Tor: The Second-Generation Onion Router

This paper asks how a browser can cross several volunteer relays without handing any one relay the complete route. Read it to see how the client extends a circuit one hop at a time, wraps each fixed-size cell in a different session key for every relay, and why those layers still cannot defeat an observer who can correlate traffic at both ends.

Reading focus: Why a relay can forward a connection while knowing only its predecessor and successor rather than both the client and destination. How telescoping circuit construction negotiates a fresh key with each relay and how layered cells expose one local step at a time. Where Tor's 2004 threat model stops: traffic timing, hostile exits, application leaks, directory trust, and early-deployment scale remain real boundaries.

USENIX Security 2004. Dingledine, Mathewson, and Syverson. 35 min read, easy difficulty.