Differentially private data sharing in a cloud federation with blockchain

Cloud federation is an emergent cloud-computing paradigm that allows services from different cloud systems to be aggregated in a single pool. To support secure data sharing in a cloud federation, anonymization services that obfuscate sensitive datasets under differential privacy have been recently proposed. However, by outsourcing data protection to the cloud, data owners lose control over their data, raising privacy concerns. This is even more compelling in multi-query scenarios in which maintaining privacy amounts to controlling the allocation of the so-called privacy budget. In this paper, we propose a blockchain-based approach that enables data owners to control the anonymization process and that enhances the security of the services. Our approach relies on blockchain to validate the usage of the privacy budget and adaptively change its allocation through smart contracts, depending on the privacy requirements provided by data owners. Prototype implementation with the Hyperledger permissioned blockchain validates our approach with respect to privacy guarantee and practicality.