Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Short Paper: Accountable Safety Implies Finality

Joachim Neu, Ertem Nusret Tas, David Tse

TL;DR

The paper studies BFT SMR protocols in partially synchronous settings, introducing accountable safety and proving that achieving $(f+1)$-accountable safety necessarily yields $f$-finality, thereby unifying prior results on finality and accountability. The authors provide formal models, adversarial settings, and a central impossibility-driven argument that even the weakest form of accountability (delay-free networks) implies finality across network models. They further relate these notions to CAP-like tradeoffs, show simplifications of earlier results, and demonstrate nuanced relationships via the SyncFin gadget, which is accountable-safe and live under synchrony but not after GST. The results guide protocol design by showing that accountability mechanisms can ensure finality, but simultaneous guarantees on liveness and accountability under partial synchrony may be impossible, emphasizing fundamental tradeoffs in PoS/BFT blockchain settings.

Abstract

Motivated by proof-of-stake (PoS) blockchains such as Ethereum, two key desiderata have recently been studied for Byzantine-fault tolerant (BFT) state-machine replication (SMR) consensus protocols: Finality means that the protocol retains consistency, as long as less than a certain fraction of validators are malicious, even in partially-synchronous environments that allow for temporary violations of assumed network delay bounds. Accountable safety means that in any case of inconsistency, a certain fraction of validators can be identified to have provably violated the protocol. Earlier works have developed impossibility results and protocol constructions for these properties separately. We show that accountable safety implies finality, thereby unifying earlier results.

Short Paper: Accountable Safety Implies Finality

TL;DR

The paper studies BFT SMR protocols in partially synchronous settings, introducing accountable safety and proving that achieving -accountable safety necessarily yields -finality, thereby unifying prior results on finality and accountability. The authors provide formal models, adversarial settings, and a central impossibility-driven argument that even the weakest form of accountability (delay-free networks) implies finality across network models. They further relate these notions to CAP-like tradeoffs, show simplifications of earlier results, and demonstrate nuanced relationships via the SyncFin gadget, which is accountable-safe and live under synchrony but not after GST. The results guide protocol design by showing that accountability mechanisms can ensure finality, but simultaneous guarantees on liveness and accountability under partial synchrony may be impossible, emphasizing fundamental tradeoffs in PoS/BFT blockchain settings.

Abstract

Motivated by proof-of-stake (PoS) blockchains such as Ethereum, two key desiderata have recently been studied for Byzantine-fault tolerant (BFT) state-machine replication (SMR) consensus protocols: Finality means that the protocol retains consistency, as long as less than a certain fraction of validators are malicious, even in partially-synchronous environments that allow for temporary violations of assumed network delay bounds. Accountable safety means that in any case of inconsistency, a certain fraction of validators can be identified to have provably violated the protocol. Earlier works have developed impossibility results and protocol constructions for these properties separately. We show that accountable safety implies finality, thereby unifying earlier results.
Paper Structure (5 sections, 6 theorems, 2 figures)

This paper contains 5 sections, 6 theorems, 2 figures.

Table of Contents

  1. Introduction
  2. Model
  3. Accountability Implies Finality
  4. Simplification of Earlier Results
  5. Finality, Accountable Safety, and Security under Partial Synchrony

Key Result

theorem 1

If a consensus protocol provides $(f+1)$-accountable safety in a delay-free network, then it also satisfies $f$-finality.

Figures (2)

  • Figure 1: Execution of a consensus protocol with four replicas. World $0$ is partially-synchronous, worlds $1$, $2$ and $3$ are delay-free. Red replica $P_4$ is adversary in all worlds. Orange replicas are adversary but do not violate the protocol rules other than delaying the sending/receiving of messages to/from the honest replica. Green replicas are honest.
  • Figure 2: Venn diagram of protocols satisfying finality, accountable safety, security under partial synchrony, and dynamic participation. The key Theorem \ref{['thm:acc-implies-fin']} of this work means that accountable safe protocols are contained in the set of final protocols.

Theorems & Definitions (11)

  • definition 1
  • definition 2
  • definition 3
  • theorem 1
  • proof : of Theorem \ref{['thm:acc-implies-fin']}
  • proposition 1: From psync-model, see also aadilemmafullv1multithresholdmultithresholdbft
  • corollary 1
  • proposition 2: From blockchain-cap-theorem, see also captheorem
  • corollary 2
  • theorem 2
  • ...and 1 more