New client puzzle outsourcing techniques for DoS resistance

Abstract

We explore new techniques for the use of cryptographic puzzles as a countermeasure to Denial-of-Service (DoS) attacks. We propose simple new techniques that permit the out-sourcing of puzzles; their distribution via a robust external service that we call a bastion. Many servers can rely on puzzles distributed by a single bastion. We show how a bastion, somewhat surprisingly, need not know which servers rely on its services. Indeed, in one of our constructions, a bastion may consist merely of a publicly accessible random data source, rather than a special purpose server. Our out-sourcing techniques help eliminate puzzle distribution as a point of compromise.