A system that allows a person to pay for goods or services by transmitting a number from one computer to another. Like the serial numbers on real dollar bills, the digital cash numbers are unique. Each one is issued by a bank and represents a specified sum of real money. One of the key features of digital cash is that, like real cash, it is anonymous and reusable. That is, when a digital cash amount is sent from a buyer to a vendor, there is no way to obtain information about the buyer. This is one of the key differences between digital cash and credit card systems. Another key difference is that a digital cash certificate can be reused. Digital cash transactions are expected to become commonplace by the year 2000. However, there a number of competing protocols, and it is unclear which ones will become dominant. Most digital cash systems start with a participating bank that issues cash numbers or other unique identifiers that carry a given value, such as five dollars. To obtain such a certificate, you must have an account at the bank; when you purchase digital cash certificates, the money is withdrawn from your account. You transfer the certificate to the vendor to pay for a product or service, and the vendor deposits the cash number in any participating bank or retransmits it to another vendor. For large purchases, the vendor can check the validity of a cash number by contacting the issuing bank.