A formal foundation for XrML

XrML is becoming a popular language in industry for writing software licenses. The semantics for XrML is implicitly given by an algorithm that determines if a permission follows from a set of licenses. We focus on a representative fragment of the language and use it to highlight some problematic aspects of the algorithm. We then correct the problems, introduce formal semantics, and show that our semantics matches the (corrected) algorithm. Finally, we consider the complexity of determining if a permission is implied by a set of XrML licenses. We show that the general problem is NP-hard, but it is polynomial-time computable for an expressive fragment of the language.