This book constitutes the thoroughly refereed post-workshop proceedings of the 6th International Workshop on Fast Software Encryption, FSE'99, held in Rome, Italy, in March 1999. The 22 revised full papers presented were carefully selected from a total of 51 submissions during two rounds of reviewing and revision. The volume is divided into sections on advanced encryption standard (AES), remotely keyed encryptions, analysis of block ciphers, miscellaneous, modes of operation, and stream ciphers.