BFT Protocol Forensics

TL;DR

This paper formalizes forensic support in BFT protocols, analyzing how minor implementation details affect the ability to identify malicious replicas, and demonstrates both capabilities and limitations across various protocols including LibraBFT.

Contribution

It provides a formal analysis of forensic support in BFT protocols, highlighting the impact of implementation details and establishing an impossibility result for certain network models.

Findings

Forensic support varies significantly with minor protocol variants.

LibraBFT exhibits strong forensic support capabilities.

In synchronous networks, forensic support is impossible for protocols with 2t+1 replicas.

Abstract

Byzantine fault-tolerant (BFT) protocols allow a group of replicas to come to a consensus even when some of the replicas are Byzantine faulty. There exist multiple BFT protocols to securely tolerate an optimal number of faults $t$ under different network settings. However, if the number of faults $f$ exceeds $t$ then security could be violated. In this paper we mathematically formalize the study of forensic support of BFT protocols: we aim to identify (with cryptographic integrity) as many of the malicious replicas as possible and in as a distributed manner as possible. Our main result is that forensic support of BFT protocols depends heavily on minor implementation details that do not affect the protocol's security or complexity. Focusing on popular BFT protocols (PBFT, HotStuff, Algorand) we exactly characterize their forensic support, showing that there exist minor variants of each protocol for which the forensic supports vary widely. We show strong forensic support capability of LibraBFT, the consensus protocol of Diem cryptocurrency; our lightweight forensic module implemented on a Diem client is open-sourced and is under active consideration for deployment in Diem. Finally, we show that all secure BFT protocols designed for $2t+1$ replicas communicating over a synchronous network forensic support are inherently nonexistent; this impossibility result holds for all BFT protocols and even if one has access to the states of all replicas (including Byzantine ones).