Most of the password based authentication protocols make use of the single authentication server for user's authentication. User's verifier information stored on the single server is a main point of susceptibility and remains an attractive target for the attacker. On the other hand, multi-server architecture based authentication protocols make it difficult for the attacker to find out any significant authentication information related to the legitimate users. In 2009, Liao and Wang proposed a

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... namic identity based remote user authentication protocol for multi-server environment. However, we found that Liao and Wang's protocol is susceptible to malicious server attack and malicious user attack. This paper presents a novel dynamic identity based authentication protocol for multi-server architecture using smart cards that resolves the aforementioned flaws, while keeping the merits of Liao and Wang's protocol. It uses two-server paradigm by imposing different levels of trust upon the two servers and the user's verifier information is distributed between these two servers known as the service provider server and the control server. The proposed protocol is practical and computational efficient because only nonce, one-way hash function and XOR operations are used in its implementation. It provides a secure method to change the user's password without the server's help. In e-commerce, the number of servers providing the services to the user is usually more than one and hence secure authentication protocols for multi-server environment are required.