Blockchain Is Dead, Long Live Blockchain! Accountable State Machine Replication for Longlasting Blockchain.

Abstract

The long-standing impossibility of reaching agreement restricts the lifespan of blockchains. In fact, the consensus on a block to be appended to any blockchain succeeds either with some probability or at the condition that two thirds of the $n$ replicas are not Byzantine. In the former case, the probability that the blockchain fails grows exponentially with the number of newly appended blocks. In the latter case, the blockchain fails as soon as a coalition bribes $f=n/3$ replicas. As a result, one may wonder whether blockchains are doomed to fail. In this paper, we answer this question in the negative by proposing the first Longlasting Blockchain system, \emph{LLB}. LLB builds upon the observation that blockchains are rarely subject to benign faults. As opposed to probabilistic blockchains, LLB solves consensus deterministically when $f<n/3$. As opposed to Byzantine fault tolerant blockchains, it resolves a series of disagreements by reducing eventually the number of deceitful replicas from $n/3\leq f<2n/3$ to $f'<n'/3$ among a new set of $n'$ replicas. To demonstrate its effectiveness, we implement two coalition attacks and a zero loss payment application that forces replicas that misbehave to reimburse conflicting transactions. Finally, LLB outperforms the raw state machine replication at the heart of Facebook's Libra and achieves performance comparable to a scalable blockchain that cannot tolerate $n/3$ failures.