Client Puzzles and Sybil Attacks

Nikita Borisov, University of Illinois, Urbana-Champaign

Client puzzles have been used to prevent denial of service attacks and to fight spam. I will talk about applying them to the problem of Sybil attacks in peer-to-peer networks. The fundamental challenge in this setting is enforcing that puzzle solutions not be reused by attackers over time. I will describe a fully decentralized scheme to achieve this goal.

In this scheme, challenges from each node are continually distributed throughout a p2p network and combined with challenges from others. The combinations are verifiable, such that any node can check whether its challenge was used by each other node, ensuring freshness of puzzles. I will show how to implement such a scheme on top of the Chord overlay and how to address the problem of node churn.


Nikita Borisov is an Assistant Professor at the University of Illinois at Urbana-Champaign. His research interests are in the area of computer privacy and security; his recent work deals with designing and analyzing anonymous peer-to-peer networks.

24 February (Friday) at 1630 hrs

Gates 4B (opposite 490)