Tor

(anonymity network)

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Tor (The Onion Router) is a free software implementation of second-generation onion routing – a system enabling its users to communicate anonymously on the Internet. Originally sponsored by the US Naval Research Laboratory, Tor became an Electronic Frontier Foundation (EFF) project in late 2004. The EFF supported Tor financially until November 2005,^[1] and continues to provide web hosting for the project.

Like all current low latency anonymity networks, Tor is vulnerable to traffic analysis from observers who can watch both ends of a user's connection^[2].

Overview

Aiming to protect its users against traffic analysis attacks, Tor operates an overlay network of "onion routers" that enable anonymous outgoing connections and anonymous "hidden" services.

Anonymous outgoing connections

Users of the Tor network run an onion proxy on their machine. This software connects out to Tor, periodically negotiating a virtual circuit through the Tor network. Tor employs cryptography in a layered manner (hence the onion analogy), ensuring perfect forward secrecy between routers. 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 the 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 TCP stream level. Applications commonly anonymised using Tor include IRC, instant messaging and browsing the Web. When browsing the Web, Tor is often coupled with Privoxy – a filtering proxy server – that aims to add privacy at the application layer.

Weaknesses

DNS leaks

As with many anonymous web surfing systems, direct DNS requests are usually still performed by many applications, without using the Tor proxy. Solutions such as the previously mentioned Privoxy or using the command "torify" included with the Tor distribution are possible solutions to this problem.^[3] 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 Tor hidden service (.onion) access to non-SOCKS aware applications. ^[4]

Traffic analysis

Steven J. Murdoch and George Danezis from University of Cambridge presented an article,^[5] in the 2005 IEEE Symposium on Security and Privacy, Oakland, California, USA, May 8–11, 2005. They presented 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 and therefore greatly reduce the anonymity provided by Tor. They have also shown that otherwise unrelated streams can be linked back to the same initiator. There have so far been no responses by the development team to this report, which on the face of it seems to destroy the whole aim of Tor.

Eavesdropping by exit nodes

In September 2007, 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.^[6] As Tor does not and by design cannot itself encrypt the traffic between an exit node and the target server, any exit node is in a position to capture any traffic which is not encrypted at the application layer, e.g. by SSL. While this does not inherently violate the anonymity of the source, it affords added opportunities for data interception by self-selected third parties, greatly increasing the risk of exposure of sensitive data by users who are careless or who mistake Tor's anonymity for security.

Etiquette and abuse

Because Tor is capable of anonymising arbitrary TCP traffic, it attracts its fair share of abuse. Routers maintain an exit policy of what traffic is and is not permitted to leave the Tor network through that node. It is possible to combat most major abuses of the Tor network using a combination of addresses and ports. Potential abuses include:

Bandwidth hogging: It is considered impolite to transfer massive amounts of data across the Tor network – the onion routers are run by volunteers using their own bandwidth at their own cost.
E-mail: Anonymous usage of SMTP (i.e., outgoing e-mail) can result in spam. Consequently the default exit policy of Tor nodes rejects outgoing connections to port 25, the port most frequently used for SMTP.

Anonymous hidden services

Although Tor's most popular feature is its provision of anonymity to clients, it can also provide anonymity to servers. By using the Tor network, it is possible to host servers in such a way that their network location is unknown. In order to access a hidden service, Tor must also be used by the client.

Hidden services are accessed through the Tor-specific .onion pseudo top-level domain. The Tor network understands this TLD and routes data anonymously to the hidden service. The hidden service then hands over to standard server software, which should be configured to listen only on non-public interfaces. Services that are reachable through Tor hidden services and the public Internet are susceptible to correlation attacks, and consequently are not really hidden.

An added advantage of Tor hidden services is that, because no public IP address is required, services may be hosted behind firewalls and NAT.

Central sites

Note that due to the nature of the network, access to links are dependent on the nodes you are connected to as well as those your tor software queries during each access attempt. As a result, it may take several tries to access these sites. Also, due to the anonymous nature of this network, sensitive material, which may be illegal in one's jurisdiction or simply distasteful, is posted more commonly than on the general Internet.

core.onion, a simple site containing links to other .onion sites, a simple forum and a comment system.
The Hidden Wiki Reloaded, a site with a wiki interface with a wide variety of subjects.
The Hidden Wiki, originally the most popular index site, it went offline in early June 2007. No particular reason was given for this service being stopped.

References

^ Tor:People (2006-10-15). Retrieved on 2007-05-21.
^ TheOnionRouter/TorFAQ. Retrieved on 2007-09-18. “Tor (like all current practical low-latency anonymity designs) fails when the attacker can see both ends of the communications channel”
^ TheOnionRouter/TorifyHOWTO - Noreply Wiki. Retrieved on 2007-04-19.
^ Tor Changelog. Retrieved on 2007-09-11.
^ Low-Cost Traffic Analysis of Tor (PDF) (2006-01-19). Retrieved on 2007-05-21.
^ Wired: Rogue Nodes Turn Tor Anonymizer Into Eavesdropper's Paradise. Retrieved on 2007-09-16.

Anonymity Bibliography Retrieved: 21 May 2007
Schneier, Bruce. Applied Cryptography. ISBN 0-471-11709-9.
Schneier, Bruce. Email Security. ISBN 0-471-05318-X.
Bacard, Andre. Computer Privacy Handbook. ISBN 1-56609-171-3.

External links

Wikinews has related news:

German police seize Tor anonymity servers

Articles

http://blog.washingtonpost.com/securityfix/2007/08/attacks_prompt_update_for_tor.html?nav=rss_blog
(French) http://www.secuser.com/alertes/2007/storm-worm-zhelatinir.htm
Dan Goodin, "Tor at heart of embassy passwords leak", 10 September 2007 article on The Register news website. (Accessed 20 September 2007).

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Tor