Efficient Computation of Algebraic Immunity for Algebraic and Fast Algebraic Attacks

In this paper we propose several efficient algorithms for assessing the resistance of Boolean functions against algebraic and fast algebraic attacks when implemented in LFSR-based stream ciphers. An algorithm is described which permits to compute the algebraic immunity d of a Boolean function with n variables in O(D) operations, for D ≈ ` n d ́ , rather than in O(D) operations necessary in all previous algorithms. Our algorithm is based on a multivariate polynomial interpolation and is extended to compute the whole annihilator space which can be used in some cases to efficiently determine the resistance against fast algebraic attacks. For assessing the vulnerability of arbitrary Boolean functions with respect to fast algebraic attacks, an efficient generic algorithm is presented that is not based on interpolation. This algorithm is demonstrated to be particularly efficient for symmetric Boolean functions. As an application it is shown that large classes of symmetric functions are very vulnerable to fast algebraic attacks despite their proven resistance against conventional algebraic attacks.