Stream cipher is an important device of the GSM system. A secure stream cipher is based on the secure key generator. Lo and Chen proposed a new key generator for the GSM network. Based on the basic architecture of their key generator, they designed three stream ciphers for different security levels of the GSM network. They claimed that the output of their key generator has a long period and satisfies randomness. Consequently, they claimed that the stream ciphers are secure. This paper shows that their key generator and stream ciphers have some weakness. Their stream cipher architectures are not secure.
International Journal of Internet Protocol Technology 1(1), pp.30-33