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Lecture notes in computer science

Optimally-Fair Exchange of Secrets via Delay Encryption and Commutative Blinding

Ivo Maffei, Andrew William Roscoe

Published Nov 30, 2023• 3 citations
Publisher

Abstract

Abstract We propose a new fair exchange protocol that takes advantage of delay encryption and commutative encryption to achieve optimal partial fairness among all protocols involving one-way messages. Our protocol consists of 3 setup messages and $$2N+1$$ 2 N + 1 exchange messages and it is fair against covert adversaries with probability $$1- \frac{1}{2N}$$ 1 - 1 2 N . We prove that this is optimal up to shortening the setup phase which is notably more efficient than existing protocols.

DOI
10.1007/978-3-031-47754-6_6

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Abstract We propose a new fair exchange protocol that takes advantage of delay encryption and commutative encryption to achieve optimal partial fairness among all protocols involving one-way messages. Our protocol consists of 3 setup messages and 2N+12N+12N+1 2 N + 1 exchange messages and it is fair against covert adversaries with probability 1−12N1- \frac{1}{2N}1−2N1​ 1 - 1 2 N . We prove that this is optimal up to shortening the setup phase which is notably more efficient than existing protocols.
Abstract We present a multi-party exchange protocol that achieves optimal partial fairness even in the presence of a dishonest majority. We demonstrate how this protocol can be applied to any type of multi-party exchange scenario where the network topology is complete. When combined with standard secure multi-party computation techniques, our protocol enables SMPC with partial fairness when a dishonest majority is involved. Fairness optimality is proven in an abstract model which applies to all protocols based on the concept of concealing the point when the secrets are exchanged. Our protocol improves known results via the use of timed-release encryption and commutative blinding.
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.
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.
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.>
Based on the verifiably encrypted signature scheme without random oracles recently proposed by Gorantla et al.,a new scheme of optimistic fair exchange of digital signatures is proposed in this paper.The two players firstly exchange their verifiably encrypted signature on agreed-upon messages then their ordinary signature after the verification of each other's verifiably encrypted signature.If one of them executes the protocol dishonestly,the other can resort to a trusted third party so as to get the purpose of fair exchange of signatures.The proposed scheme has properties of short signature,low computation and verification cost.It can realize exchange of digital signatures fairly and efficiently.
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.