This book constitutes the thoroughly refereed post-workshop proceedings of the 6th Annual International Workshop on Selected Areas in Cryptography, SAC'99, held in Kingston, Ontario, Canada in August 1999. The 17 revised full papers presented went through two rounds of reviewing and revision and were selected from 29 submissions. The papers are organized in sections on cryptosystems and pseudorandom number generators, security aspects of block cyphers, cryptoanalysis of block cyphers, efficient implementations of cryptosystems, and cryptography for network applications.