Stronger Difficulty Notions for Client Puzzles and Denial-of-Service-Resistant Protocols

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 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, the authors 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.