This book constitutes the refereed proceedings of the 4th international Information Security Conference, ISC 2001, held in Malaga, Spain in October 2001. The 37 revised full papers presented were carefully reviewed and selected from 98 submissions. The papers are organized in topical sections on key distribution, protocols, enhancing technologies, privacy, software protection, message hiding, PKI issues and protocols, hardware/software implementations, cryptanalysis and prevention, implementations, non-repudiation techniques, and contracts and auctions.