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（2^26）-byte known plaintext with success rate 93.4597% and complexity O（2^113）, which is much lower than 2^512, the complexity of exhaustive search.
Ω-protocols, introduced by Garay, Mackenzie and Yang, is a variant of S-protocols with online extractor which is a useful tool to overcome the nest effect in concurrent scenario. In this work, we construct an Ω-protocol for Hamiltonian cycle problem, and therefore, it allows us to present Ω-protocol for any NP relation. For most general NP relations, our construction of Ω-protocols is much more efficient than the informal one described by Garay et ah and we believe that the method for our construction may be of independent interest.
