Cloud storage providers such as Dropbox and Google drive heavily rely on data deduplication to save storage costs by only storing one copy of each uploaded file. Although recent studies report that whole file deduplication can achieve up to 50% storage reduction, users do not directly benefit from these savings—as there is no transparent relation between effective storage costs and the prices offered to the users.
In this paper, we propose a novel storage solution, ClearBox, which allows a storage service provider to transparently attest to its customers the deduplication patterns of the (encrypted) data that it is storing. By doing so, ClearBox enables cloud users to verify the effective storage space that their data is occupying in the cloud, and consequently to check whether they qualify for benefits such as price reductions, etc. ClearBox is secure against malicious users and a rational storage provider, and ensures that files can only be accessed by their legitimate owners. We evaluate a prototype implementation of ClearBox using both Amazon S3 and Dropbox as back-end cloud storage. Our findings show that our solution works with the APIs provided by existing service providers without any modifications and achieves comparable performance to existing solutions.