  Decentralization initiatives like Solid enable data owners to control who has
access to their data and to stimulate innovation by creating both application
and data markets. Once data owners share their data with others, though, it is
no longer possible for them to control how their data are used. To address this
issue, we propose a usage control architecture to monitor compliance with usage
control policies. To this end, our solution relies on blockchain and trusted
execution environments. We demonstrate the potential of the architecture by
describing the various workflows needed to realize a motivating use case
scenario for data markets. Additionally, we discuss the merits of the approach
from privacy, security, integrateability, and affordability perspectives.
