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Electronic and Mechanical Engineering and Information Technology

An improved protocol for optimistic multi-party fair exchange

Yi Liu, Hongli Hu

Published Aug 1, 2011• 5 citations
Publisher

Abstract

An exchange protocol is a protocol that allows two or more parties to exchange electronic information. Bao et al. proposed a multi-party fair exchange protocol based on an offline TTP. Participants in this protocol have to trust the initiator of the exchange in addition of the TTP. In this paper we present an improved optimistic multi-party fair exchange protocol in which participants do not have to trust anyone except for the TTP. Moreover, the communication overhead of our protocol is not increased.

DOI
10.1109/emeit.2011.6024052

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An exchange protocol is a protocol that allows two or more parties to exchange electronic information. Bao et al. proposed a multi-party fair exchange protocol based on an offline TTP. Participants in this protocol have to trust the initiator of the exchange in addition of the TTP. In this paper we present an improved optimistic multi-party fair exchange protocol in which participants do not have to trust anyone except for the TTP. Moreover, the communication overhead of our protocol is not increased.
For electronic items without generability or revocability, previous optimistic exchange protocols can't guarantee strong fairness. Gradual protocols assume equal computing power. And online-TTP (Third Trusted Party) protocols often suffer from communication bottle-neck. A gradual and optimistic fair exchange protocol is presented in this paper to solve the problem. Secrets are gradually exchanged with partially validation by recipients, while the probability of detecting cheats increases. Fairness doesn't depend on equal computational power assumption. TTP isn't involved unless dispute appears. The protocol provides stronger fairness than optimistic or gradual schemes.
An Optimistic fair exchange (OFE) protocol is a good way for two parties to exchange their digital items in a fair way such that at the end of the protocol, both of them receive their items or none of them receives anything. In 2008, Huang et al. presented an efficient OFE protocol, but there is a security problem in their scheme. If the trusted third party (TTP) is dishonest and colludes with the verifier, the TTP can complete the transaction without getting signer's agreement. To solve this problem, we propose an OFE protocol in which there is a tracing algorithm to detect the party who completes the transaction. Furthermore, we obtain the accountability property introduced by Huang et al. in 2011, more efficiently and in the standard model while their protocol is in the random oracle model. We prove the security of our scheme under the chosen-key model and multi-user setting in the standard model.
Multi-party fair exchange (MFE) and fair secure multi-party computation (fair SMPC) are under-studied fields of research, with practical importance. In particular, we consider MFE scenarios where at the end of the protocol, either every participant receives every other participant’s item, or no participant receives anything. We analyze the case where a trusted third party (TTP) is optimistically available, although we emphasize that the trust put on the TTP is only regarding the fairness , and our protocols preserve the privacy of the exchanged items against the TTP. In the fair SMPC case, we prove that a malicious TTP can only harm fairness, but not security . We construct an asymptotically optimal multi-party fair exchange protocol that requires a constant number of rounds (in comparison to linear) and O(n 2 ) messages (in comparison to cubic), where n is the number of participating parties. In our protocol, we enable the parties to efficiently exchange any item that can be efficiently put into a verifiable encryption (e.g., signatures on a contract). We show how to apply this protocol on top of any SMPC protocol to achieve fairness with very little overhead (independent of the circuit size). We then generalize our protocol to efficiently handle any exchange topology (participants exchange items with arbitrary other participants). Our protocol guarantees fairness in its strongest sense: even if all n-1 other participants are malicious and colluding with each other, the fairness is still guaranteed.
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.
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.