Tor is free software for enabling anonymous communication. The name is derived from an acronym for the original software project name "The Onion Router". Tor directs Internet traffic through a free, worldwide, volunteer overlay network consisting of more than seven thousand relays to conceal a user's location and usage from anyone conducting network surveillance or traffic analysis. Using Tor makes it more difficult for Internet activity to be traced back to the user: this includes "visits to Web sites, online posts, instant messages, and other communication forms". Tor's use is intended to protect the personal privacy of users, as well as their freedom and ability to conduct confidential communication by keeping their Internet activities from being monitored.
Tor does not prevent an online service from determining when it is being accessed through Tor. Tor protects a user's privacy, but does not hide the fact that Tor is being used. Some websites restrict what is allowed when using Tor. For example, Wikipedia limits the edits that can be made through Tor.
Onion routing is implemented by encryption in the application layer of a communication protocol stack, nested like the layers of an onion. Tor encrypts the data, including the next node destination IP address, multiple times and sends it through a virtual circuitcomprising 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 innermost layer of encryption and sends the original data to its destination without revealing, or even knowing, the source IP address. Because the routing of the communication is partly concealed at every hop in the Tor circuit, this method eliminates any single point at which the communicating peers can be determined through network surveillance that relies upon knowing its source and destination.
An adversary might try to de-anonymize the user by some means. One way this may be achieved is by exploiting vulnerable software on the user's computer. The NSA had a technique that targets a vulnerability – which they codenamed "EgotisticalGiraffe" – in an outdated Firefox browser version at one time bundled with the Tor package, and in general, targets Tor users for close monitoring under its XKeyscore program. Attacks against Tor are an active area of academic research, and are welcomed by the Tor Project itself. However, Tor was not only developed for a time in the early 2000s by individuals who "were on contract from DARPAand the U.S. Naval Research Laboratory", but since its inception the bulk of its funding has come from the federal government of the United States.
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, with the purpose of protecting U.S. intelligence communications online. Onion routing was further developed by DARPA in 1997.
The alpha version of Tor, developed by Syverson and computer scientists Roger Dingledine and Nick Mathewson and then called The Onion Routing project, or TOR project, launched on 20 September 2002. The first public release occurred a year later. On 13 August 2004, Syverson, Dingledine and Mathewson presented "Tor: The Second-Generation Onion Router" at the 13th USENIX Security Symposium. 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 December 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. International Broadcasting Bureau, Internews, Human Rights Watch, the University of Cambridge, Google, and Netherlands-based Stichting NLnet.
From this period onwards, the majority of funding sources came from the U.S. government.
In November 2014 there was speculation in the aftermath of Operation Onymous that a Tor weakness had been exploited. A representative of Europol was secretive about the method used, saying: "This is something we want to keep for ourselves. The way we do this, we can’t share with the whole world, because we want to do it again and again and again." A BBC source cited a "technical breakthrough" that allowed the tracking of the physical location of servers, and the number of sites that police initially claimed to have infiltrated led to speculation that a weakness in the Tor network had been exploited. This possibility was downplayed by Andrew Lewman, a representative of the not-for-profit Tor project, suggesting that execution of more traditional police work was more likely. However, in November 2015 court documents on the matter generated serious ethical security research as well as Fourth Amendment concerns.
In December 2015, The Tor Project announced that it had hired Shari Steele as its new executive director. Steele had previously led the Electronic Frontier Foundation for 15 years, and in 2004 spearheaded EFF's decision to fund Tor's early development. One of her key stated aims is to make Tor more user-friendly in order to bring wider access to anonymous web browsing.
In July 2016 the complete board of the Tor Project resigned, and announced a new board, made up of Matt Blaze, Cindy Cohn, Gabriella Coleman, Linus Nordberg, Megan Price and Bruce Schneier .