Accountability of Perfect Concurrent Signature

Concurrent signature provided a novel idea for fair exchange protocol without trusted third party. Perfect Concurrent Signature is proposed to strengthen the ambiguity of the concurrent signature. Wang et al, pointed out there exist an attack against the fairness of Perfect Concurrent Signature and proposed the improved perfect concurrent signature. This paper find that in proposed (perfect) concurrent signature protocol, no matter two party or multi-party, the signer could bind multiple messages with one keystone set but let the other signers know only one of the messages. Based on this observation, this paper proposes that accountability should be one of the security properties of (perfect) concurrent signature and we give the definition of accountability of concurrent signature. To illustrate this idea, this paper give an attack scene against the accountability of improved perfect concurrent signature proposed by Wang et al. Finally, this paper proposes an update version of perfect concurrent signature to avoid such attack.