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Securing Proof-of-Work Ledgers via Checkpointing

May/2021, To appear at: ICBC '21


Distributed ledgers based on the Proof-of-Work (PoW) paradigm are typically most vulnerable when mining participation is low. During these periods an attacker can mount devastating attacks, such as double spending or censorship of transactions. Checkpointing has been proposed as a mechanism to mitigate such 51% attacks. The core idea is to employ an external set of parties that securely run an assisting service which guarantees the ledger's properties and can be relied upon at times when the invested hashing power is low. We realize the assisting service in two ways, via checkpointing and timestamping, and show that a ledger, which employs either, is secure with high probability, even in the presence of an adversarial mining majority. We put forth the first rigorous study of checkpointing as a mechanism to protect PoW ledgers from 51% attacks. Notably, our design is the first to offer both consistency and liveness guarantees, even under adversarial mining majorities. Our liveness analysis also identifies a previously undocumented attack, namely front-running, which enables Denial-of-Service against existing checkpointed ledger systems. We showcase the liveness guarantees of our mechanism by evaluating the checkpointed version of Ethereum Classic, a blockchain which recently suffered a 51% attack, and build a federated distributed checkpointing service, which provides high assurance with low performance requirements. Finally, we prove the security of our timestamping mechanism, build a fully decentralized timestamping solution, by utilizing a secure distributed ledger, and evaluate its performance on the existing Bitcoin and Ethereum systems.