Enhanced criteria on differential uniformity and nonlinearity of cryptographically significant functions

The functions defined on finite fields with high nonlinearity are important primitives in cryptography. They are used as the substitution boxes in many block ciphers. To avoid the differential and linear attacks on the ciphers, the Sboxes must have low differential uniformity and high nonlinearity. In this paper, we generalize the notions of the differential uniformity and nonlinearity, which are called the t - th differential uniformity and the diversity of the nonlinearity , to measure the nonlinear property of the functions. We show that the Sboxes endorsed by ZUC, SNOW 3G and some lightweight block ciphers have poor performances under these new criteria. The properties and characterizations of these new notions are presented. Another contribution of this paper is to study the nonlinearity of the functions with the form F = f ∘ a , where f is from 𝔽_2^k to 𝔽_2^n and a is a linear surjection from 𝔽_2^n to 𝔽_2^m . The motivation of this study is that such a substitution-permutation composition structure is widely used in the design of modern ciphers, which is to bring the confusion and diffusion to the ciphers. We determine the nonlinearity of F for the linear function a with certain property. Using this result, we compute the diversity of the nonlinearity for F and f . It is found that the former value is greatly amplified, which weakens the ciphers against the linear attack.