The Internet (and networks overall) are currently the core media for data and knowledge exchange. XML is currently the most popular standardization for Web document representation and is rapidly becoming a standard for data representation and exchange over the Internet. One of the main issues is XML documents and in particular, storage and accessing. Among data management issues, storage and security techniques have a particular importance, since the performance of the overall XML-based Web information system relies on them. Storage issues mainly rely on the usage of typical database management systems (DBMSs), whereas XML documents can also be stored in other storage environments (such as file systems and LDAP directories) (Amer-Yahia & Fernandez, 2002; Kanne & Moerkotte, 2000; Silberschatz, Korth & Sudarshan, 2002). Additionally, in order to guarantee the security of the XML data, which are located in a variety of the above storage topologies, the majority of implementations also provide an appropriate access control. Most storage systems cooperate with access control modules implementing various models (Joshi, Aref, Ghafoor & Spafford, 2001), whereas there are few commercial access control products available. However, there are some standardized XML-based access control languages that can be adopted by most tools.
