Comments on Gateway-Oriented Password-Based Authenticated Key Exchange Protocol

A gateway-oriented password-based authenticated key exchange (GPAKE) scheme allows a client to establish an authenticated session key with a gateway via the help of an authentication server, where the client has pre-shared a password with the server. The desirable security properties of a GPAKE include session key semantic security, key privacy against the server, and password guessing attacks resistance. This letter shows that both Byun et al.'s schemes failed to commit the security requirements. The improved scheme would be proposed in the extended version.