Anonymity-revocable Fair Divisible E-cash System

The e-cash(electronic cash) is a serial cipher that issued and promised its value by the e-cash company (bank). As an ideal payment type, the electronic cash have many excellences, such as lower cost, higher safety and off-line and so on.Through the study of Brands electronic cash scheme, this paper puts forward a trustee-based anonymity-revocable fair divisible e-cash system which is an applied electronic cash scheme, and whose correctness is proved via theory. The comparison experiment with the easy come easy go divisible e-cash which is the most excellent e-cash system, shows that our scheme is superior in speed and function.The safety of electronic commence includes two sides. The one is the credit of partner of commence, the other is the safety of electronic commence. The e-cash of this paper, itself, is safe. On the other hand, it would also reduce the possibility of loss brought by participants with a bad credit to the lowest.There are two problems, which are aroused by the credit of participants. The first is over payment, which can be prevented by the trace protocol of electronic cash referred in this paper. The second problem, which can be prevented by the justice protocol, is that the vender might cheat the user during the trade process.The e-cash is an individual system of the safety, and does not depend on the public key base establishment. The e-cash can prevent the user from forging the e-cash to cheat the vender (to pay different venders with a paying e-cash and its copies). On the other hand, it can also prevent vender from forging an e–cash, according to the e-cash he receipted, and depositing the same e-cash more than once in the bank.The anonymity of our e-cash system can prevent the bank from tracing the e-cash and venders tracing the source of e-cash, which can protect the consumption privacy of users. In order to get the using admission of government, the trustee in some special condition can revoke the anonymity of e-cash, such as when the e-cash anonymity endangers the safety of the country. After putting forward solid evidences to prove the withdrawer overpaying the e-cash, the bank can ask the trustee to trace the withdrawer. Thus, the trustee would trace the withdrawer based on the paid e-cash, while tracing the paid e-cash from the withdrawer. No matter the trade is successful or not, the trade process would prevent traders of both sides from a loss, because the paying process is fair. The user can create a payment e-cash with the original one, according to expenditure needed.The paper realizes the fair trade and revocation of the anonymity when it is necessary, by translating the digital signature and introducing the trustee. But the trustee is only involved during the registration, disputation and revocation of anonymity, so that the e-cash is quite efficient. The e-cash system is an off-line system, in which the bank does not take part in the paying process between users and venders, so that the cost of trade is low. At the same time, saving e-cash in the IC card, we can even pay the taxi fee, which is an off-line service.