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IEEE Transactions on Information Theory

A fair protocol for signing contracts

Michael Ben-Or, Oded Goldreich, Silvio M. Micali, Ronald L. Rivest

Published Jan 1, 1990• 287 citations
Publisher

Abstract

Two parties, A and B, want to sign a contract C over a communication network. To do so, they must simultaneously exchange their commitments to C. Since simultaneous exchange is usually impossible in practice, protocols are needed to approximate simultaneity by exchanging partial commitments in piece-by-piece manner. During such a protocol, one party or another may have a slight advantage; a fair protocol keeps this advantage within acceptable limits. A new protocol is proposed. It is fair in the sense that, at any stage in its execution, the conditional probability that one party cannot commit both parties to the contract given that the other party can, is close to zero. This is true even if A and B have vastly different computing powers and is proved under very weak cryptographic assumptions.>

DOI
10.1109/18.50372

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Two parties, A and B, want to sign a contract C over a communication network. To do so, they must simultaneously exchange their commitments to C. Since simultaneous exchange is usually impossible in practice, protocols are needed to approximate simultaneity by exchanging partial commitments in piece-by-piece manner. During such a protocol, one party or another may have a slight advantage; a fair protocol keeps this advantage within acceptable limits. A new protocol is proposed. It is fair in the sense that, at any stage in its execution, the conditional probability that one party cannot commit both parties to the contract given that the other party can, is close to zero. This is true even if A and B have vastly different computing powers and is proved under very weak cryptographic assumptions.>
If two parties want to sign a contract C over a communication network, they must “simultaneously” exchange their commitments to C Since simultaneous exchange is usually impossible in practice, protocols are needed to approximate simultaneity by exchange partial commitments in a piece by piece manner The party of secondly sending commitments may have a slight advantage; a “fair” protocol must keep this advantage within acceptable limits The protocol for signing a contract is improved based on the research result of secure multiparty computation In this protocol, the parties obliviously transfer the signed bit and the committed bit; the other parties can prove the validity of this bit based on the confirming sub protocol, but he can't gain more information about the whole signature by accumulating the signed bit or the committed bit After the parties exchange the whole signed information, they declare the commitment about the signature respectively, and gain the whole signature of the other party At this moment, the party doesn't care about the advance quit of the other party, because he has gained the whole signed information Finally, it is proved that this protocol is quit fairness
A fair contract-signing protocol allows two potentially mistrusted parities to exchange their commitments (i.e., digital signatures) to an agreed contract over the Internet in afairway, so that either each of them obtains the other's signature, or neither party does. Based on the RSA signature scheme, a new digital contract-signing protocol is proposed in this paper. Like the existing RSA-based solutions for the same problem, our protocol is not only fair, but also optimistic, since the trusted third party is involved only in the situations where one party is cheating or the communication channel is interrupted. Furthermore, the proposed protocol satisfies a new property-abuse-freeness. That is, if the protocol is executed unsuccessfully, none of the two parties can show the validity of intermediate results to others. Technical details are provided to analyze the security and performance of the proposed protocol. In summary, we present the first abuse-free fair contract-signing protocol based on the RSA signature, and show that it is both secure and efficient.
In ACISP 04, the authors of (F. Bao et al., 2004) suggested a fair exchange protocol for electronic contract signing between two parts. The protocol is an improvement of a previous one (S. Micali, 2003) and solves the vulnerabilities of that protocol. In (F. Bao et al., 2004) one of the signers can contact a trusted third party to resolve the exchange if he can't conclude it in the usual way. However, the other signer cannot contact the trusted third party, and for this reason, this signer can't know the final state of the exchange until, if it is the case, the trusted third party contact him or her. In this paper we have improved the protocol so that both signers can contact the TTP and solve the exchange whenever they want. We propose an asynchronous and fair protocol that improves the protocol presented in (F. Bao et al., 2004) maintaining the efficiency of the original protocol.
Based on Zhang's short signature scheme without random oracles and k+1-square roots assumption, a new fair contract signing protocol with the off-line semi-Trusted Third Party is given because the disadvantages of existed fair contract signing protocol. The off-line semi-TTP intervenes into the protocol in cases where one party attempts to cheat or simply crashes to make the fair exchange optimistic. The off-line TTP need not be completely trusted, since the TTP can get neither exchanged signature when dispute is mediated. Because this protocol is based on short signature, which needs low storage and little communication, it can be used in low-bandwidth communication and low-storage environments.
Abstract. Some cryptographic tasks, such as contract signing and other related tasks, need to ensure complex, branching time security properties. When defining such properties one needs to deal with subtle problems regarding the scheduling of non-deterministic decisions, the delivery of messages sent on resilient (non-adversarially controlled) channels, fair executions (executions where no party, both honest and dishonest, is unreasonably precluded to perform its actions), and defining strategies of adversaries against all possible non-deterministic choices of parties and arbitrary delivery of messages via resilient channels. These problems are typically not addressed in cryptographic models and these models therefore do not suffice to formalize branching time properties, such as those required of contract signing protocols. In this paper, we develop a cryptographic model that deals with all of the above problems. One central feature of our model is a general definition of fair scheduling which not only formalizes fair scheduling of resilient channels but also fair scheduling of actions of honest and dishonest principals. Based on this model and the notion of fair scheduling, we provide a definition of a prominent branching time property of contract signing protocols, namely balance, and give the first cryptographic proof that the Asokan-Shoup-Waidner two-party contract signing protocol is balanced. 1
Today in the field of security there are various protocols and authentication techniques that makes an application in the network more secure and free from various attacks in the network. There are various applications which require exchange of data between parties. A fair exchange is required since the chances of attacks have been greatly increased. Hence a new protocol has been implemented for the fair exchange of data between parties called contract signing protocol. Although there are various techniques applied for the security of these contract signing protocols such as using RSA [1]. This protocol use RSA to generate digital signature for the authentication of fair exchange between two parties, also in this technique instead of using single TTP (Trusted Third Party) and the trust can be distributed on multi TTP's. But the protocol implemented here in these techniques has less security issues and the chances of attack are more and also by using multiple TTP's storage cost is increased. Hence to overcome these limitations we propose an efficient and secure protocol of contract signing using OTPK (One Time Private Key). This technique requires less storage cost since there is no need to store any key at any end.