Tor (anonymity network)

Tor

Developer(s)	The Tor Project[1]
Initial release	September 20, 2002 (2002-09-20)[2]
Stable release	0.2.1.29  (January 15, 2011; 23 days ago (2011-01-15) [3]) [+/−]
Preview release	0.2.2.22-alpha  (January 25, 2011; 13 days ago (2011-01-25) [4]) [+/−]
Written in	C
Operating system	Cross-platform
Type	Onion routing, Anonymity
License	BSD license
Website	www.torproject.org

Tor is a system intended to enable online anonymity, composed of client software and a network of servers which can hide information about users' locations and other factors which might identify them. Use of this system makes it more difficult to trace internet traffic to the user, including visits to Web sites, online posts, instant messages, and other communication forms.^[5] It is intended to protect users' personal freedom, privacy, and ability to conduct confidential business, by keeping their internet activities from being monitored.^[6] The software is open-source and the network is free of charge to use.

Though 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.^[7]

Tor is an implementation of onion routing, and works by relaying communications through a network of systems run by volunteers in various locations. Because the internet address of the sender and the recipient are not both readable at any step along the way (and in intermediate links in the chain, neither piece of information is readable), someone engaging in network traffic analysis and surveillance at any point along the line cannot directly identify which end system is communicating with which other. Furthermore, the recipient knows only the address of the last intermediate machine, not the sender. By keeping some of the network entry points hidden, Tor is also able to evade many internet censorship systems, even ones specifically targeting Tor.^[8]

Contents 1 History 2 Operation 2.1 Originating traffic 2.2 Hidden services 3 Weaknesses 4 Legal aspects 5 Implementation 6 See also 7 Footnotes 8 References 9 Further reading 10 External links

[edit] History

An alpha version of the software, with the onion routing network "functional and deployed", was announced on 20 September 2002.^[2] Roger Dingledine, Nick Mathewson, and Paul Syverson presented "Tor: The Second-Generation Onion Router" at the 13th USENIX Security Symposium on Friday, August 13, 2004.^[9]

Originally sponsored by the US Naval Research Laboratory,^[9] Tor was financially supported by the Electronic Frontier Foundation from 2004 to 2005.^[10] Tor software is now developed by the Tor Project, which since December 2006 is a 501(c)(3) research/education non-profit organization based in the United States of America that receives a diverse base of financial support.^[1][10][11]

[edit] Operation

Tor aims to conceal its users' identity and their network activity from surveillance and traffic analysis. Volunteers operate an overlay network of onion routers that employ encryption in a multi-layered manner (hence the onion routing metaphor) to ensure perfect forward secrecy between routers, thereby providing users with anonymity in network location. That anonymity extends to the operation of censorship-resistant servers via Tor's anonymous hidden service feature.^[9]

[edit] 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. The Polipo proxy server can speak the SOCKS 4 & SOCKS 5 protocols and therefore is recommended to be used together with the TOR anonymising network. Polipo is a web proxy that does HTTP 1.1 pipelining well, so it can enhance TOR's communication latency.^[12][13]

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. Polipo can speak the SOCKS protocol and does HTTP 1.1 pipelining for enhancing latencies, therefore is now recommended to be used together with the TOR anonymising network by the torproject.org.^[14][15]

On older versions of Tor (resolved May–July 2010),^[16] 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.^[17] 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.^[18]

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.^[16]

[edit] Hidden services

Tor can also provide anonymity to servers in the form of location-hidden services, which are Tor clients or relays running specially configured server software. Rather than revealing the server's IP address (and therefore its network location), hidden services are accessed through Tor-specific .onion pseudo top-level domain (TLD), or pseudomain. The Tor network understands this TLD and routes data anonymously both to and from the hidden service. Due to this lack of reliance on a public address, hidden services may be hosted behind firewalls or network address translators (NAT). A Tor client is necessary in order to access a hidden service.^[19]

Hidden services have been deployed on the Tor network beginning in 2004.^[20] Other than the database that stores the hidden-service descriptors,^[21] Tor is decentralized by design; there is no direct readable list of hidden services. There are a number of independent hidden services that serve this purpose.

Because location-hidden services do not use exit nodes, they are not subject to exit node eavesdropping. There are, however, a number of 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),^[19] uptime and downtime statistics, intersection attacks and user error.

[edit] Weaknesses

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).^[22][23]

Steven J. Murdoch and George Danezis from University of Cambridge presented an article^[24] 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.^[24] Murdoch has been working with, and funded by, Tor since 2006.

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.^[25] 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 or may not inherently violate the anonymity of the source if users mistake Tor's anonymity for end-to-end encryption they may be subject to additional risk of data interception by self-selected third parties.^[26] (The operator of any network carrying unencrypted traffic, such as the operator of a wifi hotspot or corporate network, has the same ability to intercept traffic as a Tor exit operator. End-to-end encrypted connections should be used if such interception is a concern.) Even without end-to-end encryption, Tor provides confidentiality against these local observers which may be more likely to have interest in the traffic of users on their network than arbitrary Tor exit operators.

Nonetheless, Tor and the alternative network system JonDonym (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.^[27]

Researchers from INRIA showed that Tor dissimulation technique in Bittorrent can be bypassed.^[28]

[edit] Legal aspects

According to CNet, Tors 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".^[29] Anonymizing systems such as Tor are at times used for matters that are, or may be, illegal in some countries. These include access to censored information, but also at times circumvention of laws against the criticism of heads of state, anonymous defamation, unauthorized leaks of sensitive information, distributing copyrighted works, or illegal sexual content.^[30][31]

[edit] Implementation

• The main implementation of Tor is written in the C programming language and consists of roughly 146,000 lines of source code.^[32]
• Vuze (formerly Azureus), a BitTorrent client written in Java, includes built-in Tor support.^[33]

[edit] See also

[edit] Footnotes

[edit] References

[edit] Further reading

• Black OP Security Wiki, ToR Summary of Tor Circuit Types and Vulnerabilities
• AnonWatch, Tor in Depth Security Analysis of the Tor Network
• Goodin, Dan "Tor at heart of embassy passwords leak", 2007-09-10 article on The Register news website. (Accessed 2007-09-20).
• Krebs, Brian, Attacks Prompt Update for 'Tor' Anonymity Network 2007-08-08 (Accessed 2007-10-27)
• "The Hack of the Year". The Sydney Morning Herald. 2007-11-13. http://www.smh.com.au/news/security/the-hack-of-the-year/2007/11/12/1194766589522.html?page=fullpage#contentSwap1. Retrieved 2007-11-16. 
• Zhelatin.IR (= Storm Worm) 2007-09-07 (Accessed 2007-10-27) (French)
• Make Tor go the whole hog Hacker Magazine article

[edit] External links

Wikinews has related news: German police seize Tor anonymity servers

• Official website
• list of support programs for TOR
• Tor's LiveCD FAQ
• Core Onion — Introductory point to Tor services (Tor is required to access this site)