  In this paper, we present TILT, a transparency information language and
toolkit explicitly designed to represent and process transparency information
in line with the requirements of the GDPR and allowing for a more automated and
adaptive use of such information than established, legalese data protection
policies do.
  We provide a detailed analysis of transparency obligations from the GDPR to
identify the expressiveness required for a formal transparency language
intended to meet respective legal requirements. In addition, we identify a set
of further, non-functional requirements that need to be met to foster practical
adoption in real-world (web) information systems engineering. On this basis, we
specify our formal language and present a respective, fully implemented toolkit
around it. We then evaluate the practical applicability of our language and
toolkit and demonstrate the additional prospects it unlocks through two
different use cases: a) the inter-organizational analysis of personal
data-related practices allowing, for instance, to uncover data sharing networks
based on explicitly announced transparency information and b) the presentation
of formally represented transparency information to users through novel, more
comprehensible, and potentially adaptive user interfaces, heightening data
subjects' actual informedness about data-related practices and, thus, their
sovereignty.
  Altogether, our transparency information language and toolkit allow -
differently from previous work - to express transparency information in line
with actual legal requirements and practices of modern (web) information
systems engineering and thereby pave the way for a multitude of novel
possibilities to heighten transparency and user sovereignty in practice.
