Securing Proof-of-Work Ledgers via Checkpointing

Distributed ledgers based on Proof-of-Work (PoW) are typically vulnerable when mining participation is low. During these periods, an attacker with a mining majority can mount serious attacks, like double spending or transaction censorship. Our work explores mechanisms to secure a ledger against such adversaries and puts forth the first rigorous study of checkpointing as a protection from 51% attacks. The core idea is to employ an external set of parties to finalize blocks after their creation. This idea takes the form of checkpointing and timestamping, the former ensuring low latency in a federated setting and the latter being fully decentralized. Crucially, we identify and protect against a (previously undocumented) attack, "block lead", thus our scheme is the first to guarantee liveness.