Dynamic Proofs of Retrievability from Chameleon-Hashes

Proofs of retrievability (POR) are interactive protocols that allow a verifier to check the consistent existence and availability of data residing at a potentially untrusted storage provider, e.g., a cloud. While most POR protocols strictly refer to static files, i. e., content that is read-only, dynamic PORs shall achieve the same security guarantees (existence, consistency and the possibility to retrieve the data) for content that is subject to an unlimited number of (legitimate) modifications. This work discusses how to construct such a dynamic proof of retrievability from chameleon hashes (trapdoor commitments). Like standard POR constructions, the presented scheme is sentinel-based and does audit queries via spot checking mechanism. Unlike previous schemes, however, a-posteriori insertions of new sentinels throughout the lifetime of the file is supported. This novel feature is apparently absent in any other POR scheme in the literature. Moreover, the system is designed for compatibility with XML structured data files.