On the Deniability of Some Deniable Authentication Protocols

A deniable authentication protocol enables the protocol participants to authenticate their respective peers, while able to deny their participation after the protocol execution. This protocol can be extremely useful in some practical applications such as online negotiation and electronic voting. In 2007, Ma et al. proposed a chameleon-based deniable authenticated key agreement protocol while in 2006, Lin-Chang proposed another improved deniable authentication protocol. However, we discover that both these schemes are in fact undeniable due to some flaws in their protocol design. In this paper, we prove the opposite of their claims and discuss the undeniability of their schemes in detail.