How to Spend Your E-Cash
Mira Belenkiy
Brown University
Abstract
An electronic cash (e-cash) scheme lets a user withdraw money from a
bank and then spend it anonymously. E-cash can be used only if it can
be securely and fairly exchanged for electronic goods or services. In
this work, we introduce and realize endorsed e-cash. An endorsed
e-coin consists of a lightweight endorsement x and the rest of the
coin which is meaningless without x. We reduce the problem of
exchanging e-cash to that of exchanging endorsements. We demonstrate
the usefulness of endorsed e-cash by exhibiting simple and efficient
solutions to two important problems: (1) optimistic and unlinkable
fair exchange of e-cash for digital goods and services; and (2) onion
routing with incentives and accountability for the routers. Finally,
we show how to represent a set of n endorsements using just one
endorsement; this means that the complexity of the fair exchange
protocol for n coins is the same as for one coin, making e-cash all
the more scalable and suitable for applications. Our fair exchange of
multiple e-coins protocol can be applied to fair exchanges of (almost)
any secrets.