Attack on a Constant-round Authenticated Group Key Exchange Scheme

A constant-round authenticated group key exchange for dynamic groups scheme has been proposed by Kim et al., (2004). In this paper, an impersonation attack on this scheme is proposed. It shows that any two malicious users can impersonate an honest user to agree some session keys in a new group if these two malicious users have the previous authentication transcripts of this honest user. So, this constant-round authenticated group key exchange scheme can not provide the authenticity as claimed. A proposal to repair this scheme's flaw is proposed in the end