International Association for Cryptologic Research

International Association
for Cryptologic Research


Fabien Coelho


An (Almost) Constant-Effort Solution-Verification Proof-of-Work Protocol based on Merkle Trees
Fabien Coelho
Proof-of-work schemes are economic measures to deter denial-of-service attacks: service requesters compute moderately hard functions that are easy to check by the provider. We present such a new scheme for solution-verification protocols. Although most schemes to date are probabilistic unbounded iterative processes with high variance of the requester effort, our Merkle tree scheme is deterministic, with an almost constant effort and null variance, and is computation-optimal.
Exponential Memory-Bound Functions for Proof of Work Protocols
Fabien Coelho
In Year 2005, Internet users were twice more likely to receive unsolicited electronic messages, known as spams, than regular emails. Proof of work protocols are designed to deter such phenomena and other denial-of-service attacks by requiring computed stamps based on hard-to-solve problems with easy-to-verify solutions. As cpu-intensive computations are badly hit over time by Moore's law, memory-bound computations have been suggested to deal with heterogeneous hardware. We introduce new practical, optimal, proven and possibly memory-bound functions suitable to these protocols, in which the client-side work to compute the response is intrinsically exponential with respect to the server-side work needed to set or check the challenge. One-way non-interactive solution-verification variants are also presented. Although simple implementations of such functions are bound by memory latency, optimized versions are shown to be bound by memory bandwidth instead.