Tor

Tor, short for The Onion Router, is a free, open-source software and network designed to enable anonymous communication over the Internet. It uses layered encryption and a worldwide volunteer network of relays to conceal a user’s location and activity from surveillance and traffic analysis.

When a user connects, the Tor client selects a circuit of at least three relays: a guard (entry) relay, one or more middle relays, and an exit relay. Each relay only decrypts one layer, so no single node knows both the source and destination. The exit node sends traffic to the destination on behalf of the user.

Applications include Tor Browser, which is configured for privacy and anti-tracking, and the Tor Project, a nonprofit that develops and maintains Tor software and operates some directory services. Relays are run by volunteers and organizations worldwide.

Tor is widely used to protect privacy, circumvent censorship, and enable journalism or whistleblowing in restrictive environments, but it is also used for illicit activity.

Limitations and risks include slower performance, potential end-to-end exposure if the exit node observes traffic that is not encrypted by HTTPS, and vulnerability to correlation attacks if an observer can monitor both entry and exit points. Users should enable HTTPS sites and disable active content to reduce risk, and some sites block Tor.

Legal status varies by country; Tor usage is legal in many places but prohibited or restricted in others.

Originating from the concept of onion routing developed by researchers at the U.S. Naval Research Laboratory, Tor was further developed in the early 2000s by The Tor Project founded by Roger Dingledine, Nick Mathewson, and Paul Syverson.