Cryptology ePrint Archive: Report 2010/649

Stronger difficulty notions for client puzzles and denial-of-service-resistant protocols

Douglas Stebila and Lakshmi Kuppusamy and Jothi Rangasamy and Colin Boyd and Juan Gonzalez Nieto

Abstract: Client puzzles are meant to act as a defense against denial of service (DoS) attacks by requiring a client to solve some moderately hard problem before being granted access to a resource. However, recent client puzzle difficulty definitions (Stebila and Ustaoglu, 2009; Chen et al., 2009) do not ensure that solving n puzzles is n times harder than solving one puzzle. Motivated by examples of puzzles where this is the case, we present stronger definitions of difficulty for client puzzles that are meaningful in the context of adversaries with more computational power than required to solve a single puzzle.

A protocol using strong client puzzles may still not be secure against DoS attacks if the puzzles are not used in a secure manner. We describe a security model for analyzing the DoS resistance of any protocol in the context of client puzzles and give a generic technique for combining any protocol with a strong client puzzle to obtain a DoS-resistant protocol.

Category / Keywords: cryptographic protocols / client puzzles, proof of work, denial of service resistance, protocols

Publication Info: Extended abstract appears in CT-RSA 2011, LNCS 6558, pp. 284-301, Springer, 2011.

Date: received 20 Dec 2010, last revised 19 Feb 2013

Contact author: douglas at stebila ca

Available formats: PDF | BibTeX Citation

Note: Revised one definition and theorem.

Version: 20130220:065421 (All versions of this report)

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