Money conservation via atomicity in fair Off-line E-cash

Abstract

Atomicity and fault tolerance issues are important and typically open questions for implementing a complete payment scheme. The notion of "fair off-line e-cash" (FOLC) was originally suggested as a tool for crime prevention. This paper shows that FOLC schemes not just enable better control of e-cash when things go wrong due to "criminal suspicion" and other "regulatory/legal" issues, but it can also assure atomicity which takes care of conservation of money in case of failures during transaction runtime. The added protocols are very efficient and quite simple to implement. This kind of piggybacking atomicity control over "anonymity revocation" makes good sense as both actions are done by off-line invocation of the same trustees (TTPs). The resulting solution is a comprehensive yet efficient solution to money conservation in electronic cash transactions based on FOLC schemes. The adopted recovery approach makes the involved participants (customer, bank, merchant) sure that they can \re-think" the transactions when things go wrong, implying the atomicity of the transactions. We also take an optimistic approach achieving fair exchange costing only 2-round of communicational complexity (trivially the lower bound) with no additional TTP involvement since FOLC already employs such a party.