Adaletli ve güvenli çoklu-ikili ortaklaşa hesaplama ve çok kişili adil takas

Secure computation cannot be fair in general against malicious adversaries, unless a trusted third party (TTP) is involved, or gradual-release type of costly protocols with super-constant rounds are employed. Existing optimistic fair and secure computation protocols with constant rounds are either too costly to arbitrate (e.g., the TTP may need to re-do almost the whole computation), or require the use of electronic payments or bitcoins. Furthermore, most of the existing solutions were proven secure and fair separately, which, we show, may lead to insecurity overall. Therefore, we propose two new framework for secure two-party and multi-party computation that can be applied on top of both circuit based secure computation protocols to make them fair. We show that our fairness overhead is minimal with these frameworks, compared to all known existing work, and the TTP never learns the inputs or outputs of the computation. Furthermore, our framework for the two party computation makes a protocol fair even in terms of the work performed by the two parties Alice and Bob. We also prove that the frameworks makes a circuit based secure computation protocol fair and secure simultaneously, through one simulator, which guarantees that our fairness extensions do not leak any information. The framework for fair and secure multi-party computation includes multi-party fair exchange protocol that we designed. Multi-party fair exchange (MFE) is understudied field of research, with practical importance. We examine MFE scenarios where every participant has some item, and at the end of the protocol, either every participant receives every other participant's item, or no participant receives anything. This is a particularly hard scenario, even though it is directly applicable to protocols such as fair SMPC or multi-party contract signing. 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. We construct two asymptotically optimal multi-party fair exchange protocols that require 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 one protocol, we enable the parties to efficiently exchange any item that can be efficiently put into a verifiable escrow (e.g., signatures on a contract). In our other protocol, we let the parties exchange any verifiable item, without the constraint that it must be efficiently put into a verifiable escrow (e.g., a file cannot be efficiently verifiably escrowed, but if its hash is known, once obtained, the file can be verified). We achieve this via use of electronic payments, where if an item is not obtained, the payment of its owner will be obtained in return of the item that we sent. We then generalize our protocols to handle any exchange topology efficiently. Our protocols guarantee fairness in its strongest sense: even if all n-1 other participants are malicious and colluding, fairness will hold.