A Formal Foundation for XrML

TL;DR

This paper provides a formal semantics for XrML, addressing algorithmic issues, analyzing complexity, extending the language, and comparing it to MPEG-21, thus strengthening its theoretical foundation and practical applicability.

Contribution

It introduces a formal semantics for XrML, corrects existing algorithmic problems, analyzes complexity, extends the language with negation, and compares it to MPEG-21.

Findings

The corrected semantics accurately captures the algorithm.

Determining permission implication is undecidable in general.

For an expressive fragment, the problem is polynomial-time solvable.

Abstract

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 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 captures the (corrected) algorithm. Next, we consider the complexity of determining if a permission is implied by a set of XrML licenses. We prove that the general problem is undecidable, but it is polynomial-time computable for an expressive fragment of the language. We extend XrML to capture a wider range of licenses by adding negation to the language. Finally, we discuss the key differences between XrML and MPEG-21, an international standard based on XrML.