Better Safe than Sorry: Recovering after Adversarial Majority

TL;DR

This paper introduces a recovery gadget that ensures safety during adversarial majority in blockchain protocols and restores liveness once honest majority is reestablished, applicable to protocols with certificates.

Contribution

It presents a novel recovery gadget that maintains safety during adversarial majority and recovers liveness, compatible with existing protocols like HotStuff and Tendermint.

Findings

Safety is maintained during adversarial majority in a synchronous network.

Liveness can be recovered after honest majority is restored.

The approach is applicable to protocols with certificates such as HotStuff and Tendermint.

Abstract

The security of blockchain protocols is a combination of two properties: safety and liveness. It is well known that no blockchain protocol can provide both to sleepy (intermittently online) clients under adversarial majority. However, safety is more critical in that a single safety violation can cause users to lose money. At the same time, liveness must not be lost forever. We show that, in a synchronous network, it is possible to maintain safety for all clients even during adversarial majority, and recover liveness after honest majority is restored. Our solution takes the form of a recovery gadget that can be applied to any protocol with certificates (such as HotStuff, Streamlet, Tendermint, and their variants).