This book constitutes the refereed proceedings of the 5th International Workshop on Fast Software Encryption, FSE '98, held in Paris, France, in March 1998. The 20 revised full papers presented were carefully reviewed and selected from a total of 39 submissions. The volume is divided in topical sections on cryptanalysis, new stream ciphers, design construction analysis, hash functions, pseudo-random generators, new block ciphers, and modes of operation.