Paper 2021/1379

Ofelimos: Combinatorial Optimization via Proof-of-Useful-Work \\ A Provably Secure Blockchain Protocol

Matthias Fitzi, Aggelos Kiayias, Giorgos Panagiotakos, and Alexander Russell

Abstract

Minimizing the energy cost and carbon footprint of the Bitcoin blockchain and related protocols is one of the most widely identified open questions in the cryptocurrency space. Substituting the proof-of-work (PoW) primitive in Nakamoto's longest chain protocol with a {\em proof of useful work} (PoUW) has been long theorized as an ideal solution in many respects but, to this day, the concept still lacks a convincingly secure realization. In this work we put forth Ofelimos, a novel PoUW-based block\-chain protocol whose consensus mechanism simultaneously realizes a decentralized optimization-problem solver. Our protocol is built around a novel local search algorithm, which we call Doubly Parallel Local Search (DPLS), that is especially crafted to suit implementation as the PoUW component of our blockchain protocol. We provide a thorough security analysis of our protocol and additionally present metrics that reflect the usefulness of the system. As an illustrative example we show how DPLS can implement a variant of WalkSAT and experimentally demonstrate its competitiveness with respect to a vanilla WalkSAT implementation. In this way, our work paves the way for safely using blockchain systems as generic optimization engines for a variety of hard optimization problems for which a publicly verifiable solution is desired.