Reducing confirmation reversal probability of PoW blockchains using checkpoints
Ke Wang, Hyong S. Kim · 2022
Existing PoW blockchains adopt a simple k-block confirmation rule. A block is confirmed after it is buried sufficiently deep in the blockchain. Such confirmation does not guarantee transaction finality. Specifically, after a block is confirmed, a miner may later confirm a different block in the same position of the blockchain. The previously confirmed block effectively gets reversed. A common reason for block reversal is that an adversary could privately maintain a malicious chain. The adversary releases the malicious chain once it is longer than the blockchain of at least one honest miner.In this paper, we propose a new confirmation method that relies on checkpoints to prevent the adversary from withholding its blocks. A checkpoint contains one or more blocks at the same position of the blockchain. Unlike existing protocols, we propose a simple mechanism to establish checkpoint blocks without introducing any trusted third parties. A committee is selected periodically to vote for checkpoint blocks. Miners only confirm a block when it is referenced by all checkpoint blocks at some block height, among other criteria. We prove the liveness and safety of this new confirmation rule. We show that conditioning on honest majority within a committee, miners achieve 0 confirmation reversal probability. We demonstrate that with majority mining power being honest, we could always select a proper checkpoint period to ensure the assumption of the majority of a committee being honest holds for hundreds of years on average. In the rare case where this assumption is broken, miners in the checkpoint-enabled blockchain still incur the same reversal probability as in the current k-block confirmation.