Improved secure fuzzy auditing protocol for cloud data storage

Nowadays, more and more people prefer to outsource their storage to the cloud; however, due to some accidents, cloud storage service providers may lose some data outsourced by the data owners. Thus a mechanism to ensure the outsourced cloud data remaining intact is needed for smoothly running the cloud storage service. Fuzzy cloud auditing protocol is such a mechanism running between data owners and cloud storage service providers. In these protocols, the data owner fuzzy challenges the cloud storage servers on the randomly chosen data blocks with random values, the servers need to response with corrected aggregated tag proof to pass through the auditing process. Until now, there are many fuzzy cloud auditing protocols with various interesting properties. In 2015, Yuan et al. proposed an auditing scheme supporting publicly integrity checking and dynamic data sharing with multi-user modification, which aims at allowing multiple cloud users to modify data while ensuring the cloud data's integrity. Also recently Yuan et al. proposed a public proofs of retrievability (POR) in cloud with constant cost, they showed their scheme is the first POR scheme which can simultaneously achieve public verifiability, constant communication and computational costs on users, and prove the security of their scheme. However, in this paper, we show their schemes are not secure, concretely, the tags in their schemes can be easily forged. We also give an improved fuzzy cloud auditing scheme for the data owners.