Secure Key Authentication Scheme Based on Discrete Logarithms

In the public key cryptosystems, for obtaining the public key, one can either directly request it from the recipient or retrieve it from the public key directory. In 1999, Zhan et al. proposed an improvement to Horng-Yang´s key authentication scheme based on discrete logarithms that has the same stability as Horng-Yang´s scheme, but with a more secure. The current paper, however, demonstrates that the scheme is vulnerable to public key substitution attack and theft attack, and then presents an improvement of the scheme.