Snow White: Robustly Reconfigurable Consensus and Applications to Provably Secure Proof of Stake

Abstract

We present the a provably secure proof-of-stake protocol called Snow White. The primary application of Snow White is to be used as a “green” consensus alternative for a decentralized cryptocurrency system with open enrollement. We break down the task of designing Snow White into the following core challenges:

• 1. identify a core “permissioned” consensus protocol suitable for proof-of-stake; specifically the core consensus protocol should offer robustness in an Internet-scale, heterogeneous deployment;

• 2. propose a robust committee re-election mechanism such that as stake switches hands in the cryptocurrency system, the consensus committee can evolve in a timely manner and always reflect the most recent stake distribution; and

• 3. relying on the formal security of the underlying consensus protocol, prove the full end-to-end protocol to be secure—more specifically, we show that any consensus protocol satisfying the desired robustness properties can be used to construct proofs-of-stake consensus, as long as money does not switch hands too quickly.

Snow White was publicly released in September 2016. It provides the first formal, end-to-end proof of a proof-of-stake system in a truly decentralized, open-participation network, where nodes can join at any time (not necessarily at the creation of the system). We also give the first formal treatment of a well-known issue called “costless simulation” in our paper, proving both upper- and lower-bounds that characterize exactly what setup assumptions are needed to defend against costless simulation attacks. We refer the reader to our detailed chronological notes on a detailed comparison of Snow White and other prior and concurrent works, as well as how subsequent works (including Ethereum’s proof-of-stake design) have since extended and improved our ideas.