Efficient and Proof-of-Useful-Work Friendly Local-Search for Distributed Consensus

Blockchain protocols based on the popular “Proof-of-Work” mechanism yield public transaction ledgers maintained by a group of distributed participants who solve computationally hard puzzles to earn the right to add a block. The success and widespread adoption of this mechanism has led to staggering energy consumption devoted to solving such (otherwise) “useless” puzzles. While the environmental impacts of the framework have been widely criticized, this has been the dominant distributed ledger paradigm for years.

The Ofelimos “Proof-of-Useful-Work” protocol (Fitzi et al., CRYPTO 2022) addressed this by establishing that useful combinatorial problems could replace the conventional hashing puzzles, yielding a provably secure blockchain that meaningfully utilizes the computational work that underlies the protocol. The usefulness to wastefulness ratio of Ofelimos hinges on the properties of its underlying generic distributed local-search algorithm—Doubly Parallel Local Search (DPLS). We observe that this search procedure is particularly wasteful when exploring steep regions of the solution space.

To address this issue, we introduce Frequently Rerandomized Local Search (FRLS), a new generic distributed local search algorithm that we show to be consistent with the Ofelimos architecture. While this algorithm retains ledger security, we show that it also provides compelling performance on bench-mark problems arising in practice: Concretely, state-of-art local-search algorithms for cumulative scheduling and warehouse location can be directly adapted to FRLS and we experimentally demonstrate the efficiency of the resulting algorithms.