### Extremal Generalized S--Boxes

keywords: Quasigroups, linear structures, Boolean functions, perfect nonlinearity

It is well known that there does not exist a Boolean function f: Z_2m rightarrow Z_2n satisfying both basic cryptologic criteria, balancedness and perfect nonlinearity. In citetene it was shown that, if we use as a domain quasigroup G instead of the group Z_2n, one can find functions which are at the same time balanced and perfectly nonlinear. Such functions have completely flat difference table. We continue in our previous work, but we turn our attention to the worst case when all lines of Cayley table of G define so called linear structure of f ( citeDubuc). We solve this problem in both directions: We describe all such bijections f:G rightarrow Z_2n, for a given quasigroup |G|=2n, and describe such quasigroups for a given function f.

reference: Vol. 22, 2003, No. 1, pp. 85–99