Formalizing Nakamoto-Style Proof of Stake

TL;DR

This paper provides the first machine-checked formal proof of safety and liveness for a Nakamoto-style Proof of Stake blockchain protocol using the Coq proof assistant, ensuring protocol correctness in a synchronous network.

Contribution

It introduces a formal, machine-verified proof of safety and liveness for a PoS Nakamoto-style blockchain, covering chain growth, quality, and common prefix.

Findings

Verified safety and liveness properties for the protocol

Formal proof of chain growth, quality, and common prefix

Applicable to a synchronous network with static adversaries

Abstract

Fault-tolerant distributed systems move the trust in a single party to a majority of parties participating in the protocol. This makes blockchain based crypto-currencies possible: they allow parties to agree on a total order of transactions without a trusted third party. To trust a distributed system, the security of the protocol and the correctness of the implementation must be indisputable. We present the first machine checked proof that guarantees both safety and liveness for a consensus algorithm. We verify a Proof of Stake (PoS) Nakamoto-style blockchain (NSB) protocol, using the foundational proof assistant Coq. In particular, we consider a PoS NSB in a synchronous network with a static set of corrupted parties. We define execution semantics for this setting and prove chain growth, chain quality, and common prefix which together imply both safety and liveness.