Tor (anonymity network)
Tor (previously an acronym for The Onion Router)[5] is free software network for enabling online anonymity. Tor directs Internet traffic through a free, worldwide, volunteer network consisting of more than four 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 to trace Internet activity, including "visits to Web sites, online posts, instant messages, and other communication forms", back to the user[7] and is intended to protect the personal privacy of users, as well as their freedom and ability to conduct confidential business by keeping their internet activities from being monitored.
The term "onion routing" refers to the nested layers of encryption used to anonymize and relay 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 last 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.[8]
As the 2013 anonymity-stripping attacks EgotisticalGiraffe[9] on Freedom Hosting users demonstrated, it is possible to attack Tor users indirectly, e.g., via vulnerabilities in servers and web browsers.[10] However, an NSA appraisal released by The Guardian in October of that year characterized Tor as "[s]till the King of high secure, low latency Internet anonymity" and that "[t]here are no contenders for the throne in waiting".[11]
History
An alpha version of the free 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 thirteenth USENIX Security Symposium on 13 August 2004.[12] 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.[13]
Originally sponsored by the U.S. Naval Research Laboratory,[12] 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 [15] based in the United States of America [1] 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.[16] 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,[17] including NGOs and thousands of individual sponsors.[18]
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."[19]
Foreign Policy named Dingledine, Mathewson, and Syverson among its 2012 Top 100 Global Thinkers "for making the web safe for whistleblowers."[20]
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.".[21]
Edward Snowden used the Tor Network to send information about PRISM to the Washington Post and The Guardian in June 2013.[22]
Operation
Alice's Tor client picks a random path to destination server
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.[12] Furthermore, by keeping some of the entry relays (bridge relays) secret, users can evade Internet censorship that relies upon blocking public Tor relays.[23]
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.
A Tor non-exit relay with a maximum output of 239.69 KB/s
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 often is 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.[24]
On older versions of Tor (resolved May–July 2010),[25] 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.[26]
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.[27]
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.[25]
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.[28]
Hidden services have been deployed on the Tor network since 2004.[29] Other than the database that stores the hidden-service descriptors,[30] 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),[28] uptime and downtime statistics, intersection attacks, and user error.
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).[31][32]
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.[33]
Researchers from the University of Michigan developed a network scanner allowing identification of 86 percent of live Tor “bridges” with a single scan.[34]
Bad Apple attack
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.[35] 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.[35] Murdoch has been working with—and has been funded by—Tor since 2006.
There is an attack on Tor where, if an Autonomous System (AS) exists on both path from Alice to entry relay and from exit relay to Bob, that AS is able to de-anonymize the path. In 2012, LASTor [36] proposed a method to avoid this attack. They also propose a path selection algorithm to reduce latency of communications in Tor.
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 table tracking.[37] 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.)[37]
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.[37] BitTorrent may generate as much as 40% of all traffic on Tor.[38] Furthermore, the bad apple attack is effective against insecure use of any application over Tor, not just BitTorrent.[37]
Exit nodes should not be trusted
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.[39] 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 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.[40] 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?" [41]
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.[42][43] 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.[44]
Some protocols leak IP addresses
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Researchers from French Institute for Research in Computer Science and Control (INRIA) showed that Tor dissimulation technique in BitTorrent can be bypassed.[45]
Site operators may block traffic from Tor exit nodes
Operators of Internet sites have the ability to prevent connections from Tor exit nodes, or to offer reduced functionality to Tor users. For example, it is not generally possible to edit Wikipedia 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.
Firefox / JavaScript anonymity attack on Freedom Hosting users
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, which was being exploited to send users' IP addresses and Windows computer names to the attackers.[46] News reports linked this to an FBI operation targeting Freedom Hosting's owner, Eric Eoin Marques. He 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.”[47][48] The FBI acknowledged the attack in a 12 September 2013 court filing in Dublin;[49] further technical details from a training presentation leaked by Edward Snowden showed that the codename for the exploit was EgotisticalGiraffe.[50]
Controversy over illegal activities
Tor has been described by The Economist, in relation to Bitcoin and the Silk Road, as being "a dark corner of the web."[51] It has been targeted by both the American NSA and the British GCHQ signals intelligence agencies, albeit with marginal success.[10] 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,[52] 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[53][54][55] (and has indeed been explicitly required by one such criminal enterprise),[56] the selling of controlled substances,[57] money laundering,[58] credit card fraud, and identity theft; furthermore, the black market which utilizes the Tor infrastructure operates, at least in part, in conjunction with Bitcoin.[59][60] Ironically, Tor has been used by criminal enterprises, hacktivism groups, and law enforcement agencies at cross purposes, sometimes simultaneously;[61][62] likewise, agencies within the U.S. government variously fund Tor (the U.S. State Department),[63] 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.[10]
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".[64][65] 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".[66] EFF's Surveillance Self-Defense guide includes a description of where Tor fits in a larger strategy for protecting privacy and anonymity.[67] 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.[68]
Implementations
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The main implementation of Tor is written in the C programming language and consists of roughly 146,000 lines of source code.[69]
- Tor Browser Bundle – The Tor Project's flagship product, Tor Browser Bundle includes a modified Mozilla Firefox ESR web browser pre-configured to protect anonymity without needing to install any software. It can be run from removable media and is available for Windows, Mac OS X, and Linux.[70]
- Vuze (formerly Azureus) – a BitTorrent client written in Java, includes built-in Tor support.[71]
- PAPARouter (Plug And Play Anonymity Router) – a router with built-in hardware support can anonymize several devices at once. Tor is implemented via Debian on top of the Raspberry Pi and is literally, plug and play. This Tor implementation also excludes all exit nodes in the United States, United Kingdom, Australia, New Zealand and British Commonwealth countries.[73]
- 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.[74] The applications include:
- Gibberbot – a secure, no-logging, instant messaging client that uses OTR encryption;[75]
- Orbot – a Tor implementation for Android;[76]
- Orweb – a privacy-enhanced mobile browser;[77]
- ProxyMob – a Firefox add-on that helps manage the HTTP, SOCKS, and SSL proxy settings for integration with Orbot;[78]
- 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.[79]
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