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.
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.
The concept of concurrent signatures was introduced by Chen, Kudla and Paterson in their seminal paper in Eurocrypt 2004 which allows two entities to produce two signatures in such a way that, the signer of each signature is ambiguous from any third party's point of view until the release of a secret, known as the keystone. Once the keystone is released, both signatures become binding to their respective signers concurrently. Later, a stronger notion called perfect concurrent signatures with two concrete schemes was proposed by Susilo, Mu and Zhang. In perfect concurrent signatures, although both signers are known to be trustworthy, the two signatures are still ambiguous to any third party. Unfortunately, Susilo et al.'s schemes are unfair. In this paper, we first show that in their schemes the initial signer can cheat the matching signer by carefully choosing some communication value and then propose fair perfect concurrent signature protocol to fix our attacks.
Based on the improved perfect concurrent signature,this paper proposed an abuse-free fair exchange protocol without trusted third party.This protocol avoided the flaw,by which buyer could prove to others that he has done an artificial trade with the shop,existed in other fair exchange protocols based on perfect concurrent signature.In the proposed protocol,buyer and shop bound the order information,payment information,and digital content together in the ambiguity signature which could be verified publicly after the buyer released extra keystones.And the buyer could get the key from the shop to decrypt the cipher of the digital content he wanted after he released keystones.The keystones bound the ambiguity signature,deal information of the buyer and shop together to avoid any one of the signer abuse these information.This protocol could be used in P2P network in which it's hard to find a trusted third party.Also using this protocol could avoid the risk that third party cooperating with one party to cheat others and avoid the performance bottleneck caused by third party.
Based on perfect concurrent signature, a novel fair exchange protocol was presented. In this protocol anyone can’t identify who has signed which signature when two parties have exchanged their two ambiguous signatures and rela- tive data items. Not until the initial signer publishes a secret information, are the two signatures bound to respective signer concurrently. After that, the other party sends the key of decrypting goods, and thus the two parties can fairly get each other’s data items without the trusted third party (TTP), which avoids the possibility of two parties’ collusion and the bottleneck of the TTP involvement. Through analysis, this protocol well ensures some significant properties of fair ex- change, such as fairness, non-repudiation and abuse-free etc. What’s more, it only needs four messages between two par- ties and it is very simple and highly efficient.
The concept of concurrent signatures was introduced by Chen et al. in Eurocrypt 2004 which allows two entities to produce two signatures in such a way that, the signer of each signature is ambiguous from any third party¿s point of view until the release of a secret, known as the keystone. Once the keystone is released, both signatures become binding to their respective signers concurrently. In the previous concurrent signature schemes, the roles of participants are asymmetrical, one party, which is called initial signer, needs to create the keystone fix and send the first ambiguous signature, the other party, which is called matching signer, responds to this initial signer by creating another ambiguous signature with the same keystone fix. This work mode may cause some unfairness. In this paper, we construct a perfect concurrent signature protocol for symmetric participants and prove its security. In our concept, the roles of participants are symmetrical. The keystone can not be decided by any participant and the two ambiguous signatures can be published in any order.
At Eurocrypt 2004, the concept of concurrent signatures was introduced by Chen, Kudla and Paterson. Concurrent signatures contribute a novel approach for the traditional problem of fair exchange of signatures. In this paper a general attack named the message substitute attack to the existing concurrent signatures protocol is proposed. Under the message substitute attack there is no concurrent in the existing concurrent signatures protocols including perfect concurrent signatures (PCS) protocol proposed by Susilo et al and its improvement iPCS1 protocol proposed by Guilin Wang et al. To prevent the message substitute attack, an i2PCS1 protocol which is an improvement on iPCS1 protocol is proposed. Further more an i3PCS1 protocol which is another improvement on iPCS1 protocol is proposed, the distinct advantage of i3PCS1 protocol is that its keystones are not published by the initial signer Alice finally but by the matching signer Bob, so that the matching signer Bob has some advantage over the initial signer Alice, while in almost all previous concurrent signatures protocols the initial signer Alice has some advantage. So the i3PCS1 Protocol differs from the previous concurrent signatures protocols.
We propose a fair novel concurrent signature scheme. It resolves the existing problems in traditional concurrent signature schemes, that is initial signer (A) has some advantages over matching signer (B), such as A could show B's ambiguous signature and the keystone to a third party in private to demonstrate B's binding signature but B can not, or A could keep the keystone secret and B do not have power to force A to release it. In our scheme, the keystones were produced by both A and B, that is to say, the two participants in our scheme have equal positions, no one has any priority than the other, so our scheme can achieve the fairness of both sides.