Reliable electronic commerce systems must offer fairness. In this paper, we propose a fair exchange protocol for trading electronic vouchers, which are the representation of rights to claim goods or services. This protocol enables two players to exchange vouchers stored in their smartcards fairly and efficiently. The players can exchange vouchers through a 4-round mutual communication protocol between their smartcards as long as the protocol is performed properly. If the protocol becomes unable to proceed due to misbehavior of the partner or for any other reason, the player that has fallen into the unfair condition (i.e. the player who sent its voucher but didn't receive the desired voucher) can recover fairness by performing a recovery protocol with a trusted third-party. Since the recovery protocol doesn't need the cooperation of the partner, fairness can be recovered without identifying or tracking the partner; one can trade vouchers securely even if the trading partner cannot be identified.

This paper proposes a framework based on a new architecture that allows distributed smartcards to interact with one another as well as with application programs on their hosts. Since these interactions are handled distribution-transparently through message dispatching agents deployed on each host, the smartcards can autonomously conduct distributed protocols without turning to off-card application programs. The proposed framework thus reduces the complexity of application programs and makes it easier to develop smartcard-based services that offer a high level of functionality. The feasibility of the framework is evaluated and confirmed by implementing a smartcard-based optimistic fair trading protocol for electronic vouchers on this framework.

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 exchanged, i.e., at least one item must be a "strongly generatable" item such as a digital signature. Without this requirement, only weak fairness that requires (expensive) external dispute resolution processes (e.g. trials in court) after exchange failure can be assured. This paper proposes a novel fair exchange method that enables parties to fairly exchange arbitrary items in an optimistic manner. This is achieved by realizing an optimistic non-blocking atomic commitment (NBAC) protocol between two smartcards and adapting the known result that fair exchange can be reduced to NBAC among trusted processes.

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