On the Risk of Disruption in Several Multiparty Signature Schemes

Multiparty cryptography is an important topic in contemporary cryptography. In this paper we examine the security of some multi-party signature schemes. In particular, we point out that a multisignature scheme is vulnerable to universal forgery by an insider attacker under reasonable assumptions. This attack can be applied to some generalizations as well. Then we present a universal forgery attack on two threshold group signature schemes with anonymous signers. Furthermore, we show that in two threshold multisignature schemes it can't be guaranteed that a signer can decide with whom he is going to sign a message. All attacks have in common that the protocol is disrupted. Thus they are not undetectable. However, as they can only be detected afterwards and knowledge leaked by protocol disruptions must be useless, such attacks are not acceptable in general and must be avoided. Finally, we suggest some heuristic xes.