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Security Notions and Generic Constructions for Client Puzzles

L. Chen, Paul Morrissey, Nigel Smart, Bogdan Warinschi, Security Notions and Generic Constructions for Client Puzzles. Advances in Cryptology - Asiacrypt 2009, pp. 505–523. December 2009. No electronic version available. External information


By a computational puzzle we mean a mildly dicult computational problem that requires 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. The main contribution of this paper is a formal model for the security of client puzzles. We clarify the interface that client puzzles should o er and give two security notions for puzzles. Both functionality and security are inspired by, and tailored to, the use of puzzles as a defense against DoS attacks. Our de nitions ll an important gap: breaking either of the two properties immediately leads to successful DoS attacks. We illustrate this point with an attack against a previously proposed puzzle construction. We also provide a generic construction of a client puzzle which meets our security de nitions.

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