Crypto Archeology: Unearthing Design Methodologies of DES S-Boxes

Abstract

US defense sponsored the DES program in 1971 and released it on 1977. It remained as a well-known and well-accepted de-facto standard of block ciphers until 1998. Thirty-two 4-bit DES S-Boxes are grouped in eight each having four and are put in public domain without any mention of their design methodology. Due to this discovery of substitution Boxes, S-Boxes, 4-bit, 8-bit or 32-bit, find a permanent seat in all future block ciphers. In this paper, a brief study on Crypto relevant properties of 4-bit Boolean Functions as well as 4-bit S-Boxes has been elaborated. The design Methodology of 32 4-bit DES S-Boxes have been of keen interest of this paper. The methodology of such 4-bit DES S-boxes that remained unturned due to the interference of NIST, have also been elaborated in this paper.