One of the advantages of formalizing domain knowledge in OWL ontologies is that one can use reasoning systems to infer implicit information automatically. However, it is not always straightforward to understand why certain entailments are inferred, and others are not. The popular ontology editor Protégé offers two explanation services to deal with this issue: justifications for OWL 2 DL ontologies, and proofs generated by the reasoner ELK for lightweight OWL 2 EL ontologies. Since justifications are often insufficient for explaining inferences, there is thus only little tool support for more comprehensive explanations in expressive ontology languages, and there is no tool support at all to explain why something was not derived. In this paper, we present Evee, a Java library and a collection of plug-ins for Protégé that offers advanced explanation services for both inferred and missing entailments. Evee explains inferred entailments using proofs in description logics up to ALCH. Missing entailments can be explained using counterexamples and abduction. We evaluated the effectiveness and the interface design of our plug-ins with description logic experts, ontology engineers, and students in two user studies. In these experiments, we were able to not only validate the tool but also gather feedback and insights to improve the existing designs.

This paper shows how knowledge representation and reasoning techniques can be used to support organizations in complying with the GDPR, that is, the new European data protection regulation. This work is carried out in a European H2020 project called SPECIAL. Data usage policies, the consent of data subjects, and selected fragments of the GDPR are encoded in a fragment of OWL2 called PL (policy language); compliance checking and policy validation are reduced to subsumption checking and concept consistency checking. This work proposes a satisfactory tradeoff between the expressiveness requirements on PL posed by the modeling of the GDPR, and the scalability requirements that arise from the use cases provided by SPECIAL's industrial partners. Real-time compliance checking is achieved by means of a specialized reasoner, called PLR, that leverages knowledge compilation and structural subsumption techniques. The performance of a prototype implementation of PLR is analyzed through systematic experiments, and compared with the performance of other important reasoners. Moreover, we show how PL and PLR can be extended to support richer ontologies, by means of import-by-query techniques. We prove novel tractability and intractability results related to PL, and some negative results about the restrictions posed on ontology import.

Access control policy is typically defined in terms of attributes, but in many applications it is more natural to define permissions in terms of relationships that resources, systems, and contexts may enjoy. The paradigm of relationship-based access control has been proposed to address this issue, and modal logic has been used as a technical foundation.

The Relationship-Based Access Control (ReBAC) model was recently proposed as a general-purpose access control model. It supports the natural expression of parameterized roles, the composition of policies, and the delegation of trust. Fong proposed a policy language that is based on Modal Logic for expressing and composing ReBAC policies. A natural question is whether such a language is representationally complete, that is, whether the language is capable of expressing all ReBAC policies that one is interested in expressing. In this work, we argue that the extensive use of what we call Relational Policies is what distinguishes ReBAC from traditional access control models. We show that Fong’s policy language is representationally incomplete in that certain previously studied Relational Policies are not expressible in the language. We introduce two extensions to the policy language of Fong, and prove that the extended policy language is representationally complete with respect to a well-defined subclass of Relational Policies.

The security-by-contract (SxC) framework has been recently proposed to address the trust relationship problem of the current security model adopted for mobile devices. The key idea of SxC (similar to the one of model-carrying code) is to augment mobile code with a claim on its security behavior (a contract,) that could be matched against a mobile platform policy before downloading the code. The rational is that, thanks to SxC, a digital signature does not just certify the origin of the code but also bind together the code with a contract. In this paper we address one of the key issue of the SxC paradigm, namely the contract-policy matching problem, proposing a prototype for matching policies with security claims of mobile applications. This result can be considered a key step towards the achievement of the SxC main goal: provide a semantics for digital signatures on mobile code, thus being a step in the transition from trusted code to trustworthy code.