Abstract: In designing ciphers, safety, is a problem first consindered by clesigners. The feedforward network is an important stream ciphers generators. The correlation immunity functions presented by Siegenthaler is a class of functions which ressists correlation attacks to the feedforward network stream ciphers generators.Using minterm sum form of Boolean funtions, we discuss the characteristic of correlation immunity functions from combination. This is an ideal method to structure correlation immunity function...