Security analysis of a new stream cipher

From 1st February of 2004, Europe launches the ECRYPT project, which collects lots of stream ciphers from all over the world. These new stream ciphers are unlike the tradi- tional stream ciphers that use LFSRs as basic building blocks; instead they use mo…

Security analysis of a new stream cipher

In this paper, we analyze the security of a new stream cipher-COSvd(2, 128). This cipher was proposed by E. Filiol et al. at the ECRYPT SASC’2004 (The State of the Art of Stream Ciphers). It uses clock-controlled non-linear feedback registers together with an S-box controlled by a chaotic sequence and was claimed to prevent any existing attacks. However, our analysis shows that there are some serious security flaws in the design of the S-box, resulting in heavy biased byte distribution in the keystream. In some broadcast applications, this flaw will cause a ciphertext-only attack with high success rate. Besides, there are also many security flaws in other parts of the cipher. We point out these flaws one by one and develop a divide-and-conquer attack to recover the secret keys from O(2²⁶)-byte known plaintext with success rate 93.4597% and complexity O(2¹¹³), which is much lower than 2⁵¹², the complexity of exhaustive search.