In this paper the entropy preservability of S-Box is discussed and the definitions of condition entropy and entropy preservability of nonlinear S-Box and combining function are presented.Then the sufficient and necessary conditions for judging combining function or S-Box between condition entropy and correlation immunity and the ideal entropy preservability condition of combining function and S-Box are given.Finally

the applied entropy preservability conditions of combining function and S-Box are obtained through analyzing the inherent relation between entropy and Walsh spectrum.