A new construction of differentially 4uniform $(n,n1)$functions
Claude Carlet  Department of Mathematics, LAGA, University of Paris 8, (and LAGA, University of Paris 13, CNRS), France (email) Abstract: In this paper, a new way to construct differentially 4uniform $(n,n1)$functions is presented. As APN $(n,n)$functions, these functions offer the best resistance against differential cryptanalysis and they can be used as substitution boxes in block ciphers with a Feistel structure. Constructing such functions is assumed to be as difficult as constructing APN $(n,n)$functions. A function in our family of functions can be viewed as the concatenation of two APN $(n1,n1)$functions satisfying some necessary conditions. Then, we study the special case of this construction in which the two APN functions differ by an affine function. Within this construction, we propose a family in which one of the APN functions is a Gold function which gives the quadratic differentially 4uniform $(n,n1)$function $(x,x_n)\mapsto x^{2^i+1}+x_n x$ where $x\in \mathbb{F}_{2^{n1}}$ and $x_n\in \mathbb{F}_2$ with $\gcd(i,n1)=1$. We study the nonlinearity of this function in the case $i=1$ because in this case we can use results from Carlitz which are unknown in the general case. We also give the Walsh spectrum of this function and prove that it is CCZinequivalent to functions of the form $L \circ F$ where $L$ is an affine surjective $(n,n1)$function and $F$ is a known APN $(n,n)$function for $n\leq 8$, or the Inverse APN $(n,n)$function for every $n\geq 5$ odd, or any AB $(n,n)$function for every $n>3$ odd, or any Gold APN $(n,n)$function for every $n>4$ even.
Keywords: Block ciphers, Sboxes, vectorial Boolean functions, APN functions,
dierentially 4uniform functions.
Received: May 2014; Revised: January 2015; Available Online: November 2015. 
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