PORTOS: Proof of Data Reliability for Real-World Distributed Outsourced Storage

Proofs of data reliability are cryptographic protocols that provide assurance to a user that a cloud storage system correctly stores her data and has provisioned sufficient redundancy to be able to guarantee reliable storage service. In this paper, we consider distributed cloud storage systems that make use of erasure codes to guarantee data reliability. We propose a novel proof of data reliability scheme, named PORTOS, that on the one hand guarantees the retrieval of the outsourced data in their entirety through the use of proofs of data possession and on the other hand ensures the actual storage of redundancy. PORTOS makes sure that redundancy is stored at rest and not computed on-the-fly (whenever requested) thanks to the use of time-lock puzzles. Furthermore, PORTOS delegates the burden of generating the redundancy to the cloud. The repair operations are also taken care of by the cloud. Hence, PORTOS is compatible with the current cloud computing model where the cloud autonomously performs all maintenance operations without any interaction with the user. The security of the solution is proved in the face of a rational adversary whereby the cheating cloud provider tries to gain storage savings without increasing its total operational cost.