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International Workshop on Database and Expert Systems Applications (DEXA)

Multi-party fair exchange with an off-line trusted neutral party

Feng Bao, Robert Huijie Deng, Khanh Quoc Nguyen, Vijay Varadharajan

Published Jan 1, 1999• 32 citations
Publisher

Abstract

Recently developed cryptographic techniques make it possible to construct fair exchange protocols with an off-line trusted third party (TTP). The technique is referred to as a verifiable encryption scheme (VES) and proves that a ciphertext is the result of the encryption of a specified value without revealing this value. In this paper we apply the technique to multi-party fair exchange (MPFE) protocols which leads to the trusted neutral party (TNP, it is not the third party in multi-party protocols) involved in the protocol to be off-line. Multi-party fair exchange has been studied by Asokan et al. (1996) and Franklin and Tsudik (1998) where the TNP is on-line. For off-line TNP-based MPFE, the issues are different from that of on-line TNP MPFE. We give a definition of fairness of the MPFE with off-line TNP. We then present our MPFE protocol and prove its fairness.

DOI
10.1109/dexa.1999.795294

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Recently developed cryptographic techniques make it possible to construct fair exchange protocols with an off-line trusted third party (TTP). The technique is referred to as a verifiable encryption scheme (VES) and proves that a ciphertext is the result of the encryption of a specified value without revealing this value. In this paper we apply the technique to multi-party fair exchange (MPFE) protocols which leads to the trusted neutral party (TNP, it is not the third party in multi-party protocols) involved in the protocol to be off-line. Multi-party fair exchange has been studied by Asokan et al. (1996) and Franklin and Tsudik (1998) where the TNP is on-line. For off-line TNP-based MPFE, the issues are different from that of on-line TNP MPFE. We give a definition of fairness of the MPFE with off-line TNP. We then present our MPFE protocol and prove its fairness.
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.
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 difference between the fair exchange protocols and the non-repudiation protocols is proposed. Based on the publicly verifiable secret sharing scheme and group encryption scheme, a new multi-party fair exchagne protocol with an off-line semi-Trusted Third Party(off-STTP) is presented. The off-STTP who need not be completely trusted intervenes into the exchange in case of trouble. That the off-STTP can only decipher a sub-secret key makes sure the confidentiality of exchange data and the true fairness of the barter. The participants can barter with others at their will and the exchange topology is unknown to others including the off-STTP.
Fair multi-party exchange protocol is one of the foundation electronic commerce, but at present, the study of multi-party fair exchange protocol is very weak. Aiming at solving this problem, we present a secure and efficient ID-based fair multi-party exchange protocol with off-line semi-trusted third party. Application of multi-receiver identity-based encryption and PVSS scheme ensure security and efficiency of this new protocol. And it applied to multi-party certified e-mail exchange in network environment.