Cryptanalysis of two user identification schemes with key distribution preserving anonymity

In 2004, Wu-Hsu proposed an efficient identification scheme preserving anonymity. However, Yang et al. showed that Wu-Hsu's scheme has a serious weakness, by which the service provider can learn the secret token of the user who requests services. To overcome this limitation, they further proposed a scheme to attain the same set of objectives as the previous works. Nevertheless, the two schemes still have other serious weaknesses. Accordingly, the current paper demonstrates the vulnerability of the two schemes. Furthermore, we present a method to avoid attack.