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Tor?(previously an acronym for?The Onion Router)^[5]?is?free software?for enabling?online anonymity?and resisting censorship. Tor directs Internet traffic through a free, worldwide, volunteer network consisting of more than five thousand relays^[6]?to conceal a user's location or 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".^[7]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".^[8]

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. Since 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.^[9]?The NSA has a technique that targets outdated?Firefox?browsers codenamed EgotisticalGiraffe

History[edit]

An alpha version of the free software, with the onion routing network "functional and deployed", was announced on 20 September 2002.^[1]?Roger Dingledine, Nick Mathewson, and Paul Syverson presented "Tor: The Second-Generation Onion Router" at the thirteenth?USENIX?Security Symposium on 13 August 2004.^[11]?Although the name Tor originated as an?acronym?of The Onion Routing project (TOR project), the current project no longer considers the name to be an acronym, and therefore, does not use all capital letters.^[12]

Originally sponsored by the?U.S. Naval Research Laboratory,^[11]?which had been instrumental in the early development of?onion routing?under the aegis of?DARPA, Tor was financially supported by the?Electronic Frontier Foundation?from 2004 to 2005.^[13][14]?Tor software is now developed by the Tor Project,^[15]?which has been a?501(c)(3)?research-education?nonprofit organization?^[16]?based in the United States of America since December 2006. It has a diverse base of financial support;^[13]?the?U.S. State Department, theBroadcasting Board of Governors, and the?National Science Foundation?are major contributors.^[17]?As of 2012, 80% of the Tor Project's $2M annual budget comes from the?United States government, with the?Swedish government?and other organizations providing the rest,^[18]?including?NGOs?and thousands of individual sponsors.^[13]?One of the founders of the project, Roger Dingledine, stated that the?United States Department of Defense?funds are less similar to being a?procurement contract?and are more similar to a?research grant. Andrew Lewman, the executive director of the Tor project, 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.^[19]

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."^[20]

Foreign Policy?named Dingledine, Mathewson, and Syverson among its 2012?Top 100 Global Thinkers?"for making the web safe for whistleblowers."^[21]

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."^[22]

Edward Snowden?used the Tor Network to send information about?PRISM?to the?Washington Post?and?The Guardian?in June 2013.^[23]

Operation[edit]

Alice's Tor client picks a random path to destination server

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.^[11]?Furthermore, by keeping some of the entry relays (bridge relays) secret, users can evade?Internet censorship?that relies upon blocking public Tor relays.^[24]

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 (the exit node) is the originator of the communication rather than the sender.

Originating traffic[edit]

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

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, andWorld Wide Web?browsing.

Hidden services[edit]

See also:?List of Tor hidden services

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.^[25]

Hidden services have been deployed on the Tor network since 2004.^[26]?Other than the database that stores the hidden-service descriptors,^[27]?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.

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.^[27][28]

Weaknesses[edit]

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).^[29][30]

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.^[31]

Researchers from the?University of Michigan?developed a network scanner allowing identification of 86 percent of live Tor “bridges” with a single scan.^[32]

Eavesdropping[edit]

Autonomous System (AS) eavesdropping[edit]

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 traf?c 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 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.^[33]

Exit node eavesdropping[edit]

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.^[34]?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.^[35]?Furthermore, Egerstad is circumspect about the possible subversion of Tor by intelligence agencies –

"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?"^[36]

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.^[37][38]?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.^[39]

Traffic-analysis attack[edit]

Steven J. Murdoch?and George Danezis from?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.^[40]?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.^[40]?Murdoch has been working with—and has been funded by—Tor since 2006.

Tor exit node block[edit]

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 editWikipedia?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?exemption?is obtained.

Bad Apple attack[edit]

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 tabletracking.^[41]?According to the study:

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.)^[41]

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.^[41]?BitTorrent may generate as much as 40% of all traffic on Tor.^[42]?Furthermore, the bad apple attack is effective against insecure use of any application over Tor, not just BitTorrent.^[41]

Some protocols leak IP addresses[edit]

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:^[43]

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.^[43]: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.^[43]: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.^[43]:4–5

Using these techniques researchers were able to identify other streams initiated by users, whose IP addresses were revealed.^[43]

Sniper attack[edit]

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.^[44]

Heartbleed bug[edit]

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.^[45]?586 relays later found to be susceptible to the Heartbleed bug were taken off-line as a precautionary measure.^{[46][47][48][49]}

Licit and illicit uses[edit]

Tor has been described by?The Economist, in relation to?Bitcoin?and the?Silk Road, as being "a dark corner of the web."^[50]?It has been targeted by both the American?NSA?and the British?GCHQ?signals intelligence?agencies, albeit with marginal success.^[9]?At times, anonymizing systems such as Tor are used for matters that are, or may be, illegal in some countries, e.g., Tor may be used to gain access to?censored?information, to organize political activities,^[51]?or to circumvent laws against criticism of heads of state. Tor can also be used for anonymous defamation, unauthorized?leaks?of sensitive information and?copyright infringement, the distribution of illegal sexual content,^[52][53][54]?the selling of?controlled substances,^[55]?money laundering,^[56]?credit card fraud, and?identity theft; furthermore, the?black market?which utilizes the Tor infrastructure operates, at least in part, in conjunction with?Bitcoin.^[57]?Ironically, Tor has been used by criminal enterprises,?hacktivism?groups, and law enforcement agencies at cross purposes, sometimes simultaneously;^[57][58]?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),?Radio Free Asia, and seek to subvert it.^[9][59]

Many organizations argue or acknowledge that Tor has legal, legitimate uses. In its complaint against Ross William Ulbricht of the?Silk Road?the FBI acknowledged that Tor has "known legitimate uses".^[60][61]?According to?CNET, Tor's anonymity function is "endorsed by the?Electronic Frontier Foundation?and other civil liberties groups as a method forwhistleblowers?and human rights workers to communicate with journalists".^[62]?EFF's Surveillance Self-Defense guide includes a description of where Tor fits in a larger strategy for protecting privacy and anonymity.^[63]?The Tor Project's FAQ offers supporting reasons for EFF's endorsement:

Criminals can already do bad things. Since they're willing to break laws, they already have lots of options available that provide better privacy than Tor provides....

Tor aims to provide protection for ordinary people who want to follow the law. Only criminals have privacy right now, and we need to fix that....

So yes, criminals could in theory use Tor, but they already have better options, and it seems unlikely that taking Tor away from the world will stop them from doing their bad things. At the same time, Tor and other privacy measures can fight identity theft, physical crimes like stalking, and so on.^[64]

Tor is increasingly in common use by victims of?domestic violence?and the?social workers?and agencies which assist them inasmuch as digital stalking has also increased, given the prevalence of digital media in contemporary?on-line?life.^[65]

Implementations[edit]

The main implementation of Tor is written in the?C programming language?and consists of roughly 146,000 lines of source code.^[4]

Tor Browser[edit]

Tor BrowserDeveloper(s)Stable releasePreview releaseOperating systemEngineAvailable inLicenseWebsite

Tor Browser on?Linux Mint?showing its start page - about:tor

Tor Project

3.5.4[66]?(April?8, 2014; 2 months ago)?[±]

3.6-beta-2[67]?(April?11, 2014; 2 months ago)?[±]

OS X Microsoft Windows Linux

Gecko

14 languages

GPL

www.torproject.org/projects/torbrowser.html

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.^[68][69]?It can be run from removable media and is available for Windows, Mac OS X, and Linux.^[70]

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.^[69]

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.^[71]

Firefox / JavaScript anonymity attack[edit]

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.^[72]?This attack was being exploited to send users' MAC and IP addresses and Windows computer names to the attackers.^[73][74][75]?News reports linked this to an?FBI?operation targeting Freedom Hosting's owner, Eric Eoin Marques, who was arrested on a provisional extradition warrant issued by a U.S. court on 29 July. The FBI is seeking to extradite Marques 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.”^[76][77]?The FBI acknowledged the attack in a 12 September 2013 court filing in?Dublin;^[78]?further technical details from a training presentation leaked by?Edward Snowden?showed that the codename for the exploit wasEgotisticalGiraffe.^[79]

Third party[edit]

Vuze?(formerly Azureus)?BitTorrent?client,^[80]?Bitmessage?anonymous messaging system,^[81]?and?TorChat?instant messenger, include Tor support.

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.^[82]?The applications include?ChatSecure?instant messaging client,^[83]?Orbot?Tor implementation,^[84]?Orweb privacy-enhanced mobile browser,^[85]ProxyMob?Firefox add-on^[86]?and Secure Smart Cam.^[87]

Security-focused operating systems[edit]

Several?security-focused operating systems?like GNU/Linux distributions including?Hardened Linux From Scratch,?Incognito,?Liberté Linux,?Qubes OS,?Tails,?Tor-ramdisk?andWhonix, make extensive use of Tor.^[88]

Email[edit]

Mailtor is a Tor hidden service that allows use of an E-Mail client (or webmail) to send and receive email anonymously, to email addresses inside and outside the Tor network through their?webmail?interface. Mailtor is a long-standing email service in the Tor network being non-commercial and free to use.^[89][90]?It experienced a major increase in usage during August 2013 after?TorMail?went offline. Mailtor offers?webmail,?pop3,?IMAP?and?SMTP?access to email clients, similar to its defunct counterpart, TorMail.^[91]

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