Remarks on the new attack on the filter generator and the role of high order complexity

Abstract

Filter generators are important building blocks of stream ciphers and have been studied extensively. Recently, a new attack has been proposed. In this paper, we analyze this attack using the trace representation of the output sequence y and we prove that the attack does not work always as expected. We propose a new algorithm that covers the cases that the attack cannot be applied. The new attack is as efficient as the original attack. Finally, trying to motivate the research on the nonlinear complexity of binary sequences, we present a scenario where the knowledge of the quadratic complexity of a sequence can decrease significantly the necessary for the attack amount of known keystream bits. © Springer-Verlag Berlin Heidelberg 2007.