I will describe the 2nd generation Onion Routing* system, Tor,which is freely available from http://freehaven.net/tor/. I will give backround on anonymous communication in general and on related systems. If there is time, I will also mention some of the theory in defining and measuring anonymity as well as the economic and intellectual property issues that have arisen in developing and deploying such an infrastructure.

*Onion Routing is an infrastructure for private communication over a public network, such as the Internet. It provides anonymous connections that are resistant to both eavesdropping and traffic analysis. Thus it hides not only the data being sent, but who is talking to whom. Onion Routing provides real-time, bidirectional connections that can be used anywhere a socket connection can be used. Applications, such as web browsing, e-mail, chat, and remote login require no modification to use Onion Routing.