The Blockchain Anomaly

Abstract

Most popular blockchain solutions rely on proof-of-work to guarantee that participants reach consensus on a unique block per index of the chain. As consensus is impossible in the general case, it seems that these blockchain systems require messages are delivered fast and no participant mines faster than the crowd. To date, no experimental settings have however been proposed to demonstrate this hypothesis. In this paper, we identify conditions under which these blockchain systems fail to ensure consensus and present a reproducible execution on our Ethereum private chain. To this end, we introduce the Blockchain Anomaly, the impossibility for the blockchain to guarantee that a committed transaction is not abortable. This anomaly may translate into dramatic consequences for the user of proof-of-work blockchains. Named after the infamous Paxos anomaly, this anomaly makes dependent transactions, like “Bob sends money to Carole after he received money from Alice” impossible and may lead to double spending. We also explain how the anomaly differs from a 51-percent attack and how one could avoid it by adapting the Ethereum implementation or by exploiting smart contracts.