Revisiting Difficulty Notions for Client Puzzles and DoS Resilience
Cryptographic puzzles are moderately difficult problems that can be solved by investing non-trivial amounts of computation and/or storage. Devising models for cryptographic puzzles has only recently started to receive attention from the cryptographic community as a first step towards rigorous models and proofs of security of applications that employ them (e.g. Denial-of-service (DoS) resistance). Unfortunately, the subtle interaction between the complex scenarios for which cryptographic puzzles are intended and typical difficulties associated with defying concrete security easily leads to flaws in definitions and proofs.