The Logic of XACML - Extended

TL;DR

This paper formalizes the logic behind XACML 3.0, providing a precise semantics for its policy combining algorithms and addressing previous modeling shortcomings.

Contribution

It introduces a new logic capturing XACML's principles and proves the equivalence of different formalization approaches, improving understanding of access control policies.

Findings

Formal logic for XACML 3.0 established

Equivalence of alternative policy combining algorithms proven

Identified shortcomings in previous formalizations

Abstract

We study the international standard XACML 3.0 for describing security access control policy in a compositional way. Our main contribution is to derive a logic that precisely captures the idea behind the standard and to formally define the semantics of the policy combining algorithms of XACML. To guard against modelling artefacts we provide an alternative way of characterizing the policy combining algorithms and we formally prove the equivalence of these approaches. This allows us to pinpoint the shortcoming of previous approaches to formalization based either on Belnap logic or on D-algebra.