This volume is based on a course held several times, andagain in 1993, at the ESAT Laboratorium of the Department ofElectrical Engineering at the Katholieke Universiteit Leuvenin Belgium. These courses are intended for both researchersin computer security and cryptography and for practitionersin industry and government.The contributors of the 1991 course were invited to submitrevised and updated versions of their papers for inclusionin 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, andbetween technical developments and standardization issues.Some of the topics are public key cryptography, hashfunctions, secure protocols, digital signatures, securityarchitectures, network security, and data encryptionstandards (DES).