On the suitability of power functions as S-boxes for symmetric cryptosystems
I present some results towards a classification of power functions that are Almost Perfect Nonlinear (APN), or equivalently differentially 2-uniform, over F2n for infinitely many positive integers n. APN functions are useful in constructing S-boxes in AES-like cryptosystems. An application of a theorem by Weil  on absolutely irreducible curves shows that a monomial x m is not APN over F2n for all sufficiently large n if a related two variable polynomial has an absolutely irreducible factor defined over F2. I will show that the latter polynomial’s singularities imply that except in five cases, all power functions have such a factor. Three of these cases are already known to be APN for infinitely many fields. The last two cases are still unproven. Some specific cases of power functions have already been known to be APN over only finitely many fields, but they also follow from the results below.