Paper 2022/201

Enig: Player Replaceable Finality Layers with Optimal Validity

Simon Holmgaard Kamp and Jesper Buus Nielsen and Søren Eller Thomsen and Daniel Tschudi

Abstract

We present two new provably secure finality layers for Nakamoto style blockchains. One is for partially synchronous networks and the other is for networks with periods of synchrony. Both protocols are player replaceable and therefore enjoy protection against denial of service attacks when run with a proof-of-stake lottery to elect the parties. The finality layers are proven secure to run on top of any Nakamoto style blockchain which has a property called \emph{finality friendliness}. Both finality layers improve on all existing provably secure finality layers in terms of communication complexity or security. A proof-of-stake finality layer has $v$-validity if whenever it declares a block $B$ final then honest parties holding a fraction $v$ of the stake had $B$ on the longest chain. Validity is important to prevent that the finality layer finalises blocks that were not ``good'' according to the Nakamoto style blockchain. We prove upper bounds on the achievable validity in partially synchronous networks and networks with periods of synchrony. Both our finality layers match these upper bounds.