Keywords: Cloud storage, formal methods, attribute based signature, attribute based encryption, data and user privacy. Abstract: Storing data in the Cloud makes challenging data's security and users' privacy. To address these problems cryptographic protocols are usually designed. Cryptographic primitives have to guarantee some security properties such that data and user privacy or authentication. Attribute-Based Signature (ABS) and Attribute-Based Encryption (ABE) are very suitable for storing data on an untrusted remote entity. In this work, we formally analyze the Ruj et al. protocol of cloud storage based on ABS and ABE schemes. We clarify several ambiguities in the design of this protocol and model the protocol and its security properties with ProVerif an automatic tool for the verification of cryptographic protocols. We discover an unknown attack against user privacy. We propose a correction, and automatically prove the security of the corrected protocol with ProVerif.