  Some protected password change protocols were proposed. However, the previous
protocols were easily vulnerable to several attacks such as denial of service,
password guessing, stolen-verifier and impersonation atacks etc. Recently,
Chang et al. proposed a simple authenticated key agreement and protected
password change protocol for enhancing the security and efficiency. In this
paper, authors shall show that password guessing, denial of service and
known-key attacks can work in their password change protocol. At the same time,
authors shall propose a new password change protocol to withstand all the
threats of security.
