Background and Definition
The covering radius bound states that the nonlinearity of any -function satisfies
A function is called bent if it achieves this bound with equality.
Since nonlinearity is a CCZ-invariant, CCZ-equivalence (and therefore also EA-equivalence and affine equivalence) preserves the property of a function of being bent.
The algebraic degree of any bent -function is at most .
An -function is bent if and only if all of its derivatives for are balanced. In this sense, bent functions are also referred to as perfect nonlinear (PN) functions.
Bent (PN) -functions exist only for even and . Conversely, for any pair of integers satisfying this hypothesis, there exists a bent -function.
- Nyberg K. Perfect nonlinear S-boxes. InWorkshop on the Theory and Application of of Cryptographic Techniques 1991 Apr 8 (pp. 378-386). Springer, Berlin, Heidelberg.