Constructing Large Cryptographically Strong S-boxes

While there is evidence that large substitution boxes (S-boxes) have better cryptographic properties than small S-boxes, they are much harder to design. The difficulty arises from the relative scarcity of suitable boolean functions as the size of the S-box increases. We describe the construction of cryptographically strong 5×5 S-boxes using near-bent boolean functions of five variables. These functions, where the number of variables is odd, possess highly desirable cryptographic properties and can be generated easily and systematically. Moreover, the S-boxes they compose are shown to satisfy all the important design criteria. Further, we feel that it is possible to generalize near-bent functions to any odd number of variables, thereby making construction of yet larger S-boxes feasible.