Randomness analysis and generation of key-derived s-boxes

Abstract

Although many ciphers use fixed, close to ideal, s-boxes (like AES e.g.), random s-boxes offer an interesting alternative since they have no underlying structure that can be exploited in cryptanalysis. For this reason, some cryptosystems generate pseudorandom s-boxes as a function of the key (key-derived). We analyse the randomness properties of key-derived s-boxes generated by some popular cryptosystems like the RC4 stream cipher, and the Blowfish and Twofish block ciphers with the aim of establishing if this kind of s-boxes are indistinguishable from purely random s-boxes. For this purpose we have developed a custom software framework to generate and evaluate random and key-derived s-boxes. We also detail and analyse several mechanisms for the generation of proper key-derived s-boxes, including fixed-point filtering and different sizes based on 8 × 8 s-boxes.