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The anonymity routing network Tor is frequently in the news these days, which makes it a good case to read up on the fascinating technologies behind it. Tor stands for The Onion Router as its technology is based on the onion routing principles. These principles were first described by Goldschlag, et al., from the Naval Research Lab, in their 1996 paper on Hiding Routing Information. Almost immediately work started on addressing a number of omissions in the original work in what became known as the second-generation onion router. Tor is the implementation of such a second generation router and has a number of fascinating features. The paper describing Tor is also very interesting from a practitioners point of view as it deals with the system complexities of implementing the router at scale.

Hiding Routing Information, David M. Goldschlag, Michael G. Reed, and Paul F. Syverson, in the proceeding of the Workshop on Information Hiding, Cambridge, UK, May, 1996.

Tor: The Second-Generation Onion Router, Roger Dingledine, Nick Mathewson and Paul Syverson, in Proceedings of the 13th USENIX Security Symposium, August 2004