Understanding DES: The Classic Data Encryption Standard

DES -- Data Encryption Standard -- has been the workhorse of modern cryptography for many decades. It has never been compromised mathematically (not in the open literature, at least), yet, its design notes were never made public either. Many who use it are unaware of how it works. Here we open the DES box and find inside a repetition of sub-boxes in which very simple primitives are at work: substitution, transposition, split, concatenation, and bit-wise operation. DES inside teaches us that complexity is comprised of a lot of simplicity.