User data has become a valuable economic resource for many companies. While on the one hand user data may help companies improve their services and gain a better understanding of potential customers, on the other hand, the proliferation of user data is usually accompanied by a loss of user privacy. In the case of platforms like Facebook, Twitter, and Google, users are essentially trading their personal information for services and are often unaware of the potential consequences this trade may have on their private life. This paper presents an approach that allows users to store their data on their own smartphone and have full control over their data, allowing them to specify which companies or organizations obtain their personal data. At the same time, the blockchain technology will be used to realize a system for reliable negotiations involving user data without a central entity. Our approach makes it impossible for either the data provider or data collector to perform manipulations or cheat once a data exchange contract has been signed.
