  In 2004, Das, Saxena and Gulati proposed a dynamic ID-based remote user
authentication scheme which has many advantage such as no verifier table, user
freedom to choose and change password and so on. However the subsequent papers
have shown that this scheme is completely insecure and vulnerable to many
attacks. Since then many schemes with improvements to Das et al's scheme has
been proposed but each has its pros and cons. Recently Yan-yan Wang et al. have
proposed a scheme to overcome security weaknesses of Das et al.'s scheme.
However this scheme too is vulnerable to various security attacks such as
password guessing attack, masquerading attack, denial of service attack.
