Pardon me if this has already been mentioned here -- I haven't noticed it -- but an article of considerable interest to this group has just been published.. The May 1994 issue of the IBM Journal of Research and Development contains "The Data Encryption Standard (DES) and its strength against attacks" by Don Coppersmith of IBM Research, who participated in the design and testing of DES, particularly in the design of the famous S-boxes. Included in the paper is a discussion of differential cryptanalysis and what was done to thwart that kind of attack. Peter Capek