E. Weippl:
"Interactivity - A Possible Answer to Fundamental Security Threats to E-Learning Content: Securing E-Textbooks";
in: "Proceedings of ED-MEDIA", AACE, 2004, 363 - 369.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC04968632