Security analysis of a flexible forward-secure signature scheme

Huang and Chang proposed a flexible forward-secure signature scheme in International Journal of Computer Science and Network Security in 2006. In this paper, we give the security analysis of this scheme and point out this scheme doesn´t satisfy the forward security. The main reason is that some secret parameters are used and these secret parameters should be viewed as a part of secret key. If an adversary gets the secret key in one time period, she can forge the signatures pertaining to past time periods. In order to analyse the security, we also introduce the security of forward-secure signature scheme.