In 2010, Sood-Sarje-Singh proposed two dynamic ID-based remote user authentication schemes. The first scheme is a security improvement of Liao et al.’s scheme and the second scheme is a security improvement of Wang et al.’s scheme. In both cases, the authors claimed that their schemes can resist many attacks. However, we find that both schemes have security flaws. In addition, their schemes require a verification table and time-synchronization, making the schemes unfeasible and unsecured for electronic services. In order to remedy the security flaws of Sood et al.’s schemes, we propose a robust scheme which resists the well-known attacks and achieves all the desirable security goals.

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