Efficient memory bound puzzles using pattern databases

TL;DR: In this paper, the applicability of memory bound puzzles from a new perspective is investigated and constructions based on heuristic search methods are derived from a more algorithmic foundation, and as a result, allow us to easily tune parameters that impact puzzle creation and verification costs.

Abstract: CPU bound client puzzles have been suggested as a defense mechanism against connection depletion attacks. However, the wide disparity in CPU speeds prevents such puzzles from being globally deployed. Recently, Abadi et. al. [1] and Dwork et. al. [2] addressed this limitation by showing that memory access times vary much less than CPU speeds, and hence offer a viable alternative. In this paper, we further investigate the applicability of memory bound puzzles from a new perspective and propose constructions based on heuristic search methods. Our constructions are derived from a more algorithmic foundation, and as a result, allow us to easily tune parameters that impact puzzle creation and verification costs. Moreover, unlike prior approaches, we address client-side cost and present an extension that allows memory constrained clients (e.g., PDAs) to implement our construction in a secure fashion.