Multi-file proofs of retrievability for cloud storage auditing

Cloud storage allows clients to store a large amount of data with the help of storage service providers (SSPs). Proof-of-retrievability(POR) protocols allow one server to prove to a verifier the availability of data stored by some client. Shacham et al. presented POR protocols based on homomorphic authenticators and proved security of their schemes under a stronger security model, which requires the existence of an extractor to retrieve the original file by receiving the program of a successful prover. When using their POR protocol with public verifiability to verify the availability of multiple files separately, the number of pairing operations computed by a verifier is linear with the number of files. To improve the heavy burden on the verifier, we introduce a notion called multi-proof-of-retrievability(MPOR), allowing one verifier to verify the availability of multiple files stored by a server in one pass. We also design a MPOR protocol with public verifiability by extending the work of Shacham et al. The advantage of our MPOR scheme is that computational overhead of a verifier in our scheme is constant, independent of the number of files. Nevertheless, the soundness of our MPOR protocol is proved under a relatively weak security notion. In particular, analysis of our MPOR protocol shows that each file can be extracted in expected polynomial time under certain restriction on the size of processed files.