On Fairness in Exchange Protocols

Cagil Can Oniz

In this thesis, the problem of fair exchange on specific cases is addressed. The main idea of fair exchange is as follows: Two entities that do not trust each other want to exchange some arbitrary data over a communication network. Since they do not trust each other, neither party wants to transmit their own data before receiving the other entity's data. Even though either party could prove an unjust situation after termination of the protocol, if they are in different countries, solving disputes may require time and money due to the bureaucracy of international laws. In this thesis, a special application of fair exchange, a fair e-commerce protocol for large e-goods is designed and implemented. The proposed protocol provides a method for fair exchange of e-money to e-products, and a method for verifying the contents of the exchanged items. The presented protocol is efficient such that when none of the parties tries to cheat, only three messages are sufficient. In case of disputes, three more messages are needed. Furthermore, in most of the previously proposed protocols in the literature, e-goods are transferred multiple times among some entities. This situation is too costly when e-goods are large. In the presented protocol, e-goods are transferred only once. Another important property of the protocol is the anonymity of the customer; no information about the customers shopping habits can be gathered through the protocol. The implementation results show that the protocol is efficient and secure and that small number of cryptographic operations is sufficient. In addition to the fair e-commerce protocol, another special application of fair exchange, a fair multimedia exchange protocol using a different method is designed and implemented. This protocol is designed due to different requirements of different applications. In the fair multimedia exchange protocol, two entities want to exchange some multimedia files such as video or audio files. This protocol requires lower security and has a different a lower degree of fairness as compared to the fair e-commerce protocol. Fair multimedia exchange protocol uses a baby-step approach in which the probability of protocol completion is gradually increased over several cycles. In baby-step approach protocols, entities exchange pieces of the items, which they want to barter. At protocol completion, the complete items are formed by using the pieces exchanged.

Jianying Zhou, Robert Huijie Deng, Feng Bao

Nicolás Gonzaléz-Deleito, Olivier Markowitch

Montri Apiromvorakarn, Yongyuth Permpoontanalarp

Fair exchange is an electronic data exchange protocol that allows both sender and receiver to exchange information such that either all of parties have the exchanged information or none of them do. All existing approaches to fair exchange achieve the fair exchange property between a single sender and a receiver. However, they are unable to provide the fair exchange for all senders. A malicious receiver can deny receiving messages from some legitimate senders but do receive messages from conspired senders. Such unfair exchange is devastating in many applications such as electronic auction where the conspired sender can be the bid winner easily. We propose a fair exchange protocol which offers a new property, namely undeniable fairness for all senders. We show that our protocol is practical and secure for the fair exchange system.

Jia-Chng Loh, Swee‐Huay Heng, Syh‐Yuan Tan

The notion of optimistic fair exchange (OFE) protocol was proposed by Asokan et al. in order to solve the problem of exchanging items or services in a fair manner between a signer and a verifier. There are various OFE protocols and variants in the literature, and they possess different security properties. In this paper, we provide an overview on the development of OFE protocol and its variants, namely, ambiguous OFE, perfect ambiguous OFE, privacy preserving OFE, accountable OFE, and group-oriented OFE. In precise, we investigate the security properties, evolutions and relationships among them. We also provide an evolution tree diagram to present their developments as well as three comparison analysis tables to compare and contrast their security properties.