Tor (network)

Tor is a free overlay network for enabling anonymous communication. It is built on free and open-source software run by over seven thousand volunteer-operated relays worldwide, as well as by millions of users who route their Internet traffic via random paths through these relays. This technique is called onion routing.

Using Tor makes it more difficult to trace a user's Internet activity by preventing any single point on the Internet (other than the user's device) from being able to view both where traffic originated from and where it is ultimately going to at the same time. This conceals a user's location and usage from anyone performing network surveillance or traffic analysis from any such point, protecting the user's freedom and ability to communicate confidentially.

The core principle of Tor, known as onion routing, was developed in the mid-1990s by United States Naval Research Laboratory employees, mathematician Paul Syverson, and computer scientists Michael G. Reed and David Goldschlag, to protect American intelligence communications online. Onion routing is implemented by means of encryption in the application layer of the communication protocol stack, nested like the layers of an onion. The alpha version of Tor, developed by Syverson and computer scientists Roger Dingledine and Nick Mathewson and then called The Onion Routing project (which was later given the acronym "Tor"), was launched on 20 September 2002. The first public release occurred a year later.

In 2004, the Naval Research Laboratory released the code for Tor under a free license, and the Electronic Frontier Foundation (EFF) began funding Dingledine and Mathewson to continue its development. In 2006, Dingledine, Mathewson, and five others founded The Tor Project, a Massachusetts-based 501(c)(3) research-education nonprofit organization responsible for maintaining Tor. The EFF acted as The Tor Project's fiscal sponsor in its early years, and early financial supporters included the U.S. Bureau of Democracy, Human Rights, and Labor and International Broadcasting Bureau, Internews, Human Rights Watch, the University of Cambridge, Google, and Netherlands-based Stichting NLnet.

Over the course of its existence, various Tor vulnerabilities have been discovered and occasionally exploited. Attacks against Tor are an active area of academic research that is welcomed by The Tor Project itself.

Tor enables its users to surf the Internet, chat and send instant messages anonymously, and is used by a wide variety of people for both licit and illicit purposes. Tor has, for example, been used by criminal enterprises, hacktivism groups, and law enforcement agencies at cross purposes, sometimes simultaneously; likewise, agencies within the U.S. government variously fund Tor (the U.S. State Department, the National Science Foundation, and – through the Broadcasting Board of Governors, which itself partially funded Tor until October 2012 – Radio Free Asia) and seek to subvert it. Tor was one of a dozen circumvention tools evaluated by a Freedom House-funded report based on user experience from China in 2010, which include Ultrasurf, Hotspot Shield, and Freegate.

Tor is not meant to completely solve the issue of anonymity on the web. Tor is not designed to completely erase tracking but instead to reduce the likelihood for sites to trace actions and data back to the user.

Tor can also be used for illegal activities. These can include privacy protection or censorship circumvention, as well as distribution of child abuse content, drug sales, or malware distribution.