Cryptography Reference
In-Depth Information
accumulator, composed of the identification sequences of all members except
her own. This value is not secret; it can be published to third parties in advance,
if need be. Moreover, the value W all of the accumulator, composed for all
members, is published by a third party. If somebody has doubts about Alice's
membership, they can ask for her identification sequence and compute the
accumulator from that sequence and W Alice . If the result is W all , then Alice has
to be a member; otherwise she is a spy. Since identification sequences are made
known upon request only, no member list can be revealed by trial and error.
The security of this protocol can be compared with that of digital signatures
using a centralized service, except that it can do without the latter. It is a nice
example of showing that cryptology can still discover simple and useful things
even today.
6.6.7 Electronic Money
Just hearing the buzzword 'electronic money' will cause many people to think:
'Yet another technical gadget that no one needs!' It is understandable since
nobody wants to move our daily payment system to the Internet for the time being,
even though advertising has been going into raptures about electronic malls.
Information has become merchandise, and this merchandise is shipped increas-
ingly over the Internet. It would be natural to also be able to pay for it on
the Internet. There is certainly a lot of speculation going on about the vision
of teleshopping and customers paying electronically. I'd rather not forecast
whether and how fast this vision will gain acceptance in the real world.
But long before electronic money is widely used, cryptologists need to have
appropriate algorithms and protocols ready. Otherwise it could happen that poor
protocols are sold as the latest craze and the 'real' (bad) hackers celebrate for
there's real money to be made.
There is a large number of approaches and products concerning electronic
checks or coins. To keep within the scope and volume of this topic, I will
introduce and discuss one single protocol, namely one by Chaum, Fiat, and
Naor [Chaum]. This protocol itself uses several of the protocols previously
introduced, and it is pretty clever. We will only look at checks, not coins, since
we don't want to deal with the additional problem of change.
Requirements for Electronic Money
Let's first consider several requirements electronic checks have to meet in any
event.
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