The frameworks for protecting security and privacy can be effective only if common users—with no training in computer science or logic—increase their awareness and control over the policy applied by the systems they interact with. Towards this end, we introduce a mechanism for answering why, why-not, how-to, and what-if queries on rule-based policies for trust negotiation. Our framework is lightweight and scalable but it fulfills the main goals of modern explanation facilities. We adopt a novel tabled explanation structure, that simultaneously shows local and global (intra-proof and inter-proof) information, thereby facilitating navigation. Answers are focussed by removing irrelevant parts with suitable heuristics.
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