Cryptanalysis and improvement of an extended chaotic maps-based key agreement protocol

Very recently, Lee et?al. (C.?Lee, C.?Chen, C.?Wu, S.?Huang, An extended chaotic maps-based key agreement protocol with user anonymity, Nonlinear Dynamics, doi:10.1007/s11071-011-0247-4) proposed a chaotic maps-based key agreement protocol with user anonymity and claimed their protocol could resist various attacks. In this paper, we will point out that Lee et?al.??s protocol suffers from three weaknesses: (1)?inability of resisting the privileged insider attack; (2)?inability of resisting the denial-of-service attack; and (3)?inability of providing anonymity. To overcome the weaknesses, we also proposed an improved protocol. The analysis shows our protocol is more suitable for practical applications.