A note on ``Improved Fast Correlation Attacks on Stream Ciphers"
In SAC'08, an improved fast correlation attack on stream ciphers was proposed. This attack is based on the fast correlation attack proposed at Crypto'00 and combined with the fast Walsh transform. However, we found that the attack results are wrong. In this paper, we correct the results of the attack algorithm by analyzing it theoretically. Also we propose a threshold of the valid bias.