  Controlled Query Evaluation (CQE) has been recently studied in the context of
Semantic Web ontologies. The goal of CQE is concealing some query answers so as
to prevent external users from inferring confidential information. In general,
there exist multiple, mutually incomparable ways of concealing answers, and
previous CQE approaches choose in advance which answers are visible and which
are not. In this paper, instead, we study a dynamic CQE method, namely, we
propose to alter the answer to the current query based on the evaluation of
previous ones. We aim at a system that, besides being able to protect
confidential data, is maximally cooperative, which intuitively means that it
answers affirmatively to as many queries as possible; it achieves this goal by
delaying answer modifications as much as possible. We also show that the
behavior we get cannot be intensionally simulated through a static approach,
independent of query history. Interestingly, for OWL 2 QL ontologies and policy
expressed through denials, query evaluation under our semantics is first-order
rewritable, and thus in AC0 in data complexity. This paves the way for the
development of practical algorithms, which we also preliminarily discuss in the
paper.
