SBoxScope: A Meta S-box Strength Evaluation Framework for Heterogeneous Confusion Boxes
Abrar Ahmad, Muddassar Farooq, Muhammad Amin · 2016
In cipher algorithms -- both block or streaming -- the most important non-linear component is a confusion box (commonly termed as s Substitution box or an S-box). The designers of cipher algorithms create an S-box on the basis of a unique formal model, as a result, its parameters -- including its size -- are different. Consequently, it becomes a daunting task for a cryptanalyst to conduct a comparative study to analyze, in a scientific yet unbiased manner, the cryptographic strength of these heterogeneous S-boxes. The major contribution of this paper is SBoxScope -- a meta S-Box strength evaluation framework -- that enables designers and analysts to evaluate cryptographic strength of heterogeneous S-boxes. The framework consists of two layers: (1) White Box Layer analyzes the contents of an S-box and calculates 8 relevant parameters (5 core and 3 auxiliary) and then normalizes them to draw conclusions about the strength of an S-box, (2) Black Box Layer assumes that no knowledge is available about the contents of an S-box, rather, it gives a predefined input bit stream to each S-box and then applies NIST tests to measure 10 parameters. Finally, the two layer are augmented that empowers an analyst to make a decision about the strength of an S-box after analyzing 18 different parameters. In this paper, we have evaluated 9 S-boxes of five well known cipher algorithms: AES, MARS, Skipjack, Serpent and Twofish.