An authorization model for multimedia digital libraries

In this paper we present a generalized authorization model for multimedia digital libraries. Our aim is to support the enforcement of access control requirements of the original data sources without the need to create a new, unified model for the digital library. We integrate the three most widely used access control models (i.e., mandatory, discretionary, and role-based) within a single framework, allowing seamless accesses to data protected by these security models. In particular, we address the access control needs of continuous media data while supporting quality of service (QoS) requirements and preserving operational semantics. The technical core of the paper focuses on the development of metadata and the corresponding metastructure to represent authorization policies and QoS requirements and shows their applicabilty to continuous media. We define our security objects based on the Synchronized Multimedia Integration Language (SMIL), which controls multimedia presentations. Following the synchronization constructs par and seq of SMIL, we define a normal form for multimedia streams, called SMIL normal form. SMIL normal form provides a syntax-independent representation of semantically equivalent multimedia data. SMIL normal form compositions are extended (decorated) with RDF statements, representing security and QoS metadata. Interpretation of these statements and, therefore, the authorization and QoS requirements of the decorated multimedia object are defined by the metastructure, represented as a DAML+OIL ontology. We propose the concept of generalized subject that encompasses all access permissions of a given user regardless of the multiple permissions in different access control models. Finally, we develop methods to generate secure views for each generalized subject and retrieve them using a secure multimedia server.