Table 2: Comparison of security attributes.

Security attributesSchemes
Li-Hwang’s [19]Das’s [26]An’s [27]Ours

Resist online     guessing attackNoNoNoYes
Resist offline     guessing attackNoNoNoYes
Resist user impersonation attackNoNoNoYes
Resist server impersonation attackNoNoNoYes
Provides mutual authenticationNoNoNoYes
Provides   change facilityYesYesNoYes
Provides user anonymity NoNoNoYes