This book constitutes the refereed proceedings of the 6th International workshop on Cryptographic Hardware and Embedded Systems, CHES 2004, held in Cambridge, MA, USA in August 2004.The 32 revised full papers presented were carefully reviewed and selected from 125 submissions. The papers are organized in topical sections on side channels, modular multiplication, low resources, implementation aspects, collision attacks, fault attacks, hardware implementation, and authentication and signatures.