Tor, short for The Onion Router, is free and open-source software for enabling anonymous communication. It directs Internet traffic through a free, worldwide, volunteer overlay network, consisting of more than six thousand relays, for concealing a user's location and usage from anyone conducting network surveillance or traffic analysis. Using Tor makes it more difficult to trace the Internet activity to the user. Tor's intended use is to protect the personal privacy of its users, as well as their freedom and ability to conduct confidential communication by keeping their Internet activities unmonitored.
The core principle of Tor, 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 U.S. intelligence communications online. Onion routing is implemented by 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 later simply became "Tor", as an acronym for the former name), 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 of The Tor Project 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 attacks and weaknesses have been discovered and occasionally used. Attacks against Tor are an active area of academic research which is welcomed by the Tor Project itself.
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