This volume contains papers presented at the fourth working conference on Communications and Multimedia Security (CMS'99), held in Leuven, Belgium from September 20-21, 1999. The Conference, arrangedjointly by Technical Committees 11 and 6 of the International Federation of Information Processing (IFIP), was organized by the Department of Electrical Engineering of the Katholieke Universiteit Leuven. The name "Communications and Multimedia Security" was used for the first time in 1995, when Reinhard Posch organized the first in this series of conferences in Graz, Austria, following up on the...