Tor, which stands for The Onion Router, is free and open-source software that allows for anonymous communication. It routes Internet traffic over a free, global, volunteer overlay network of over 6,000 relays to mask a user's location and usage from network monitoring or traffic analysis. Tor makes tracing a user's Internet activities more difficult. Tor's intended use is to preserve its users' personal privacy, as well as their freedom and capacity to communicate discreetly, by employing Tor exit nodes to mask their IP addresses.
Tor's main premise, onion routing, was created in the mid-1990s by workers of the United States Naval Research Laboratory, mathematician Paul Syverson, and computer scientists Michael G. Reed and David Goldschlag, to safeguard American intelligence communications online. Onion routing is accomplished via encryption in the communication protocol stack's application layer, which is layered like the layers of an onion. Tor's initial version was released on September 20, 2002, by Syverson and computer scientists Roger Dingledine and Nick Mathewson under the name The Onion Routing project (later shortened to "Tor" as an abbreviation for the previous name). A year later, the first public release took place.
Tor's source code was provided under a free license by the Naval Research Laboratory in 2004, and the Electronic Frontier Foundation (EFF) began financing Dingledine and Mathewson to continue its development. Dingledine, Mathewson, and five others created The Tor Project in 2006, a 501(c)(3) research-education nonprofit corporation located in Massachusetts that is responsible for Tor's upkeep. Early funding backers of The Tor Project included the US Bureau of Democracy, Human Rights, and Labor, Internews, Human Rights Watch, the University of Cambridge, Google, and the Netherlands-based Stichting NLnet.
Various Tor exploits and vulnerabilities have been found and used during the course of its existence. Attacks against Tor are an active topic of academic study that the Tor Project encourages.
Tor allows its users to access the Internet, communicate, and send instant messages anonymously, and it is used for both licit and illegal reasons. Tor has, for example, been used at cross purposes by criminal enterprises, hacktivism groups, and law enforcement agencies, sometimes concurrently; likewise, agencies within the US government both fund and seek to subvert Tor (the US State Department, the National Science Foundation, and – through the Broadcasting Board of Governors, which itself partially funded Tor until October 2012 – Radio Free Asia).
Tor is not intended to address the problem of online anonymity fully. Tor is not intended to entirely erase footprints, but rather to make it more difficult for sites to trace activities and data back to the user.
Tor is sometimes used for illicit purposes. These can include things like privacy protection or censorship evasion, as well as the spread of child abuse content, drug sales, or virus distribution. According to one estimate,[by whom?] "on average, on a country/day basis, 6.7 percent of Tor network users connect to Onion/Hidden Services that are disproportionately utilized for criminal activities."
Tor has been described as "a dark corner of the web" by The Economist in regard to Bitcoin and Silk Road. It has been targeted with little success by the American National Security Agency and the British GCHQ signals intelligence services, and with more success by the British National Crime Agency in Operation Notarise. GCHQ, on the other hand, has been employing a program called "Shadowcat" for "end-to-end encrypted access to VPS through SSH using the Tor network."
According to the Tor Project, Tor users include "regular individuals" who want to keep their Internet activity hidden from websites and marketers, persons concerned about cyber-spying, and users who want to avoid censorship, such as activists, journalists, and military personnel. Tor has around four million users as of November 2013. According to the Wall Street Journal, around 14 percent of Tor's traffic originated in the United States in 2012, with those in "Internet-censoring nations" constituting its second-largest user base. Tor is increasingly being utilized by victims of domestic abuse, as well as the social workers and agencies who aid them, despite the fact that shelter employees may or may not have had professional cyber-security training.
According to a briefing issued by the Parliamentary Office of Science and Technology in March 2015, "there is widespread agreement that banning online anonymity systems entirely is not seen as an acceptable policy option in the United Kingdom," and that "even if it were, there would be technical challenges." The report also stated that Tor "plays only a minor role in the online viewing and distribution of indecent images of children" (due in part to its inherent latency); it was praised for its use by the Internet Watch Foundation, the utility of its onion services for whistleblowers, and its circumvention of China's Great Firewall.
Tor separates identity and routing in order to conceal its users' identities and online behavior from monitoring and traffic analysis. It is an onion routing system that encrypts and then randomly bounces messages around a network of relays maintained by volunteers all over the world. These onion routers use multi-layered encryption (thus the onion metaphor) to provide complete forward secrecy between relays, giving users anonymity in a network location.
Tor's anonymous onion service capability allows for the hosting of censorship-resistant material. Furthermore, by concealing some of the entrance relays (bridge relays), users can avoid Internet censorship based on the banning of public Tor relays.
Because the sender's and recipient's IP addresses are not both in cleartext at any point along the journey, anyone eavesdropping at any point along the communication channel cannot immediately identify both ends. Furthermore, to the recipient, the final Tor node (called the exit node) seems to be the originator of the message, rather than the sender.
SOCKS-aware apps can be set to route network traffic over the SOCKS interface of a Tor instance, which is listening on TCP port 9050 (for standalone Tor) or 9150 (for Tor Browser bundle) at localhost. Tor builds virtual circuits via the Tor network on a regular basis in order to multiplex and onion-route traffic to its destination. Once within a Tor network, data is routed from router to router throughout the circuit until it reaches an exit node, where the cleartext packet is accessible and delivered to its original destination. The communication seems to originate from the Tor exit node when viewed from the destination.
Tor differs from most other anonymity networks in that it operates at the Transmission Control Protocol (TCP) stream level. Internet Relay Chat (IRC), instant messaging, and World Wide Web surfing are examples of applications whose traffic is widely anonymized using Tor.
Tor may also be used to anonymize webpages and other servers. Onion services are servers that are set to only accept inbound connections via Tor (formerly, hidden services). An onion service is accessible by an onion address, rather than disclosing a server's IP address (and consequently its network location). The Tor network recognizes these addresses by seeking up their accompanying public keys and introduction points in the network's distributed hash table. It can transport data to and from onion services, even those behind firewalls or network address translators (NAT), while maintaining both parties' anonymity. Tor is required to use these onion services.
The attack against Tor traffic analysis
Passive and aggressive traffic-analysis attacks are available. The attacker uses the passive traffic-analysis approach to extract characteristics from the traffic of a certain flow on one side of the network and looks for those features on the other side of the network. In the active traffic-analysis method, the attacker modifies the timings of packets in a flow according to a specific pattern and searches for that pattern on the other side of the network; thus, the attacker can link flows on one side of the network to flows on the other side of the network, breaking the anonymity. Although temporal noise is introduced into the packets, it is demonstrated that there are active traffic analysis algorithms that are resistant to such noise.
Passive and aggressive traffic-analysis attacks are available. The attacker uses the passive traffic-analysis approach to extract characteristics from the traffic of a certain flow on one side of the network and looks for those features on the other side of the network. In the active traffic-analysis method, the attacker modifies the timings of packets in a flow according to a specific pattern and searches for that pattern on the other side of the network; thus, the attacker can link flows on one side of the network to flows on the other side of the network, breaking the anonymity. Although temporal noise is introduced into the packets, it is demonstrated that there are active traffic analysis algorithms that are resistant to such noise.
At the 2005 IEEE Symposium on security and privacy, Steven Murdoch and George Danezis from the University of Cambridge presented an article 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 tactics significantly weaken Tor's anonymity. Murdoch and Danezis have also demonstrated that seemingly unrelated streams may be traced back to the same originator. However, this attack does not expose the identity of the original user. Since 2006, Murdoch has collaborated with and been financed by Tor.
Internet site operators can block traffic from Tor exit nodes or give limited functionality to Tor users. For example, while using Tor or an IP address that is also used by a Tor exit node, it is often not feasible to edit Wikipedia. The BBC's iPlayer service restricts the IP addresses of all known Tor guards and exit nodes, but not relays and bridges.
The Tor Browser is the Tor Project's main product. Steven J. Murdoch designed it as the Tor Browser Bundle and launched it in January 2008. Tor Browser is made out of a customized version of Mozilla Firefox ESR, the TorButton, TorLauncher, NoScript, and HTTPS Everywhere Firefox extensions, and the Tor proxy. Tor Browser may be launched from removable media. It is compatible with Microsoft Windows, macOS, Android, Linux, and IOS.
DuckDuckGo is the default search engine (to version 4.5, Startpage.com was its default). The Tor Browser runs Tor background processes and directs traffic over the Tor network automatically. When a session ends, the browser deletes privacy-sensitive data such as HTTP cookies and browsing history. This reduces online tracking and canvas fingerprinting while also preventing the formation of a filter bubble.
To facilitate downloads from locations where visiting the Tor Project URL may be problematic or banned, a GitHub repository containing links to releases stored on other sites is maintained.
The Tor Project published Tor Messenger Beta on October 29, 2015, an instant messaging software based on Instantbird that includes and uses Tor and OTR by default. Tor Messenger, like Pidgin and Adium, supports various instant messaging protocols; however, it does so without relying on libpurple, instead implementing all chat protocols in the memory-safe language JavaScript.
According to Toms Hardware's Lucian Armasu, the Tor Project shut down the Tor Messenger project in April 2018 for three reasons: "Instabird" (sic) developers withdrew support for their own software, insufficient resources, and known metadata difficulties. The Tor Messenger developers claimed that resolving any future vulnerabilities would be hard owing to the project's reliance on obsolete software dependencies.
Tor is supported by the Vuze (previously Azureus) BitTorrent client, the Bitmessage anonymous messaging system, and the TorChat instant messenger. OnionShare enables people to transfer files through Tor.
To increase the security of mobile communications, the Guardian Project is actively creating a free and open-source suite of applications and firmware for the Android operating system. ChatSecure instant messaging client, Orbot Tor implementation, Orweb (discontinued) privacy-enhanced mobile browser, Orfox, the mobile equivalent of the Tor Browser, ProxyMob Firefox add-on, and ObscuraCam are among the programs.
Onion Browser is an open-source, privacy-enhancing iOS web browser that makes use of Tor. The iOS App Store has it, and the source code is accessible on GitHub.
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