Algorithmic Toolkit for Linearization of S-Boxes

Linearization is a cryptanalysis technique in which a nonlinear function (an S-box) is represented by an affine mapping on a certain subset of inputs. Its variants were applied to analyze Keccak, LowMC, RAIN and AIM. In these primitives, the S-boxes are either very small (up to 5 bits) or are very specific monomial functions over a binary field. Linearization of arbitrary S-boxes was never practically explored due to the lack of theoretic, algorithmic, and cryptanalytic understanding.

For the first time, we develop an algorithmic toolkit which allows one to compute strong linearizations of S-boxes, when they exist. For up to $n=8$ bits, our algorithms are able to find provably the best possible approximations, while for larger S-boxes it is feasible to obtain good approximations together with meaningful upper bounds. We apply our algorithms to a variety of S-boxes from existing primitives, to monomial functions, to so-called APN functions, and to 16-bit Super-Sboxes. We obtain interesting results raising many new open questions and open up new research directions, as well as a foundation for developing cryptanalytic attacks.

To advance the cryptanalytic utility of linearization, we study and solve the problem of covering an S-box with multiple approximations. As an application, we derive a generic linearization approach for the CICO problem (constrained-input-constrained-output) over SPN-based permutations (Substitution-Permutation Networks) with general linear layers. This is the first such general cryptanalysis based on the existence of a strong linearization of the S-box.