An Algorithm for Allocating User Requests to Licenses in the OMA DRM System

Abstract

Nikolaos TRIANTAFYLLOU†a) , Student Member, Petros STEFANEAS † †b) , and Panayiotis FRANGOS †c) , Nonmembers SUMMARYThe Open Mobile Alliance (OMA) Order of Rights Object Evaluation algorithm causes the loss of rights on contents under certain circumstances. By identifying the cases that cause this loss we suggest an algebraic characterization, as well as an ordering of OMA licenses. These allow us to redesign the algorithm so as to minimize the losses, in a way suitable for the low computational powers of mobile devices. In addition we provide a formal proof that the proposed algorithm fulfills its intent. The proof is conducted using the OTS/CafeOBJ method for verifying invariant properties. Barth and Mitchel [7] first identified this loss of rights and argued that a correct algorithm should behave monotonically, i.e., if a set of rights is allowed by a set of licenses then this set is also allowed by a set of more flexible licenses (licenses that contain at least the same rights * * ). We suggest an algorithm to overcome the unintentional loss of rights without designing complex and computationally heavy DRM agents that reallocate actions to rights. There exist some cases where our algorithm is not fully monotonic and this is intentional. In the case where some form of loss is inevitable we prompt the user to decide which rights he prefers, so that we may ascertain that those to be lost are the least desired. While this can cause the algorithm to behave non-monotonically, we believe that in this special case an algorithm that respects the desires of the user is preferable to a fully monotonic one.The paper is organized as follows: Sect. 2 briefly overviews related work and gives a short comparison with ours. Section 3 presents OMAs algorithm and the loss of execution rights. In Sect. 4 we give an introduction to order sorted algebra and present our new algorithm. Section 5 introduces the reader to the concepts of Observation Transition Systems (OTS) and the algebraic specification language CafeOBJ. Also in Sect. 5 a specification of a DRM agent using the suggested algorithm is presented and used to prove formally that such a system does not suffer from unintentional loss of rights. Finally Sect. 6 concludes the paper. Related WorkDRM licenses can be regarded as special cases of authorization policies, where the properties have been widely studied in the literature. The discussed problem is similar to the policy reachability problem, i.e., given a policy and a domain, what the sequence of actions that leads to a state satisfying a goal property is. In [16] the authors analyze reachability and availability properties in Administrative Role-Based Access Control policies and prove that reachability analysis is PSPACE-complete for their domain. Dougherty et al., [19], study reachability, availability and containment queries for dynamic access control policies in Datalog and prove that reachability is in NLOGSPACE. * * Note that in this context we are only interested in the monotonicity of the existence of rights an...

Software engineering applications of the OTS/CafeOBJ algebraic specification method

“…In [102] we suggested and in this chapter we will present an algorithm to overcome the unintentional loss of rights without designing complex and computationally heavy DRM agents that reallocate actions to rights. There exist some cases where our algorithm is not fully monotonic and this is intentional.…”