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Journal of Communications

Novel fair exchange protocol based on concurrent signature

Yang Yi-xian

Published Jan 1, 2008• 2 citations
OpenAlex

Abstract

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.

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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.
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.
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.
We present protocols for fair exchange of electronic data (digital signatures, payment and confidential data) between two parties A and B. Novel properties of the proposed protocols include: 1) offline trusted third party (TTP), i.e., TTP does not take part in the exchange unless one of the parties behaves improperly; 2) only three message exchanges are required in the normal situation; 3) true fair exchange, i.e., either A and B obtain each other's data or no party receives anything useful; no loss can be incurred to a party no matter how maliciously the other party behaves during the exchange. This last property is in contrast to previously proposed protocols with offline TTP ([1] and [21]), where a misbehaving party may get another party's data while refusing to send his document to the other party, and the TTP can provide affidavits attesting to what happened during the exchange. To our knowledge, the protocols presented here are the first exchange protocols which use offline TTP and at the same time guarantee true fair exchange of digital messages. We introduce a novel cryptographic primitive, called the Certificate of Encrypted Message Being a Signature (CEMBS), as the basic building block of the fair exchange protocols. It is used to prove that an encrypted message is a certain party's signature on a public file, without revealing the signature. We also give two examples to show in detail how the certificate can be constructed.
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.
The paper proposes a new fair document exchange protocol.In such a fair exchange scenario,each party owns a valuable secret message and they want to exchange their secrets in a fairness way so that at the end of the transaction,either both of them get the other’s secret or neither of them gets the other’s secret.Compared with previous fair exchange protocols,the protocol holds confidentiality,that is,except the two participating party,no one even the Trusted Third Party(TTP) can get exchanged items.
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.