Keyphrases optimistic fair exchange third party asynchronous protocol practical fair exchange protocol reliable communication channel optimistic approach optimistic fair exchange protocol useful technique temporary failure protocol run communication channel previous solution central feature. Powered by:. Garay, J. In: Wiener, M. Jakobsson, M. In: Guillou, L. In: Kim, K. PKC Mao, W. In: Mu, Y. ACISP Markowitch, O. In: Davida, G. ISC In: Syverson, P.
FC Pfitzmann, B. Pagnia, H. Schoenmakers, B. Goldwasser, S. Iosup, A. Katz, J. Lindell, Y. In: Malkin, T. CT-RSA Markowitch, O. In: Syverson, P. FC Merkle, R. In: Pomerance, C. Micali, S. Patent, No. Naor, M. Pagnia, H. Paillier, P. In: Stern, J. Shamir, A. Shmatikov, V. This is due to lack of state variables to maintain the actual status of the protocol.
Furthermore, an inconsistency can be found in its explanation for 3 parties. In the 3-party version, B and A cancel the protocol in this order but continue with its execution. If the protocol stops before the last step, A and B have the signed contract, but C can obtain only a cancel token instead.
If priority to the cancel token is assigned by the arbitrator, then the protocol is fair, since C presents its cancel token to the arbitrator in case of dispute and the arbitrator settles that the contract has been cancelled. But if the last step occurs, then now, all get the signed contract but A and B have cancel tokens. In this case priority to the finished state should be assigned, as the honest party acting properly C got the contract.
Nevertheless, the ar- bitrator does not know who is the honest party, and cannot assign priority to the tokens. This is a serious contradiction. And this happens with any sequence e. As shown above, parties cheat about their protocol state when contacting the TTP. Obviously, the TTP can detect it, but can do nothing to repair the situation. Moreover, in the original work there is no dispute resolution process defined for the multi-party version, which makes it more difficult to explicitly resolve possible conflicts.
We claim that in any design of a contract signing protocol, a well-defined dispute resolution process has to be provided. Previous work mainly focused on two-party contract signing. In some applications, however, a contract may need to be signed by multiple parties. In this paper, we analyzed an optimistic N -party contract signing protocol, and pointed out its security problem.
This clearly demonstrates that if we want a bank deposit to be made with several beneficiaries, further re- search is needed on the multi-party fair exchange protocols and more concretely on multi-party contract-signing protocols. Part of this research contemplates formal verification and analysis of existing solutions. This finding encourages us to study and improve existing multi-party contract signing solutions.
References 1. Opti- mistic synchronous multi-party contract signing. Asokan, Matthias Schunter, and Michael Waidner. Optimistic protocols for multi-party fair exchange.
Optimistic protocols for fair exchange. ACM Press, Asokan, Victor Shoup, and Michael Waidner. Optimistic fair exchange of digital signatures. Efficient and practical fair exchange protocols with off-line ttp. IEEE, May Birgit Baum-Waidner. Optimistic asynchronous multi-party contract signing with reduced number of rounds.
Springer, Birgit Baum-Waidner and Michael Waidner. Round-optimal and abuse-free multi- party contract signing. Ben-Or, O. Goldreich, S. Micali, and R.
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