In the fair exchange problem, mutually untrusting parties must securely exchange digital goods. A fair exchange protocol must ensure that no combination of cheating or failures will result in some goods being delivered but not others, and that all goods will be delivered in the absence of cheating and failures. This paper proposes two novel randomized protocols for solving fair exchange using s...

Contract signing is a practical application of the fair exchange of digital signatures. This application used to be realized by directly adopting the results of the fair exchange of signatures, which do not completely meet the requirements of the signing of a secret contract. The assistance of a trusted third party (TTP) and some cryptographic technology are required to allow two parties to exc...

Motivated by the great success and adoption of Bitcoin, a number of cryptocurrencies such as Litecoin, Dogecoin, and Ethereum are becoming increasingly popular. Although existing blockchainbased cryptocurrency schemes can ensure reasonable security for transactions, they do not consider any notion of fairness. Fair exchange allows two players to exchange digital “items”, such as digital signatu...

Contract signing protocols have been proposed and analyzed for more than three decades now. One of the main problems that appeared while studying such schemes is the impossibility of achieving both fairness and guaranteed output delivery. As workarounds, cryptographers have put forth three main categories of contract signing schemes: gradual release, optimistic and concurrent or legally fair sc...

The fair exchange problem is key to trading electronic items in systems of mutually untrusted parties. In modern variants of such systems, each party is equipped with a tamper proof security module. The security modules trust each other but can only communicate by exchanging messages through their host parties. These hosts are untrusted and could intercept and drop those messages. We describe a...

Some electronic commerce transactions are inherently performed between more than two parties. In this context, it is thus important to determine whether the underlying fair exchange protocols allowing the secure implementation of such transactions enable participants to exclude other entities from a protocol execution. This is an important point that has not been sufficiently addressed when ana...

In this paper we present techniques by which each mix in a mix network can be paid for its services by message senders, in a way that ensures fairness and without sacrificing anonymity. We describe a payment mechanism for use in mix networks, and use this payment scheme in fair exchange mechanisms for both connection-based and messagebased mix networks. In connection-based mix networks, our pro...

A currency exchange system is introduced for a multi-currency electronic money trial, in which smartcardbased electronic moneys (two currencies) are circulated over the Internet. Considering the need to support our existing single currency electronic money protocol, the exchange method proposed herein offers fairness between untrusted parties based on the “optimistic approach”. This paper descr...