Tor (anonymity network)
|Developer(s)||The Tor Project |
|Initial release||20 September 2002|
|Stable release||0.2.3.25 (3 December 2012) [±]|
|Preview release||0.2.4.11-alpha (19 March 2013 ) [±]|
|Type||Onion routing, Anonymity|
Tor (originally short for The Onion Router) is free software for enabling online anonymity. Tor directs Internet traffic through a free, worldwide volunteer network consisting of thousands of relays to conceal a user's location or usage from anyone conducting network surveillance or traffic analysis. Using Tor makes it more difficult to trace Internet activity, including "visits to Web sites, online posts, instant messages and other communication forms", back to the user  and is intended to protect users' personal privacy, freedom, and ability to conduct confidential business by keeping their internet activities from being monitored.
"Onion Routing" refers to the layers of the encryption used. The original data, including its destination, are encrypted and re-encrypted multiple times, and sent 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 last layer of encryption and sends the original data, without revealing or even knowing its sender, to the destination. This method reduces the chance of the original data being understood in transit and, more notably, conceals the routing of it.
An alpha version of the software, with the onion routing network "functional and deployed", was announced on 20 September 2002. Roger Dingledine, Nick Mathewson and Paul Syverson presented "Tor: The Second-Generation Onion Router" at the 13th USENIX Security Symposium on 13 August 2004. Although the name Tor originated as an acronym of The Onion Routing project, the current project no longer considers the name to be an acronym, and therefore does not use capital letters.
Originally sponsored by the U.S. Naval Research Laboratory (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. Tor software is now developed by the Tor Project, which has been a 501(c)(3) research-education nonprofit organization  based in the United States of America  since December 2006. It has a diverse base of financial support; the U.S. State Department, the Broadcasting Board of Governors, and the National Science Foundation are major contributors. 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, including NGOs and thousands of individual sponsors.
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."
Jacob Appelbaum in 2013 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." 
Tor aims to conceal its users' identities and their network 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. 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 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 
Users of a Tor network run an onion proxy on their machine. The Tor software periodically negotiates a virtual circuit through the Tor network, using multi-layer encryption, ensuring perfect forward secrecy. At the same time, the onion proxy software presents a SOCKS interface to its clients. SOCKS-aware applications may be pointed at Tor, which then multiplexes the traffic through a Tor virtual circuit.
Once inside a Tor network, the traffic is sent from router to router, ultimately reaching an exit node at which point the cleartext packet is available and is forwarded on to its original destination. Viewed from the destination, the traffic appears to originate at the Tor exit node.
Tor's application independence sets it apart from most other anonymity networks: it works at the Transmission Control Protocol (TCP) stream level. Applications whose traffic is commonly anonymised using Tor include Internet Relay Chat (IRC), instant messaging, and World Wide Web browsing. When browsing the Web, Tor is often coupled with Polipo or Privoxy proxy servers. Privoxy is a filtering proxy server that aims to add privacy at the application layer. The Polipo proxy server can speak the SOCKS 4 & SOCKS 5 protocols and does HTTP 1.1 pipelining well, so it can enhance Tor's communication latency. TorProject.org therefore recommends that Polipo be used together with the Tor anonymising network.
On older versions of Tor (resolved May–July 2010), as with many anonymous web surfing systems, direct Domain Name System (DNS) requests are usually still performed by many applications, without using a Tor proxy. This allows someone monitoring a user's connection to determine (for example) which WWW sites they are viewing using Tor, even though they cannot see the content being viewed. Using Privoxy or the command "torify" included with a Tor distribution is a possible solution to this problem. Additionally, applications using SOCKS5 – which supports name-based proxy requests – can route DNS requests through Tor, having lookups performed at the exit node and thus receiving the same anonymity as other Tor traffic.
As of Tor release 0.2.0.1-alpha, Tor includes its own DNS resolver which will dispatch queries over the mix network. This should close the DNS leak and can interact with Tor's address mapping facilities to provide the Tor hidden service (
.onion) access to non-SOCKS-aware applications.
Hidden services 
Tor can also provide anonymity to websites and other servers. Servers configured to receive inbound connections 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 those hosted behind firewalls or network address translators (NAT), while preserving the anonymity of both parties. Tor is necessary to access hidden services.
Hidden services have been deployed on the Tor network beginning in 2004. Other than the database that stores the hidden-service descriptors, Tor is decentralized by design; there is no direct readable list of all hidden services, though a number of hidden services catalog publicly known onion addresses.
Because hidden services do not use exit nodes, they are not subject to exit node 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.
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).
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. 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. However, this attack fails to reveal the identity of the original user. Murdoch has been working with—and has been funded by—Tor since 2006.
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) have 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. 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.)
The results presented in the bad apple attack research paper are based on an actual 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. BitTorrent may generate as much as 40% of all traffic on Tor, which means a significant number of Tor users are potentially at risk. Furthermore, the bad apple attack is effective against insecure use of any application over Tor, not just BitTorrent.
In September 2007, Dan Egerstad, a Swedish security consultant, revealed that he had intercepted usernames and passwords for a large number of email accounts by operating and monitoring Tor exit nodes. 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 which does not use end-to-end encryption such as 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. 
Nonetheless, 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.
Researchers from INRIA showed that Tor dissimulation technique in BitTorrent can be bypassed.
In October 2011, a research team from ESIEA (a French engineering school) 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 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.
Legal aspects 
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". It has also been described by The Economist, in relation to Bitcoin and the Silk Road, as being "a dark corner of the web." Anonymizing systems such as Tor are at times 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, 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, the selling of controlled substances, money laundering, credit card fraud and identity theft; the black market which utilizes the Tor infrastructure operates, at least in part, in conjunction with Bitcoin, and Tor itself has been used by criminal enterprises, hacktivism groups (such as Anonymous), and law enforcement agencies at cross purposes, sometimes simultaneously.
- The main implementation of Tor is written in the C programming language and consists of roughly 146,000 lines of source code.
- Vuze (formerly Azureus), a BitTorrent client written in Java, includes built-in Tor support.
- Routers with built-in hardware support for Tor are currently under development through the Torouter project. The code is currently in the alpha phase of development, and runs on top of the OpenWrt platform.
- 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. The applications include: Gibberbot — a secure, no-logging, instant messaging client that uses OTR encryption; Orbot — a Tor implementation for Android; Orweb — a privacy-enhanced mobile browser; ProxyMob — a Firefox add-on that helps manage the HTTP, SOCKS, and SSL proxy settings for integration with Orbot; and Secure Smart Cam — a set of privacy enhancing tools that offers encryption of stored images, face detection and blurring, and secure remote sync of media over slow networks.
- The TAILS Linux Distribution includes Tor as part of its web browser.
See also 
- Anonymous P2P
- Anonymous remailer
- Arm (software)
- Free Haven Project
- Freedom of information
- The Hidden Wiki
- Incognito (operating system)
- Internet censorship
- Internet privacy
- OpenNet Initiative
- Portable Tor
- Proxy server
- The Silk Road (anonymous marketplace)
- Vidalia project
- xB Machine
- XeroBank Browser
- "Tor Project: Core People". The Tor Project. Retrieved 17 July 2008.
- Dingledine, Roger (20 September 2002). "pre-alpha: run an onion proxy now!". or-dev mailing list. http://archives.seul.org/or/dev/Sep-2002/msg00019.html. Retrieved 17 July 2008.
- ChangeLog in Tor Project Git repository
- Jordi Domingo-Pascual; Yuval Shavitt; Steve Uhlig (14 June 2011). Traffic Monitoring and Analysis: Third International Workshop, TMA 2011, Vienna, Austria, April 27, 2011, Proceedings. Springer. pp. 113–. ISBN 978-3-642-20304-6. Retrieved 6 August 2012.
- Glater, Jonathan D. (25 January 2006). "Privacy for People Who Don't Show Their Navels". The New York Times. Retrieved 13 May 2011.
- The Tor Project. "Tor: anonymity online". Retrieved 9 January 2011.
- Dingledine, Roger; Mathewson, Nick; Syverson, Paul (13 August 2004). "Tor: The Second-Generation Onion Router". Proc. 13th USENIX Security Symposium. San Diego, California. http://www.usenix.org/events/sec04/tech/dingledine.html. Retrieved 17 November 2008.
- "Tor FAQ: Why is it called Tor?". The Tor Project. Retrieved 1 July 2011.
- "Tor: Sponsors". The Tor Project. Retrieved 11 December 2010.
- "Make a Donation". The Tor Project. Retrieved 31 May 2011.
- Privacy software, criminal use – Page 2 – Boston.com. Articles.boston.com (8 March 2012). Retrieved 20 April 2012.
- Fowler, Geoffrey A. (2012-12-17). "Tor: An Anonymous, And Controversial, Way to Web-Surf - WSJ.com". Online.wsj.com. Retrieved 2013-05-19.
- The Tor Project, Inc. (2013-04-03). "Donate to Tor". Tor Project. Retrieved 2013-05-19.
- "2010 Free Software Awards announced". Retrieved 23 March 2011.
- "The FP Top 100 Global Thinkers". Foreign Policy. 26 November 2012. Archived from the original on 28 November 2012. Retrieved 28 November 2012.
- "Interview uncut: Jacob Appelbaum".
- "Tor: Bridges". The Tor Project. Retrieved 9 January 2011.
- "TheOnionRouter/TorFAQ". The Tor Project. Retrieved 28 December 2010.
- "Ubuntuusers.de – Tor einrichten und verwenden.". Retrieved 28 December 2010.
- "Tor Changelog". Retrieved 11 September 2007.
- "TheOnionRouter/TorifyHOWTO – Noreply Wiki". Retrieved 19 April 2007.
- "RFC 1928 – SOCKS Proxy Protocol, Version 5". Retrieved 4 August 2008.
- Øverlier, Lasse; Paul Syverson (21 June 2006). "Locating Hidden Servers" (PDF). Proceedings of the 2006 IEEE Symposium on Security and Privacy. IEEE Symposium on Security and Privacy. Oakland, CA: IEEE CS Press. p. 1. doi:10.1109/SP.2006.24. ISBN 0-7695-2574-1. http://www.onion-router.net/Publications/locating-hidden-servers.pdf. Retrieved 8 June 2008.
- The Tor Project, Inc. "Tor: Hidden Service Protocol, Hidden services". Torproject.org. Retrieved 9 January 2011.
- "One cell is enough to break Tor's anonymity". Tor website. 18 February 2009. Retrieved 9 January 2011.
- "TheOnionRouter/TorFAQ". Retrieved 18 September 2007. "Tor (like all current practical low-latency anonymity designs) fails when the attacker can see both ends of the communications channel"
- "Low-Cost Traffic Analysis of Tor" (PDF). 19 January 2006. Retrieved 21 May 2007.
- Le Blond, Stevens; Manils, Pere; Chaabane, Abdelberi; Ali Kaafar, Mohamed; Castelluccia, Claude; Legout, Arnaud; Dabbous, Walid; Inria (March 2011). "One Bad Apple Spoils the Bunch: Exploiting P2P Applications to Trace and Profile Tor Users" (PDF). National Institute for Research in Computer Science and Control. Retrieved 13 April 2011.
- McCoy, Damon; Bauer, Kevin; Grunwald, Dirk; Kohno, Tadayoshi; Sicker, Douglas (2008). "Shining Light in Dark Places: Understanding the Tor Network" (PDF). Proceedings of the 8th International Symposium on Privacy Enhancing Technologies (Berlin: Springer-Verlag). pp. 63–76. doi:10.1007/978-3-540-70630-4_5. ISBN 978-3-540-70629-8.
- Zetter, Kim (10 September 2007). "Rogue Nodes Turn Tor Anonymizer Into Eavesdropper's Paradise". Wired. Retrieved 16 September 2007.
- "Tor hack proposed to catch criminals". Retrieved 1 February 2008.
- Herrmann, Dominik; Wendolsky, Rolf; Federrath, Hannes (13 November 2009). "Website Fingerprinting: Attacking Popular Privacy Enhancing Technologies with the Multinomial Naïve-Bayes Classifier". Proceedings of the 2009 ACM Cloud Computing Security Workshop (CCSW). New York: Association for Computing Machinery (ACM). Retrieved 2 September 2010.
- Pere Manils, Abdelberi Chaabane, Stevens Le Blond, Mohamed Ali Kaafar, Claude Castelluccia, Arnaud Legout, Walid Dabbous (April 2010). Compromising Tor Anonymity Exploiting P2P Information Leakage.
- "Tor anonymizing network Compromised by French researchers". The Hacker News. Thehackernews.com. 24 October 2011. Retrieved 10 December 2011.
- "Announcement on 01net.com". Retrieved 17 October 2011.(French)
- Rumors of Tor's compromise are greatly exaggerated | The Tor Blog. Blog.torproject.org. Retrieved 20 April 2012.
- Soghoian, Chris (16 September 2007). "Tor anonymity server admin arrested". CNET News. Retrieved 17 January 2011.
- "Bitcoin: Monetarists Anonymous". The Economist. 2012-09-29. Retrieved 2013-05-19.
- Cochrane, Nate (2 February 2011). "Egyptians turn to Tor to organise dissent online". Scmagazine.com.au. Retrieved 10 December 2011.
- Cleaning up Tor on broadband.com
- Jones, Robert (2005). Internet forensics. O'Reilly. pp. P133. ISBN 0-596-10006-X.
- 'Dark Net' Kiddie Porn Website Stymies FBI Investigation. Gawker.com (11 June 2012). Retrieved 6 August 2012.
- The Underground Website Where You Can Buy Any Drug Imaginable. Gawker.com (1 June 2011). Retrieved 20 April 2012.
- Goodin, Dan. (16 April 2012) Feds shutter online narcotics store that used TOR to hide its tracks. Arstechnica.com. Retrieved 20 April 2012.
- How online black markets work – CSO Online – Security and Risk. CSO Online (30 April 2012). Retrieved 6 August 2012.
- How online black markets work – CSO Online – Security and Risk. CSO Online (30 April 2012). Retrieved 6 August 2012.
- Hunting for child porn, FBI stymied by Tor undernet. Muckrock (8 June 2012). Retrieved 6 August 2012.
- Luckey, Robin. "Tor". Ohloh. Retrieved 16 April 2010.
- "Tor". Vuze. Retrieved 3 March 2010.
- Simonite, Tom (22 December 2010). "Home Internet with Anonymity Built In". Technology Review. Retrieved 14 May 2011.
- "Torouter Project". Retrieved 10 May 2011.
- "About". The Guardian Project. Retrieved 10 May 2011.
- "Gibberbot: Secure Instant Messaging". The Guardian Project. Retrieved 10 May 2011.
- "Orbot: Mobile Anonymity + Circumvention". The Guardian Project. Retrieved 10 May 2011.
- "Orweb: Privacy Browser". The Guardian Project. Retrieved 10 May 2011.
- "ProxyMob: Firefox Mobile Add-on". The Guardian Project. Retrieved 10 May 2011.
- "Obscura: Secure Smart Camera". The Guardian Project. Retrieved 10 May 2011.
Further reading 
- AnonWatch, Tor in Depth Security Analysis of the Tor Network
- Goodin, Dan "Tor at heart of embassy passwords leak", 10 September 2007 article on The Register news website.. Retrieved 20 September 2007.
- Krebs, Brian, Attacks Prompt Update for 'Tor' Anonymity Network 8 August 2007. Retrieved 27 October 2007
- "The Hack of the Year". The Sydney Morning Herald. 13 November 2007. Retrieved 16 November 2007.
- Zhelatin.IR (= Storm Worm) 7 September 2007. Retrieved 27 October 2007 (French)
- Make Tor go the whole hog Hacker Magazine article
|Wikimedia Commons has media related to: Tor project|
- Official website
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- Core Onion — Introductory point to Tor services (Tor is required to access this site)