Cryptography Reference
In-Depth Information
mail. It is not clear, however, that you can completely eliminate
every way of communicating.
The simplest example for sending a message is to tie up some
shared resource like a printer. If you want to send a '1' to a
friend, then you print a file at 12:05 and tie up the printer. If
you want to send a '0', then you print the file at 12:30. The other
person checks the availability of the printer. This may not be a
fast method, but it could work. The speed of the channel de-
pends on the shared system resources and the accuracy of de-
tection. Obviously one way to defend against covert channels
is to create timing errors, but then that just creates other prob-
lems.
Some beginning sources are [NCS93, PN93, MM92]
Digital Cash
There are many different ways to exchange money over
digital wires, but some of the most interesting systems offer
complete anonymity. People are able to spend their money
without fear of records being kept. This is a fairly neat trick
because digital cash must be counterfeit-resistant. Paper cash
achieves this goal when it is printed with a sophisticated press.
Digital copies, on the other hand, are easy to make. If peo-
ple can copy files of numbers meant to represent cash, then
anonymity would seem to allow people the freedom to coun-
terfeit without being caught.
The cleverest schemes involve a complicated spending system
that forces the spender to reveal part of their identity. If the
spender tries to use a bill twice, enough of the identity should
be revealed to expose the criminal.
Anonymous Voting
People often want to cast their votes anony-
mously because this can prevent coercision. Paper ballots are
generally successful if no one checks the ballot before they en-
ter the box. Providing the same accountability and security is
no simple feat.
Interest in this topic is very high and there are enough good al-
gorithms to justify a separate topic. K. Sako and J. Kilian [SK95],
for instance, modified the Mixmaster protocol described in
Chapter 10 to provide a simple way for people to cast their vote.
Each person can check the tally and compare their vote to the
recorded vote to guarantee that the election was fair.
Many of the newer systems rely upon the homomorphic en-
cryption systems that allow manipulation of encrypted data.
