When accessing remote services over public networks, a user authentication mechanism is required because these activities are executed in an insecure communication environment. Recently, Wang et al. proposed an authentication and key agreement scheme preserving the privacy of secret keys and providing user anonymity. Later, Chang et al. indicated that their scheme suffers from two security flaws. First, it cannot resist DoS (denial-of-service) attack because the indicators for the next session are not consistent. Second, the user password may be modified by a malicious attacker because no authentication mechanism is applied before the user password is updated. To eliminate the security flaws and preserve the advantages of Wang et al.'s scheme, we propose an improvement in this paper.
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