The Onion Router (Tor) is a free software implementation of second-generation onion routing enabling Internet anonymity by thwarting network traffic analysis. Roger Dingledine, Nick Mathewson, and Paul Syverson presented "Tor: The Second-Generation Onion Router" at the 13th USENIX Security Symposium on Friday, August 13, 2004.

Tor employs cryptography in a multi-layered manner (hence the Onion routing analogy), ensuring perfect forward secrecy between routers. A user of the Tor network runs a proxy server on his computer. Internet-facing software can then access Tor through a SOCKS interface. Once inside a Tor network, the traffic is sent from router to router, the Tor software periodically negotiating a virtual circuit through the Tor network, ultimately reaching an exit node at which point the cleartext packet is forwarded on to its original destination. Viewed from the destination, the traffic appears to originate at the Tor exit node.

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. The United States government, for example, has the capability to monitor any broadband Internet traffic using devices mandated by the Communications Assistance For Law Enforcement Act (CALEA) and can therefore monitor both ends of a US-based Tor connection. While Tor does provide protection against traffic analysis, it cannot prevent traffic confirmation (also called end-to-end correlation).

Originally sponsored by the US Naval Research Laboratory, Tor became an Electronic Frontier Foundation (EFF) project in late 2004 and the EFF supported Tor financially until November 2005. Tor software is now developed by the Tor Project, which since December 2006 is a 501(c)(3) research/education non-profit organization based in the United States of America that receives a diverse base of financial support.