The Availability-Accountability Dilemma and its Resolution via Accountability Gadgets
This addresses a critical security and scalability problem for blockchain systems like Ethereum 2.0, offering a novel resolution to a fundamental trade-off.
The paper identifies an availability-accountability dilemma in Byzantine fault tolerant consensus protocols under dynamic participation, where no protocol can be both accountably-safe and live, and resolves it by constructing an accountability gadget that ensures liveness for the full ledger and accountability for a checkpointed prefix, with experiments on over 4000 nodes showing better latency than the current insecure solution Gasper.
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.