The Availability-Accountability Dilemma and its Resolution via Accountability Gadgets

TL;DR

This paper addresses the availability-accountability dilemma in Byzantine fault tolerant protocols with dynamic participation, proposing a novel accountability gadget that ensures both liveness and accountability, demonstrated through Ethereum 2.0 implementation and large-scale experiments.

Contribution

It introduces a provably secure accountability gadget that resolves the availability-accountability dilemma, enabling dynamic participation while maintaining accountability and liveness.

Findings

The gadget achieves both liveness and accountability in dynamic environments.

Implementation on Ethereum 2.0 shows improved latency over Gasper.

Experimental results with 4000+ nodes demonstrate scalability and security.

Abstract

For applications of Byzantine fault tolerant (BFT) consensus protocols where the participants are economic agents, recent works highlighted the importance of accountability: the ability to identify participants who provably violate the protocol. At the same time, being able to reach consensus under dynamic levels of participation is desirable for censorship resistance. We identify an availability-accountability dilemma: in an environment with dynamic participation, no protocol can simultaneously be accountably-safe and live. We provide a resolution to this dilemma by constructing a provably secure optimally-resilient accountability gadget to checkpoint a longest chain protocol, such that the full ledger is live under dynamic participation and the checkpointed prefix ledger is accountable. Our accountability gadget construction is black-box and can use any BFT protocol which is accountable under static participation. Using HotStuff as the black box, we implemented our construction as a protocol for the Ethereum 2.0 beacon chain, and our Internet-scale experiments with more than 4000 nodes show that the protocol achieves the required scalability and has better latency than the current solution Gasper, which was shown insecure by recent attacks.