Security Notions and Generic Constructions for Client Puzzles

Computational puzzles are mildly difficult computational problems that require resources (processor cycles, memory, or both) to solve. Puzzles have found a variety of uses in security. In this paper we are concerned with client puzzles: a type of puzzle used as a defense against Denial of Service (DoS) attacks. Before engaging in a resource consuming protocol with a client, a server demands that the client solves a freshly generated client puzzle. Despite their widespread use, the lack of formal models for security of client puzzles prevents a full analysis of proposed puzzles and, more importantly, prevents rigorous proofs for the effectiveness of puzzles as a DoS defense. The main contribution of this paper is a formal model for the security of client puzzles as a stepping stone towards solving the above problems. We clarify the interface that client puzzles should offer and give two security notions for puzzles. Both functionality and security are inspired by, and tailored to, the ...

• Type: Conference
• Year: 2009
• Where: ASIACRYPT
• Authors: Liqun Chen, Paul Morrissey, Nigel P. Smart, Bogdan Warinschi
• Keywords: ASIACRYPT 2009, Client Puzzles, Computational Puzzles, Cryptology, Dos Attack