This volume is based on a course held several times, and again in 1993, at the ESAT Laboratorium of the Department of Electrical Engineering at the Katholieke Universiteit Leuven in Belgium. These courses are intended for both researchers in computer security and cryptography and for practitioners in industry and government. The contributors of the 1991 course were invited to submit revised and updated versions of their papers for inclusion in a book. This volume is the final result; it is well- balanced between basic theory and real life applications, between mathematical background and juridical aspects, and between technical developments and standardization issues. Some of the topics are public key cryptography, hash functions, secure protocols, digital signatures, security architectures, network security, and data encryption standards (DES).