text: 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.
  @cite_2: The recent increase in reported incidents of surveillance and security breaches compromising users' privacy call into question the current model, in which third-parties collect and control massive amounts of personal data. Bit coin has demonstrated in the financial space that trusted, auditable computing is possible using a decentralized network of peers accompanied by a public ledger. In this paper, we describe a decentralized personal data management system that ensures users own and control their data. We implement a protocol that turns a block chain into an automated access-control manager that does not require trust in a third party. Unlike Bit coin, transactions in our system are not strictly financial -- they are used to carry instructions, such as storing, querying and sharing data. Finally, we discuss possible future extensions to block chains that could harness them into a well-rounded solution for trusted computing problems in society.
summary: 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.