A Formal Foundation for XrML

XrML is becoming a popular language in industry forwriting software licenses. The semantics for XrML is implicitlygiven by an algorithm that determines if a permissionfollows from a set of licenses. We focus on a representativefragment of the language and use it to highlightsome problematic aspects of the algorithm. We then correctthe problems, introduce formal semantics, and showthat our semantics matches the (corrected) algorithm. Finally,we consider the complexity of determining if a permissionis implied by a set of XrML licenses. We show thatthe general problem is NP-hard, but it is polynomial-timecomputable for an expressive fragment of the language.