Proof of Storage-Time: Efficiently Checking Continuous Data Availability
Giuseppe Ateniese, Long Chen, Mohammard Etemad, Qiang Tang · 2020
A high-quality outsourced storage service is crucial for many existing applications.For example, hospitals and data centers need to guarantee the availability of their systems to perform routine daily activities.Such a system should protect users against downtime and ensure data availability over time.Continuous data availability is a critical property to measure the quality of an outsourced storage service, which implies that outsourced data is continuously available to the server during the entire storage period.We formally study the Proof of Storage-Time (PoSt), the notion initially proposed in the Filecoin whitepaper, which enables a verifier to audit the continuous data availability of an outsourced storage service.We provide a formal security model of PoSt and generic constructions that are proven secure under our definition.Moreover, our concrete instantiation can yield a PoSt protocol with an extremely efficient verification: a single hash computation to verify a proof of size around 200 bits.This makes our scheme applicable even in the decentralized storage marketplace enabled by blockchain.The above discussion highlights a central question we would like to study in this article: How can we efficiently monitor storage providers to ensure outsourced data is continuously available?Naive attempts.Proof of data possession (PDP) [8] and proof of retrievability (PoR) [31] are cryptographic protocols that enable a client to efficiently verify whether data stored at the cloud provider is intact and available.The original PoR scheme, based on hidden sentinels, worked only for encrypted files and a limited number of challenges.PDP, based on homomorphic tags, had no such restrictions and offered public verifiability where everybody, not just the data owner, can verify the proofs.The field evolved rapidly and schemes with better efficiency[44], [9], [17], [45], [5], or advanced properties [50], [51], [20] were introduced.New PoR schemes