Secure Payment System Of Electronic Cash

E-cash is a new electronic payment mechanism, which has many advantages, such as protecting anonymity of user, keeping from ceasing to pay and overdraft. As an electronic equivalent of paper currency, it has five basic functions which are measurement of value, means of circulation, means of store, means of payment, general currency. E-cash can be circulated, deposited and withdrawed through Internet or public information flat. Its emergence will contribute greatly to development of Electronic Bussiness. This thesis presents a solution scheme of e-cash system. It focuses on solving some comparatively difficult problems in e-cash system: divisibility problem , and strives to improve the system efficiency. First the thesis introduces e-currency in the mass. Based on such subject it introduces research status of e-cash. In succession it describes a secure and efficient e-cash system. This system is a modification one coming from Brands scheme (Brands scheme is considered as one of the most efficient e-cash scheme), and it uses a certification scheme based on ElGamal digital signature. The certification scheme is an advanced one of a previous scheme based on ElGamal digital signature as inverse operations eliminated and is efficient with high security. In the addition, the divisibility of e-cash is used to address the traditional problem of change in a trade, that is, to break up a big coin into parts to accomplish payment. However, this thesis transforms divisibility problem to exact payment problem which means that a user withdraws many coins of different denominations during withdrawal period, then uses a subset of these coins to provide the extra payment during payment period. How to get the optimal set of coins and how to accomplish payment with the set are the keys to solve the divisibility problem. The solution of above two problems is using greedy payment algorithm and N-divisibility algorithm. By discussing them in two situations such as being unlimited and being limited in denomination, it proves above two algorithm feasibility and efficiency respectively.