Cryptography Reference
In-Depth Information
transactions can be handled much faster on the Net than by current accounting
methods.
In contrast, the load on the network is a big deal. Every payment transaction
requires a multi-step dialogue between Alice and her bank, between Alice and
Bob, and between Bob and the bank. Not all of this has to be done online
though. Another downside is that this protocol doesn't allow for change. This
means that Alice has to create a separate check for each 'odd' amount and have
it signed blindly by her bank. Interferences in the line to the bank, or in the
bank itself, will have an immediate hindering effect on the sales transaction,
which is currently not the case with ATM cards or clearing checks.
A popular topic in magazine articles seems to be the catastrophe that Alice's
hard disk 'goes up in smoke'. I even came across the argument that compa-
nies that restore data from destroyed disks will enjoy a strong boom. That's
computer-technical nonsense. Electronic payment systems will initially be han-
dling small amounts for which such an expensive operation wouldn't be worth-
while. By the time amounts get bigger users will probably have learned that
every once in a while they should back up their data — even immediately when
writing checks for 10 000 dollars and more. Also, it shouldn't be a problem to
have one's bank stop payment of a check with a particular number, provided
one has memorized that number.
Bottom Line
Let's leave it at that. You probably haven't paid for this topic by e-cash, and
you are likely to pay for the next in the same manner. I find it more interest-
ing to look at how sophisticated cryptographic protocols can be. The protocol
introduced above uses three other protocols: digital signatures, blind signa-
tures, and secret splitting. The structure of digital coin protocols is even more
complicated since they have to remain completely anonymous, and coins have
to be divisible. The first protocol of this type required a transfer of about
200 Mbytes per payment. There is intensive ongoing research on practicable
and secure protocols. At the EUROCRYPT '98, Chan, Frankel, and Tsiou-
nis [ChanFrTsi] introduced a practicable and mostly anonymous method that
can be done with 300 bytes per payment and requires only small computation
times.
At any rate, the Chaum - Fiat - Naor protocol guarantees Alice's anonymity.
That's a new feature that conventional check-clearance systems cannot offer.
