))[3] (25 October 2014 (2014-10-25))
Preview release[4] (30 October 2014 (2014-10-30))
Development status Active
Written in C[5]
Operating system
Size 2–3 MB
Type Onion routing, Anonymity
License BSD
Website .org.torprojectwww

Tor (previously an acronym for The Onion Router)[6] is free software for enabling online anonymity and resisting censorship. It is designed to make it possible for users to surf the Internet anonymously, so their activities and location cannot be discovered by government agencies, corporations, or anyone else.

Tor directs Internet traffic through a free, worldwide, volunteer network consisting of more than five thousand relays[7] to conceal a user's location and usage from anyone conducting network surveillance or traffic analysis. Using Tor makes it more difficult for Internet activity to be traced back to the user: this includes "visits to Web sites, online posts, instant messages, and other communication forms".[8] Tor's use is intended to protect the personal privacy of users, as well as their freedom and ability to conduct confidential communication by keeping their Internet activities from being monitored. An extract of a Top Secret appraisal by the National Security Agency (NSA) characterized Tor as "the King of high secure, low latency Internet anonymity" with "no contenders for the throne in waiting".[9]

The term "onion routing" refers to application layers of encryption, nested like the layers of an onion, used to anonymize communication. Tor encrypts the original data, including the destination IP address, multiple times and sends it through a virtual circuit comprising successive, randomly selected Tor relays. Each relay decrypts a layer of encryption to reveal only the next relay in the circuit in order to pass the remaining encrypted data on to it. The final relay decrypts the innermost layer of encryption and sends the original data to its destination without revealing, or even knowing, the source IP address. Because the routing of the communication is partly concealed at every hop in the Tor circuit, this method eliminates any single point at which the communication can be de-anonymized through network surveillance that relies upon knowing its source and destination.

An adversary unable to defeat the strong anonymity that Tor provides may try to de-anonymize the communication by other means. One way this may be achieved is by exploiting vulnerable software on the user's computer.[10] The NSA has a technique that targets outdated Firefox browsers codenamed EgotisticalGiraffe,[11] and targets Tor users in general for close monitoring under its XKeyscore program.[12][13] Attacks against Tor are an active area of academic research[14][15] which is welcomed by Tor itself.[16]


  • History 1
  • Reception and impact 2
  • Usage 3
  • Operation 4
    • Originating traffic 4.1
    • Hidden services 4.2
  • Weaknesses 5
    • Eavesdropping 5.1
      • Autonomous System (AS) eavesdropping 5.1.1
      • Exit node eavesdropping 5.1.2
    • Traffic-analysis attack 5.2
    • Tor exit node block 5.3
    • Bad Apple attack 5.4
    • Some protocols leak IP addresses 5.5
    • Sniper attack 5.6
    • Heartbleed bug 5.7
  • Implementations 6
    • Tor Browser 6.1
      • Firefox / JavaScript anonymity attack 6.1.1
    • Third party 6.2
    • Security-focused operating systems 6.3
  • See also 7
  • References 8
  • Footnotes 9
  • External links 10


A cartogram illustrating Tor usage

The core principle behind Tor, "onion routing," was developed in the mid-1990s by U.S. Naval Research Laboratory employees, mathematician Paul Syverson and computer scientists Michael Reed and David Goldschlag, with the purpose of protecting U.S. intelligence communications online. Onion routing was further developed by DARPA in 1997.[17][18][19]

The alpha version of Tor, developed by Syverson and computer scientists Roger Dingledine and Nick Mathewson[20] and then called The Onion Routing project or TOR project, launched on 20 September 2002.[1][21] On 13 August 2004, Syverson, Dingledine and Mathewson presented "Tor: The Second-Generation Onion Router" at the 13th USENIX Security Symposium.[22] In 2004, the Naval Research Laboratory released the code for Tor under a free licence, and the Electronic Frontier Foundation (EFF) began funding Dingledine and Mathewson to continue its development.[20]

In December 2006, Dingledine, Mathewson and five others founded The Tor Project, a

  • Official website
    • Official blog

External links

  • Anonymity Bibliography Retrieved: 21 May 2007
  • Bacard, Andre. Computer Privacy Handbook.  


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See also

Several security-focused operating systems like GNU/Linux distributions including Hardened Linux From Scratch, Incognito, Liberté Linux, Qubes OS, Tails, Tor-ramdisk and Whonix, make extensive use of Tor.[122]

Security-focused operating systems

The Guardian Project is actively developing a free and open-source suite of application programs and firmware for the Android operating system to help make mobile communications more secure.[116] The applications include ChatSecure instant messaging client,[117] Orbot Tor implementation,[118] Orweb privacy-enhanced mobile browser,[119] ProxyMob Firefox add-on[120] and ObscuraCam.[121]

Vuze (formerly Azureus) BitTorrent client,[114] Bitmessage anonymous messaging system,[115] and TorChat instant messenger, include Tor support.

Third party

The FBI, in Operation Torpedo, has been targeting Tor hidden servers since 2012, such as in the case of Aaron McGrath, who was sentenced to 20 years for running three hidden Tor servers containing child pornography.[113]

In August 2013, it was discovered that the Firefox browsers in many older versions of the Tor Browser Bundle were vulnerable to a JavaScript attack, as NoScript was not enabled by default.[105] This attack was being exploited to send users' MAC and IP addresses and Windows computer names to the attackers.[106][107][108] News reports linked this to a United States Federal Bureau of Investigation (FBI) operation targeting Freedom Hosting's owner, Eric Eoin Marques, who was arrested on a provisional extradition warrant issued by a United States court on 29 July. The FBI is seeking to extradite Marques out of Ireland to Maryland on four charges — distributing, conspiring to distribute, and advertising child pornography — as well as aiding and abetting advertising of child pornography. The warrant alleges that Marques is “the largest facilitator of child porn on the planet.”[109][110] The FBI acknowledged the attack in a 12 September 2013 court filing in Dublin;[111] further technical details from a training presentation leaked by Edward Snowden showed that the codename for the exploit was EgotisticalGiraffe.[112]

Firefox / JavaScript anonymity attack

Tor also allows access to websites not available for normal browsers. However, normal websites can still be accessed.

Following the series of global surveillance disclosures, Stuart Dredge of The Guardian recommended using Tor Browser to avoid eavesdropping and retain privacy on the internet.[104]

Tor Browser automatically starts Tor processes and routes traffic through the Tor network. Upon termination of a session the browser deletes privacy-sensitive data such as cookies and browsing history.[102]

Tor Browser (previously known as Tor Browser Bundle or TBB) is the Tor Project's flagship product. It consists of a modified Mozilla Firefox ESR web browser, the TorButton, TorLauncher, NoScript and HTTPS Everywhere Firefox extensions and the Tor proxy.[101][102] It can be run from removable media and is available for Windows, Mac OS X, and Linux.[103]

Tor Browser
Tor Browser on Linux Mint showing its start page - about:tor
Developer(s) Tor Project
Stable release 4.0.2[99] (3 December 2014 (2014-12-03))
Preview release 4.5-alpha-1[100] (17 November 2014 (2014-11-17))
Development status Active
Operating system
Engine Gecko
Size 26–33 MB
Available in 15 languages
Type Onion routing, Anonymity, Web browser, Feed reader
License GPL
Website .html/torbrowser/

Tor Browser

The main implementation of Tor is written mostly in the C programming language and consists of roughly 340,000 lines of source code.[5]


The Heartbleed OpenSSL bug disrupted the Tor network for several days in April 2014 while private keys were renewed. The Tor Project recommended that Tor relay operators and hidden service operators revoke and generate fresh keys after patching OpenSSL, but noted that Tor relays use two sets of keys and that Tor's multi-hop design minimizes the impact of exploiting a single relay.[94] 586 relays later found to be susceptible to the Heartbleed bug were taken off-line as a precautionary measure.[95][96][97][98]

Heartbleed bug

Jensen et al., describe DDoS attack targeted at the TOR node software, together with defences against that attack and 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.[93]

Sniper attack

Using these techniques researchers were able to identify other streams initiated by users, whose IP addresses were revealed.[92]

Inspection of BitTorrent control messages
Tracker announces and Extension Protocol handshakes may optionally contain client's IP address. Analysis of collected data revealed that 35% and 33% of respective messages contained real addresses of clients.[92]:3
Hijacking trackers' responses
Due to lack of encryption or authentication in communication between tracker and peer typical man-in-the-middle attack allows attacker to determine peer's IP address and even verify the distribution of content. This attack works when Tor is used only for tracker communication.[92]:4
Exploiting distributed hash tables (DHT)
This attack exploits the fact that DHT connections through Tor are impossible, so attacker is able to reveal target's IP address by looking it up in DHT even if target uses Tor to connect to other peers.[92]:4–5

Researchers from French Institute for Research in Computer Science and Control (INRIA) showed that Tor dissimulation technique in BitTorrent can be bypassed by attacker controlling Tor exit node. The study was conducted by monitoring 6 exit nodes for a period of 23 days. Researches used three attack vectors:[92]

Some protocols leak IP addresses

The results presented in the bad apple attack research paper are based on an attack in the wild launched against the Tor network by the authors of the study. The attack targeted six exit nodes, lasted for 23 days, and revealed a total of 10,000 IP addresses of active Tor users. This study is particularly significant because it is the first documented attack designed to target P2P file sharing applications on Tor.[90] BitTorrent may generate as much as 40% of all traffic on Tor.[91] Furthermore, the bad apple attack is effective against insecure use of any application over Tor, not just BitTorrent.[90]

This attack against Tor consists of two parts: (a) exploiting an insecure application to reveal the source IP address of, or trace, a Tor user and (b) exploiting Tor to associate the use of a secure application with the IP address of a user (revealed by the insecure application). As it is not a goal of Tor to protect against application-level attacks, Tor cannot be held responsible for the first part of this attack. However, because Tor's design makes it possible to associate streams originating from secure application with traced users, the second part of this attack is indeed an attack against Tor. We call the second part of this attack the bad apple attack. (The name of this attack refers to the saying 'one bad apple spoils the bunch.' We use this wording to illustrate that one insecure application on Tor may allow to trace other applications.)[90]

In March 2011, researchers with the Rocquencourt, France based National Institute for Research in Computer Science and Control (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 use 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.[90] According to the study:

Bad Apple attack

Operators of Internet sites have the ability to prevent traffic from Tor exit nodes, or to offer reduced functionality to Tor users. For example, it is not generally possible to edit WorldHeritage when using Tor, or when using an IP address that also is used by a Tor exit node, due to the use of the TorBlock MediaWiki extension, unless an is obtained.

Tor exit node block

University of Cambridge presented an article at the 2005 IEEE Symposium 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.[89] 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.[89] Murdoch has been working with—and has been funded by—Tor since 2006.

Traffic-analysis 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.[86][87] The technique they describe requires creating a map of Tor network nodes, controlling one third of them, and then acquiring their encryption keys 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-based 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 that these rumours of Tor's compromise are greatly exaggerated.[88]

"If you actually look in to where these Tor nodes are hosted and how big they are, some of these nodes cost thousands of dollars each month just to host because they're using lots of bandwidth, they're heavy-duty servers and so on. Who would pay for this and be anonymous?"[85]

In September 2007, Dan Egerstad, a Swedish security consultant, revealed that he had intercepted usernames and passwords for a large number of e-mail accounts by operating and monitoring Tor exit nodes.[83] As Tor does not, and by design cannot, encrypt the traffic between an exit node and the target server, any exit node is in a position to capture any traffic passing through it that does not use end-to-end encryption such as SSL or TLS. While this may not inherently breach the anonymity of the source, traffic intercepted in this way by self-selected third parties can expose information about the source in either or both of payload and protocol data.[84] Furthermore, Egerstad is circumspect about the possible subversion of Tor by intelligence agencies –

Exit node eavesdropping

If an Autonomous System (AS) exists 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 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 path selection algorithm on client side. In this paper, they also improve latency by choosing shorter geographical paths between client and destination.[82]

Autonomous System (AS) eavesdropping


Researchers from the University of Michigan developed a network scanner allowing identification of 86 percent of live Tor “bridges” with a single scan.[81]

In spite of known weaknesses and attacks listed here, Tor and the alternative network system JonDonym (Java Anon Proxy, JAP) are considered more resilient than alternatives such as VPNs. Were a local observer on an ISP or WLAN to attempt to analyze the size and timing of the encrypted data stream going through the VPN, Tor, or JonDo system, the latter two would be harder to analyze, as demonstrated by a 2009 study.[80]

Like all current low-latency anonymity networks, 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).[78][79]


Hidden services could be also accessed from a standard web browser without client-side connection to the Tor network via such services like Tor2web.[77]

Because hidden services do not use exit nodes, connection to a hidden service is encrypted end-to-end and not subject to eavesdropping. There are, however, security issues involving Tor hidden services. For example, services that are reachable through Tor hidden 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.[75][76]

Hidden services have been deployed on the Tor network since 2004.[74] Other than the database that stores the hidden-service descriptors,[75] Tor is decentralized by design; there is no direct readable list of all hidden services, although a number of hidden services catalog publicly known onion addresses.

Tor can also provide anonymity to websites and other servers. Servers configured to receive inbound connections only through Tor are called hidden services. Rather than revealing a server's IP address (and thus its network location), a hidden service is accessed through its onion address. The Tor network understands these addresses and can route data to and from hidden services, even to those hosted behind firewalls or network address translators (NAT), while preserving the anonymity of both parties. Tor is necessary to access hidden services.[73]

Hidden services

Tor's application independence sets it apart from most other anonymity networks: it works at the Transmission Control Protocol (TCP) stream level. Applications whose traffic is commonly anonymised using Tor include Internet Relay Chat (IRC), instant messaging, and World Wide Web browsing.

A Tor non-exit relay with a maximum output of 239.69 KB/s

A Tor user's SOCKS-aware applications can be configured to direct their network traffic through a Tor instance's SOCKS interface. Tor periodically creates virtual circuits through the Tor network through which it can multiplex 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.

A visual depiction of the traffic between some Tor relay nodes from the open source packet sniffing program EtherApe

Originating traffic

Because the internet address 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 (called the exit node), rather than the sender, is the originator of the communication.

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 a multi-layered manner (hence the onion metaphor) to ensure perfect forward secrecy between relays, thereby providing users with anonymity in network location. That anonymity extends to the hosting of censorship-resistant content via Tor's anonymous hidden service feature.[22] Furthermore, by keeping some of the entry relays (bridge relays) secret, users can evade Internet censorship that relies upon blocking public Tor relays.[72]

Infographic about how Tor works, by EFF
Illustration of the Deep Web, Tor given as an example


In 2014 the EFF's Eva Galperin told BusinessWeek magazine 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.”[33]

In its complaint against Ross William Ulbricht of the Silk Road the FBI acknowledged that Tor has "known legitimate uses".[67][68] According to CNET, Tor's anonymity function is "endorsed by the Electronic Frontier Foundation and other civil liberties groups as a method for whistleblowers and human rights workers to communicate with journalists".[69] EFF's Surveillance Self-Defense guide includes a description of where Tor fits in a larger strategy for protecting privacy and anonymity.[70] The Tor Project's FAQ offers supporting reasons for EFF's endorsement:

[66] Tor's executive director, Andrew Lewman, said in August 2014 that agents of the NSA and the GCHQ have anonymously provided Tor with bug reports.[65][10], and seek to subvert it.Radio Free Asia likewise, agencies within the U.S. government variously fund Tor (the U.S. State Department), the National Science Foundation, and (via the Broadcasting Board of Governors, which itself partially funded Tor until October 2012), [64][63] groups, and law enforcement agencies at cross purposes, sometimes simultaneously;hacktivism Ironically, Tor has been used by criminal enterprises, [63] utilizes the Tor infrastructure at least in part, in conjunction with market; the identity theft, credit card fraud[62] bank fraud, [61],money laundering [60],controlled substances selling [59][58][57], distribution of illegal sexual content,copyright infringement of sensitive information and leaks Tor can be used for anonymous defamation, unauthorized [56][55] 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."[54] in its Operation Notarise.National Crime Agency and more successfully by the British [10] agencies, albeit with marginal success,signals intelligence GCHQ and the British NSA It has been targeted by both the American [53], as being "a dark corner of the web."Silk Road and the Bitcoin, in relation to The Economist or to circumvent laws against criticism of heads of state. Tor has been described by [52] Tor is used for matters that are, or may be, illegal in some countries, e.g., to gain access to

Tor enables 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.[47] The Tor Project says Tor's users include "normal people" who want to keep their internet activities private from websites and advertisers, people concerned about cyber-spying, users who are evading censorship such as activists and journalists, and military professionals. As of late 2014 Tor had about four million users.[48] 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.[49] Tor is increasingly used by victims of New Yorker, ProPublica and the Intercept to protect the privacy of whistleblowers.[51]


Critics say Tor is not as secure as it claims,[43] 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.[20] 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.[10] 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."[44] When Tor users are arrested, it is typically due to human error, not to the core technology being hacked or cracked. [45]"On 7th November 2014, 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.[46]

As of 2012, 80% of The Tor Project's $2M annual budget came from the NGOs and thousands of individual sponsors.[27][41] One of the founders of the project, Roger Dingledine, stated that the United States Department of Defense funds are more similar to a research grant than a procurement contract. Andrew Lewman, the executive director of Tor, stated that even though it accepts funds from the U.S. federal government, the Tor service did not necessarily collaborate with the NSA to reveal identities of users.[42]

Advocates for Tor say that it supports freedom of expression, including in countries where the Internet is heavily censored, by protecting the privacy and anonymity of internet users. However because the project was originally developed on behalf of the U.S. intelligence community and continues to receive U.S. government funding, it has been criticized as "more resembl[ing] a spook project than a tool designed by a culture that values accountability or transparency."[20]

In 2014, the Russian government opened a public bid worth $111,000 to "study the possibility of obtaining technical information about users and users' equipment on the Tor anonymous network".[38][39]

In June 2013, whistleblower Edward Snowden used Tor to send information about PRISM to the Washington Post and The Guardian.[37]

In 2013, Jacob Appelbaum 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."[36]

In 2012, Foreign Policy magazine named Dingledine, Mathewson, and Syverson among its Top 100 Global Thinkers "for making the web safe for whistleblowers."[35]

In March 2011, The Tor Project was awarded the Free Software Foundation's 2010 Award for Projects of Social Benefit on the following grounds: "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 and more recently Egypt."[34]

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 women who have been threatened with violence or abuse by stalkers.[29] The U.S. National Security Agency (NSA) has called Tor "the king of high-secure, low-latency internet anonymity,"[10] and BusinessWeek magazine has described it as "perhaps the most effective means of defeating the online surveillance efforts of intelligence agencies around the world."[30] Other media have described Tor as "a sophisticated privacy tool,"[31] "easy to use"[32] and "so secure that even the world's most sophisticated electronic spies haven't figured out how to crack it."[33]

Reception and impact

[28][27][26][25][24], and Netherlands-based, University of Cambridge, the Human Rights Watch, Internews, International Broadcasting Bureau in its early years, and early financial supporters of The Tor Project included the U.S. fiscal sponsor acted as The Tor Project's EFF The [23]

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