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Undeniable fair exchange

Montri Apiromvorakarn, Yongyuth Permpoontanalarp

Published Sep 22, 2006• 1 citation
OpenAlex

Abstract

Fair exchange is an electronic data exchange protocol that allows both sender and receiver to exchange information such that either all of parties have the exchanged information or none of them do. All existing approaches to fair exchange achieve the fair exchange property between a single sender and a receiver. However, they are unable to provide the fair exchange for all senders. A malicious receiver can deny receiving messages from some legitimate senders but do receive messages from conspired senders. Such unfair exchange is devastating in many applications such as electronic auction where the conspired sender can be the bid winner easily. We propose a fair exchange protocol which offers a new property, namely undeniable fairness for all senders. We show that our protocol is practical and secure for the fair exchange system.

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Fair exchange is an electronic data exchange protocol that allows both sender and receiver to exchange information such that either all of parties have the exchanged information or none of them do. All existing approaches to fair exchange achieve the fair exchange property between a single sender and a receiver. However, they are unable to provide the fair exchange for all senders. A malicious receiver can deny receiving messages from some legitimate senders but do receive messages from conspired senders. Such unfair exchange is devastating in many applications such as electronic auction where the conspired sender can be the bid winner easily. We propose a fair exchange protocol which offers a new property, namely undeniable fairness for all senders. We show that our protocol is practical and secure for the fair exchange system.
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.
A fair exchange protocol enable two parties to exchange secrets with fairness, so that neither can gain any information advantage by quitting prematurely or otherwise misbehaving. Therefore a fair exchange is the most important for electronic transactions between untrusted parties. To design new fair exchange, after describing basic concepts, definitions and existing protocols and designing a non-interactive OT protocol using ELGamal's public key system, I will design new protocol to support fair exchange. In my designed new protocol, untrusted parties exchange secrets obliviously and verify that their received secrets are true by using transformed Zero Knowledge Interactive Proof extended to duplex. At this time, concerned two parties can't decrypt the other's ciphertext. After all of the steps, two parties can do it. It is the most important to provide perfect fairness and anonymity to untrusted parties in this protocol.
Fair exchange protocol is an electronic commerce protocol that ensures that no player gains an unfair advantage over the other player by misbehaving misrepresenting or prematurely aborting the protocol. From 2000,Ray et al. proposed a series of fair exchange protocols based on the theory of cross validation. One of important contributions is that by using a pair of compatible keys to encrypt one product respectively,Ray's protocols allow a customer to verify that the product he is about to receive from the merchant is the same as the product he ordered,before the customer pays for the product. However,it is shown that in fact malicious merchant can easily provide the customer false product such that the authenticating equality is satisfied. The result is that the customer will be cheated and pay for the false product. In addition,improvement is provided to avoid the identified attacks.
We present new protocols for two parties to exchange documents with fairness, i.e., such that no party can gain an advantage by quitting prematurely or otherwise misbehaving. We use a third party that is L‘semi-trusted”, in the sense that it may misbehave on its own but will not conspire with either of the main parties. In our solutions, disruption by any one of the three parties will not allow the disrupter gain any useful new information about the documents. Our solutions are efficient and can be based on any of several cryptographic assumptions (e.g., factoring, discrete log, graph isomorpbism). We also discuss the application of our techniques to electronic commerce protocols to achieve fair payment.
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.