Hooks: A Simple and Modular Checkpointing Protocol for Blockchains

This paper presents Hooks, a modular checkpointing protocol for chain-based (a.k.a. Nakamoto-style) blockchains, which enables trustless bootstrap thanks to its public verifiability. A hook is a pointer from a block into one of its descendent blocks implemented by a digital signature. In this way, it provides a forward analogue to the traditional parent (hash) links that point backwards. Broadly speaking, our protocol uses a quorum of hooks to create checkpoints. Our protocol differs from other checkpointing protocols as it does not attempt to solve an agreement problem. Instead, it extends the safety guarantees offered by the input blockchain ${\mathcal{B}}$ and does so while requiring only a weak notion of safety for ${\mathcal{B}}$. As a trade-off, our protocol may not be live when our safety requirements are not met, namely, we prevent safety issues when possible and converts them to liveness issues otherwise. Our construction is efficient, easy to implement and to integrate, thus making it an appealing enhancement for many blockchain systems.