What is a digital cash system?
Many instances in society feel that the old-fashioned payment system
with physical ''paper'' cash has a far too large overhead in the
administration. Therefore some electronic system seems appropriate.
Existing systems such as ''plastic'' cards reduces the overhead but
have one great disadvantage: The integrity of the user is not in
any way protected. For instance it is totally with in reach for
some malicious party to keep track of a persons travel habits,
his purchases etc.
This has led to a definition of electronic cash systems as a
set of protocols between three parties, the user,
the bank and the vendor. The protocols must simultaneously
satisfy the requirements of all parties, namely:
- The bank/vendor must be protected against any kind of fraud,
even frauds by a coalition of the other parties. Examples of such
frauds are counterfeiting and double-spending. In the later case
a user tries to spend an authentic ''coin'' twice.
- The users anonymity must be protected in the same sense.
Not only should it be infeasible to connect a certain user
to a certain payment, it should also be infeasible to
link any set of payments to a common user.
- For efficiency reasons, the system should be off-line, i.e.
no contact with the bank during payment is needed. As an
example it should be possible to pay a highway fee without
slowing down your car. Naturally the overall efficiency of
the system should be as high as possible.
The security aspects of the systems, 1 and 2 above, should be
provable under standard complexity theoretic assumptions
(such as P <> NP).
There are several suggested systems today. However there seems
to be a trade off between efficiency and security that is hard
to overcome.
References:
Brands: "Off-Line Cash Transfer by Smart Cards". Proceedings CARDIS 94.
Brands: "An efficient off-line electronic cash system based on the representation
problem". CWI Report CS-R9323 1993.
Chaum, Fiat & Naor: "Untraceable Electronic Cash". Proceedings CRYPTO 88.
Franklin & Yung: "Secure and Efficient Off-Line Digital Money". Proceedings ICALP 92.
Nice overviews in english and swedish respectively are available in:
Chaum: "Achieving Electronic Privacy". Scientific American, August 1992.
Hellberg: "Elektroniska kontantbetalningar off-line". TELE 1/94.
Mats Näslund <matsn@nada.kth.se>, September 1994