Cryptography Reference
In-Depth Information
reputation server . If bad behavior comes to light, the service can ap-
proach the reputation server and ask it to spring open the trap door.
This will allow the server to ban the bad person when they return.
[JKTS07]
The field of digital cash
is a rich world of
mathematical
techniques for settling
debts anonymously. All
of the algorithms are
much more
sophisticated and secure
than the simple one
presented here. See
[CFN93, Way95b,
Way97a] and many
others.
The actual algorithm is too complicated to describe in this space,
but it is based on a technique known as hash chains ,asequenceof
values produced by repeatedly hashing a number. That is,
m 0 is cho-
sen at random and then
m i−1 . This technique might be com-
bined with the anonymous coin by presenting
m i =
h
(
m i with the anony-
mous coin on the first visit. The server does not need to renew the
service at each visit by providing another anonymous coin because
it can use the hash chain to keep track of the good users.
On the next trip, Alice presents
m i−1
to the server which verifies it
by looking at to see that
h
(
m i−1 )=
m i .Ifitdoes,theserverdiscards
on the list of hash chains corresponding
to good anonymous users. If problems emerge later, the server just
shuts down a particular chain. Bad behavior is banned again.
m i and replaces it with
m i−1
10.8 Anonymous Auction Protocols
In anonymous and semi-anonymous networks, it is often difficult to
conduct traditional business when the traditional solution requires
some form of identification. Auctions, for instance, usually demand
that all of the interested parties appear before each other so everyone
can watch the action. Real estate auctions are held on the steps of
the court house for just this reason. When people want to remain
anonymous during an auction, they usually need to send some proxy
or rely on the auction house to keep their identity a secret.
Ross Anderson and Frank Stajano showedhow the Diffie-Hellman
key exchange algorithm could be extended to offer anonymous bid-
ding over a network. [SA00] Their auction takes place continuously
and requires the various bidders to chime in with their bids at the
right time, much like a traditional auction held in a hall. The iden-
tities, though, are hidden from each other and even from the seller
too. When a winner is finally chosen, the seller and the buyer can
quickly establish an encrypted communication channel to negotiate
when and how to trade the goods for cash. If the goods are digital,
the buyer and seller don't even need to reveal their identities to each
other.
In the original paper, the
th round of the auction begins at time
i
it
is the amount of time between rounds. For the sake of
simplicity, let the price be
where
t
f
(
i
) . Since this is a Diffie-Hellman-based
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