In conventional micropayment schemes, a seller must have checked if the electronic coin was double-spent, and have demanded the change to the issuing bank in order to give it to a buyer in case of need. Therefore whenever the seller did a business transaction with the buyer, the seller must have contacted with the issuing bank. In these conventional schemes, the more sellers and buyers participate in business transactions, the longer the turnaround time are getting to finish the payment after the sellers send the electronic coins.
This thesis proposes a client-isolated scheme in which sellers do not have to contact with the issuing bank to check whether the coin was double-spent or not and demand the change. The client-isolated system prevents the system users from accessing the client and hacking the important information monitoring the system. And this scheme uses the public-key encryption method to protect the electronic coin against various eavesdroppers. Although they get the encrypted electronic coins, eavesdroppers cannot get any available information from them because they do not have the private key. This scheme makes sellers have always enough coins in order to provide the change for buyers like the business transactions of off-line small shops. A seller is not influenced by the number of other sellers because the seller does not have to contact with the issuing bank and does business with the only buyers. Irrespective of increasing sellers, the turnaround time can be kept stable, and any seller and buyer can transact business with each other in a simpler way.
