Welcome to the resource topic for 2024/312
Title:
Trapdoor Memory-Hard Functions
Authors: Benedikt Auerbach, Christoph U. Günther, Krzysztof Pietrzak
Abstract:Memory-hard functions (MHF) are functions whose evaluation provably requires a lot of memory. While MHFs are an unkeyed primitive, it is natural to consider the notion of trapdoor MHFs (TMHFs). A TMHF is like an MHF, but when sampling the public parameters one also samples a trapdoor which allows evaluating the function much cheaper.
Biryukov and Perrin (Asiacrypt’17) were the first to consider TMHFs and put forth a candidate TMHF construction called Diodon that is based on the Scrypt MHF (Percival, BSDCan’09). To allow for a trapdoor, Scrypt’s initial hash chain is replaced by a sequence of squares in a group of unknown order where the order of the group is the trapdoor. For a length n sequence of squares and a group of order N, Diodon’s cumulative memory complexity (CMC) is O(n^2\log N) without the trapdoor and O(n \log(n) \log(N)^2) with knowledge of it.
While Scrypt is proven to be optimally memory-hard in the random oracle model (Alwen et al., Eurocrypt’17), Diodon’s memory-hardness has not been proven so far. In this work, we fill this gap by rigorously analyzing a specific instantiation of Diodon. We show that its CMC is lower bounded by \Omega(\frac{n^2}{\log n} \log N) which almost matches the upper bound. Our proof is based Alwen et al.'s lower bound on Scrypt’s CMC but requires non-trivial modifications due to the algebraic structure of Diodon. Most importantly, our analysis involves a more elaborate compression argument and a solvability criterion for certain systems of Diophantine equations.
ePrint: https://eprint.iacr.org/2024/312
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