How to confirm blocks in PoW blockchains

Existing PoW blockchains adopt a simple k-block confirmation rule. A block is confirmed after it is buried sufficiently deep in the blockchain. The selection of the required depth k is largely based on empirical experiences and lacks a theoretical guarantee. We ask a fundamental question in PoW blockchains: when and how should miners confirm a block in PoW blockchains? We say a block is in the consensus state if it is included in every miner’s blockchain. The issue of the current k-block rule is that it does not depend on consensus. A block buried k-block deep may not be in the consensus state. We propose that a block should not be confirmed before the miner believes it to be in the consensus state. We present a consensus-inference protocol for miners to infer whether a block is in the consensus state locally. Based on the inference protocol, we further describe a new consensus-aware confirmation rule. Compared to the existing k-block rule, we show that the new rule achieves lower confirmation reversal probability under the same confirmation latency. The reduction could be orders of magnitude when multiple blocks are mined on average per network delay. Correspondingly, we further show that the new rule could improve the scalability of current blockchains by as much as 90% in common scenarios.