Tor: the second generation onion router

This is the original Dingledine et al's Tor paper. Please consider it required reading.

INFO6003 (2013) Tor: the second generation onion router -- Paper

This is the original Dingledine et al's Tor paper. Please consider it required reading.

Hiding routing information

This is the original "onion routing" paper, it explains in the details how onions are built and work. This is optional reading, although I strongly advise you to read the Introduction and section 3 "Onions"