| Developer(s) | The Tor Project[1] |
|---|---|
| Initial release | 20 September 2002[2] |
| Stable release | 0.2.2.35 (16 December 2011) [±] |
| Preview release | 0.2.3.10-alpha (16 December 2011[3][4]) [±] |
| Written in | C |
| Operating system | Cross-platform |
| Type | Onion routing, Anonymity |
| License | BSD license |
| Website | www.torproject.org |
Tor (short for The onion router) is a system intended to enable online anonymity. Tor client software routes Internet traffic through a worldwide volunteer network of servers in order 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[5] and is intended to protect users' personal freedom, privacy, and ability to conduct confidential business by keeping their internet activities from being monitored.
"Onion routing" refers to the layered nature of the encryption service: The original data are encrypted and re-encrypted multiple times, then sent through successive Tor relays, each one of which decrypts a "layer" of encryption before passing the data on to the next relay and, ultimately, its destination. This reduces the possibility of the original data being unscrambled or understood in transit.[6]
The Tor client is free software and usage of the Tor network is free of charge.
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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 13 August 2004.[7] 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.[8]
Originally sponsored by the US Naval Research Laboratory,[7] Tor was financially supported by the Electronic Frontier Foundation from 2004 to 2005.[9] Tor software is now developed by the Tor Project, which has been a 501(c)(3) research/education nonprofit organization[10] based in the United States of America[1] since December 2006 and receives a diverse base of financial support.[9]
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."[11]
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 throughout 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.[7] Furthermore, by keeping some of the entry relays (bridge relays) secret, users can evade Internet censorship which relies upon blocking public Tor relays.[12]
Because the internet address of the sender and the recipient are not both in cleartext at any hop along the way at non-exit (or "middle") relays neither piece of information is in cleartext, such that 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.
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.[13][14]
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.[13][14]
On older versions of Tor (resolved May–July 2010),[15] 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.[16] 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.[17]
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.[15]
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.[18]
Hidden services have been deployed on the Tor network beginning in 2004.[19] Other than the database that stores the hidden-service descriptors,[20] 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),[18] 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).[21][22]
Steven J. Murdoch and George Danezis from University of Cambridge presented an article[23] 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.[23] 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 usage 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.[24] 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 usage 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.)[24]
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.[24] BitTorrent may generate as much as 40% of all traffic on Tor,[25] 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.[24]
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.[26] 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.[27] (The operator of any network carrying unencrypted traffic, such as the operator of a wi-fi 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 (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.[28]
Researchers from INRIA showed that Tor dissimulation technique in Bittorrent can be bypassed.[29]
In October 2011, a research team from the French engineering school Esiea claimed to have discovered a way to compromise the Tor network by decrypting communication passing over it.[30][31] 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 both Denial-of-service attack and packet spinning methods. No technical analysis has been made available yet for the public or the Tor developers to analyze. Eric Filiol and his team intend to release this information at the upcoming PacSec and Hackers to Hackers conferences.[32]
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".[33] Anonymizing systems such as Tor are at times used for matters that are, or may be, illegal in some countries. For instance, Tor may be used to gain access to censored information, to organize political activities,[34] or to circumvent laws against criticism of heads of state. At the same time, Tor also allows anonymous defamation, unauthorized leaks of sensitive information, distributing copyrighted works, or illegal sexual content.[35][36]