Cryptography Reference
In-Depth Information
5.8 Digital Cash Schemes
Some rhyme a neebor's name to lash;
Some rhyme
(
vain thought!
)
for needfu' cash;
Some rhyme to court the countra clash,
An' raise a din;
For me, an aim I never flash;
I rhyme for fun.
Robert Burns (1759-1796)
, Scottish poet
from
To J. S
[mith] (1786), st. 5
Anyone living in the modern world, having access to a computer and a source
of funds, is aware of
e-commerce
, that is, basicallydoing business bycomputer.
This mayinclude the purchasing of goods or services from an Internet site,
or conducting banking transactions, for instance. All of this requires the use
of
digital cash schemes
that comprise a collection of protocols for transferring
money. These are modeled on “hard cash” and its use, as well as credit card or
debit card use. Naturally, we want secure transactions, and this involves many
facets.
In our discussion below, we will simplifythe general scenario byassuming
one customer, one vendor, one bank, but potentiallymanyadversaries seeking to
steal your money. What makes digital cash and e-commerce possible are hybrid
cryptosystems, PKC and its digital signatures, for security and authentication,
combined with SKC for eKcient transfer of funds. For instance, an example of
the use of digital cash is the bank aKxing its digital signature to an electronic
moneyorder via its private key, and the vendor verifying this byusing the bank's
public key. In this fashion, the customer may withdraw funds from the bank to
paythe vendor both of whom are assured that the proper amounts are securely
withdrawn as payment for the goods or services.
Before describing our digital cash scheme, we must be familiar with some
terminologyfrom e-commerce. In particular, we are going to describe a scheme
called
ECash
as a relativelysimple template for Internet purchases, which in-
volves Bob as the customer, the bank, and the vendor.
E-commerce Terminology
Bank
The
bank
represents anyelectronic bank that is connected to the
Internet and plays an essential but unobtrusive role. The bank has an RSA
modulus
n
, a private key
d
that it keeps secure, and a public key
e
.
Coins
A coin is a pair of integers (
m, m
d
)(mod
n
) where
m
is the unique
identification number of the coin. The value
m
d
(mod
n
)isthe
bank's signature
or
special digital stamp
on the coin. Coins will typically have different denomi-
nations, but for simplicitywe will assume that the denomination is the same for
each of them, e.g., $100. To ensure a coin is used onlyonce, the bank records all
identification numbers
m
in its
used coin database
.If
m
has been so recorded
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