The security of filter generators is provided by the filter function. For the resistance to algebraic attack, functions with maximum algebraic immunity were used for designing filter functions. We find that the existing algebraic immune functions have a strong property of rotation symmetry and present a rotation-symmetric attack on the filter functions. We also discuss the rotation-symmetric property of filter functions and its influence on the rotation-symmetric attack. After the survey of the vulnerability of algebraic immunity function to the rotation-symmetric attack, we give a new criterion for the choice of filter function.
