1,066 research outputs found
GEM: a Distributed Goal Evaluation Algorithm for Trust Management
Trust management is an approach to access control in distributed systems
where access decisions are based on policy statements issued by multiple
principals and stored in a distributed manner. In trust management, the policy
statements of a principal can refer to other principals' statements; thus, the
process of evaluating an access request (i.e., a goal) consists of finding a
"chain" of policy statements that allows the access to the requested resource.
Most existing goal evaluation algorithms for trust management either rely on a
centralized evaluation strategy, which consists of collecting all the relevant
policy statements in a single location (and therefore they do not guarantee the
confidentiality of intensional policies), or do not detect the termination of
the computation (i.e., when all the answers of a goal are computed). In this
paper we present GEM, a distributed goal evaluation algorithm for trust
management systems that relies on function-free logic programming for the
specification of policy statements. GEM detects termination in a completely
distributed way without disclosing intensional policies, thereby preserving
their confidentiality. We demonstrate that the algorithm terminates and is
sound and complete with respect to the standard semantics for logic programs.Comment: To appear in Theory and Practice of Logic Programming (TPLP
On definability of team relations with k-invariant atoms
We study the expressive power of logics whose truth is defined over sets of assignments, called teams, instead of single assignments. Given a team X, any k-tuple of variables in the domain of X defines a corresponding k-ary team relation. Thus the expressive power of a logic L with team semantics amounts to the set of properties of team relations which L-formulas can define. We introduce a concept of k-invariance which is a natural semantic restriction on any atomic formulae with team semantics. Then we develop a novel proof method to show that, if L is an extension of FO with any k-invariant atoms, then there are such properties of (k+1)-ary team relations which cannot be defined in L. This method can be applied e.g. for arity fragments of various logics with team semantics to prove undefinability results. In particular, we make some interesting observations on the definability of binary team relations with unary inclusion-exclusion logic.publishedVersionPeer reviewe
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