In ownership-based access control frameworks with the possibility of delegating permissions and administrative rights, chains of delegated accesses will form. There are different ways to treat these delegation chains when revoking rights, which give rise to different revocation schemes. Hagström et al. [8] proposed a framework for classifying revocation schemes, in which the different revocation schemes are defined graph-theoretically; they motivate the revocation schemes in this framework by presenting various scenarios in which the agents have different reasons for revocating. This paper is based on the observation that there are some problems with Hagström et al.'s definitions of the revocation schemes, which have led us to propose a refined framework with new graph-theoretic definitions of the revocation schemes. In order to formally study the merits and demerits of various definitions of revocation schemes, we propose to apply the axiomatic method originating in social choice theory to revocation schemes. For formulating an axiom, i.e. a desirable property of revocation frameworks, we propose a logic, Trust Delegation Logic (TDL), with which one can formalize the different reasons an agent may have for performing a revocation. We show that our refined graph-theoretic definitions of the revocation schemes, unlike Hagström et al.'s original definitions, satisfy the desirable property that can be formulated using TDL.

The recent unprecedented popularity of food recommender applications has raised several issues related to the ethical, societal and legal implications of relying on these applications. In this paper, in order to assess the relevant ethical issues, we rely on the emerging principles across the AI&Ethics community and define them tailored context specifically. Considering the popular Food Recommender Systems (henceforth F-RS) in the European market cannot be regarded as personalised F-RS, we show how merely this lack of feature shifts the relevance of the focal ethical concerns. We identify the major challenges and propose a scheme for how explicit ethical agendas should be explained. We also argue how a multi-stakeholder approach is indispensable to ensure producing long-term benefits for all stakeholders. After proposing eight ethical desiderata points for F-RS, we present a case-study and assess it based on our proposed desiderata points.

Abstract. The complexity of input/output logic has been sparsely developed. In this paper we study the complexity of four existing input/output logics. We show that the lower bound of the complexity of the fulfillment problem of these input/output logics is coNP, while the upper bound is either coNP, or P N P .

Abstract. Input/output logics are abstract structures designed to represent conditional norms. The complexity of input/output logic has been sparsely developed. In this paper we study the complexity of input/output logics. We show that the lower bound of the complexity of the fulfillment problem of 4 input/output logics is coNP, while the upper bound is either coNP or P NP . 3

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