Tor: Design
The Design Document (published at Usenix Security 2004) gives our justifications and security analysis for the Tor design:
The Specification aims to give developers enough information to build a compatible version of Tor:
Look at the slides from the Codecon 2004 talk, or listen to the audio from the talk. You can also peruse the Black Hat 2004 slides or Defcon 2004 slides.