  Blockchain has the potential to revolutionize the way we store, use, and
process data. Information on most blockchains can be viewed by every node
hosting the blockchain, which means that most blockchains cannot handle private
data. Decentralized databases exist that guarantee privacy by encrypting user
data with the user's private key, but this prevents easy data sharing. However,
in many real world applications, from student data to medical records, it is
desirable that user data is anonymously searchable. In this paper we present a
novel system that gives users ownership over their data while at the same time
enabling them to make their data searchable within previously agreed upon
limits. Our system implements a strong notion of ownership using a
self-sovereign identity system and a weak notion of ownership using multiple
centralized databases together with a blockchain and a tumbling process. We
discuss applications of our methods to university's student records and medical
data.
