The recent approval of the General Data Protection Regulation (GDPR) imposes new data protection requirements on data controllers and processors with respect to the processing of European Union (EU) residents' data. These requirements consist of a single set of rules that have binding legal status and should be enforced in all EU member states. In light of these requirements, we propose in this paper the use of a blockchain-based approach to support data accountability and provenance tracking. Our approach relies on the use of publicly auditable contracts deployed in a blockchain that increase the transparency with respect to the access and usage of data. We identify and discuss three different models for our approach with different granularity and scalability requirements where contracts can be used to encode data usage policies and provenance tracking information in a privacy-friendly way. From these three models we designed, implemented, and evaluated a model where contracts are deployed by data subjects for each data controller, and a model where subjects join contracts deployed by data controllers in case they accept the data handling conditions. Our implementations show in practice the feasibility and limitations of contracts for the purposes identified in this paper.
An example of users' data protection and privacy policy enforcement on a blockchain is provided in @cite_2 . The authors propose to control access permissions to private data collected by a service (e.g., location from a mobile phone) through a Bitcoin blockchain. Every time a users subscribes to a service a new transaction specifies the access permissions and another contains the hash of the data, which are stored in a off-chain database. Policies encoded in a protocol executed by the blockchain grant or deny access to the data referenced in the chain. Although this solution is in part similar to ours, their proposed policies only specify simple allow deny enforcement (i.e. white blacklisting) without the possibility to express more complex policy conditions. Moreover, scalability is not taken into consideration: issuing minimum two transactions for every subscription of a user application to an online service would easily saturate a Bitcoin network even considering few service providers.
@cite_2
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Access Control systems are used in computer security to regulate the access to critical or valuable resources. The rights of subjects to access such resources are typically expressed through access control policies, which are evaluated at access request time against the current access context. This paper proposes a new approach based on blockchain technology to publish the policies expressing the right to access a resource and to allow the distributed transfer of such right among users. In our proposed protocol the policies and the rights exchanges are publicly visible on the blockchain, consequently any user can know at any time the policy paired with a resource and the subjects who currently have the rights to access the resource. This solution allows distributed auditability, preventing a party from fraudulently denying the rights granted by an enforceable policy. We also show a possible working implementation based on XACML policies, deployed on the Bitcoin blockchain.