A Simple Attack on a Recently Introduced Hash-Based Secure User Authentication Scheme

User authentication is an important service in network security. Recently, several user authentication protocols have been proposed. However, a scheme which withstands all known attacks is not yet available. The Lee-Li-Hwang (LLH) authentication scheme [3] was proposed to circumvent the guessing attack in the Peyravian-Zunic (PZ) password scheme [6]. However, Yoon, Ryu, and Yoo (YRY) [9] discovered that the LLH scheme still suffers from the denial of service attack, and proposed an enhancement for the LLH scheme to solve its security problems. More recently, Ku, Chiang, and Chang (KCC) [2] demonstrated that the YRY scheme is vulnerable to the offline guessing and the stolen-verifier attacks. In this paper, we show that the YRY scheme is also vulnerable to the denial-ofservice attack. Furthermore, it was also claimed in [2] that the YRY scheme cannot achieve backward secrecy. We show in this paper that this claim is not entirely valid.