  In today's digital landscape, the Web has become increasingly centralized,
raising concerns about user privacy violations. Decentralized Web
architectures, such as Solid, offer a promising solution by empowering users
with better control over their data in their personal `Pods'. However, a
significant challenge remains: users must navigate numerous applications to
decide which application can be trusted with access to their data Pods. This
often involves reading lengthy and complex Terms of Use agreements, a process
that users often find daunting or simply ignore. This compromises user autonomy
and impedes detection of data misuse. We propose a novel formal description of
Data Terms of Use (DToU), along with a DToU reasoner. Users and applications
specify their own parts of the DToU policy with local knowledge, covering
permissions, requirements, prohibitions and obligations. Automated reasoning
verifies compliance, and also derives policies for output data. This
constitutes a ``perennial'' DToU language, where the policy authoring only
occurs once, and we can conduct ongoing automated checks across users,
applications and activity cycles. Our solution is built on Turtle, Notation 3
and RDF Surfaces, for the language and the reasoning engine. It ensures
seamless integration with other semantic tools for enhanced interoperability.
We have successfully integrated this language into the Solid framework, and
conducted performance benchmark. We believe this work demonstrates a
practicality of a perennial DToU language and the potential of a paradigm shift
to how users interact with data and applications in a decentralized Web,
offering both improved privacy and usability.
