Optimistic fair exchange (OFE) is a protocol for solving the problem of exchanging items or services in a fair manner between two parties, a signer and a verifier, with the help of an arbitrator which is called in only when a dispute happens between the two parties. In almost all the previous work on OFE, after obtaining a partial signature from the signer, the verifier can present it to others...

Fairly exchanging digital content is an everyday problem. It has been shown that fair exchange cannot be done without a trusted third party (called the Arbiter). Yet, even with a trusted party, it is still non-trivial to come up with an efficient solution, especially one that can be used in a p2p file sharing system with a high volume of data exchanged. We provide an efficient optimistic fair e...

This paper addresses the security of optimistic fair exchange in a multi-user setting. While the security of public key encryption and public key signature schemes in a single-user setting guarantees the security in a multi-user setting, we show that the situation is different in the optimistic fair exchange. First, we show how to break, in the multi-user setting, an optimistic fair exchange sc...

We describe a generic protocol for fair exchange of electronic goods with non-repudiation. Goods can be signatures (i.e., non-repudiation tokens of public data), confidential data, or payments. The protocol does not involve a third party in the exchange in the faultless case but only for recovery.

An Optimistic Fair Exchange (OFE) protocol is a good way for two parties to exchange their digital items in a fair way such that at the end of the protocol execution, both of them receive their items or none of them receive anything. In an OFE protocol there is a semi-trusted third party, named arbitrator, which involves in the protocol if it is necessary. But there is a security problem when a...

Protocol for fair exchange of digital signatures is essential in many applications including contract signing, electronic commerce, or even peer-to-peer file sharing. In such a protocol, two parties, Alice and Bob, would like to exchange digital signatures on some messages in a fair way. It is known that a trusted arbitrator is necessary in the realization

Fair exchange protocols are important in realizing safe electronic commerce. In particular, optimistic fair exchange protocols, which involve a trusted third party only when mutual communication between exchanging parties fails, are the most promising development because of their efficiency. Unfortunately, however, existing optimistic protocols place restrictions on the items that can be exchan...

Fair exchange protocol aims to allow two parties to exchange digital items in a fair manner. It is well-known that fairness can only be achieved with the help of a trusted third party, usually referred to as arbitrator. A fair exchange protocol is optimistic if the arbitrator is not involved in the normal execution of the fair exchange process. That is, its presence is necessary only when one o...