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.
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.
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.
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.
A perfect concurrent signature protocol proposed by Huang et al in 2007 is analyzed.Two attacks are given out to show that there are two weaknesses in Huang's protocol: firstly,both participators A and B can change the signed messages after the concurrent signature protocol is completed;secondly,both A and B can forge a concurrent signature themselves.To prevent such forgery attacks,an improved protocol is proposed by adding authentication to the keystone information,and then the security of the improvement is analyzed.
When using digital signatures, we need to deal with the problem of fairness of information exchange. To solve this problem, Chen, etc. introduced a new conception which is named concurrent signatures in Eurocrypt'04. Using concurrent signatures scheme, two entities in the scheme can generate two ambiguous signatures until one of the entities releases additional information which is called keystone. After the keystone is released, the two ambiguous signatures will be bound to their real signers at the same time. In order to provide a method to solve the fairness problem of quantum digital signatures, we propose a new quantum concurrent signature scheme. The scheme we proposed does not use a trusted third party in a quantum computing environment, and has such advantages as no need to conduct complex quantum operations and easy to implement by a quantum circuit. Quantum concurrent signature improves the theory of quantum cryptography, and it also provides broad prospects for the specific applications of quantum cryptography.
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.