Secret Broadcasts - Disappearing Cryptography - page 228

determine who is broadcasting. Presumably, there would be rules
against broadcasting when another person is using the DC network
so it would be possible to unwind the chain far enough to reveal who
that person might be.
This can be facilitated if everyone sends out a digital signature
of the block of coin flips. Each person in the network has access to
two keys— theirs and their neighbor's. The best solution is to have
someone sign the coin flips of their neighbor. When the process is
unwound, this prevents them from lying about their coin flips to pro-
tect themselves. Forcing everyone to sign their neighbor's coin flips
prevents people from lying about their own coin flips and changing
the signature.
This tracing can be useful if only one person uses the DC network
for a purpose that offends a majority of the participants—perhaps to
send an illegal threat. If one person is trying to jam the communi-
cations of another, however, then it reveals both senders. The only
way to determine which one is legitimate is to produce some rules
for when members can start broadcasting. The first one would be
the legitimate sender. The one who began broadcasting afterward
would be the jammer.

11.4 Summary

Dining Cryptographers networks offer a good opportunity to provide
unconditional security against traffic analysis. No one can detect
the broadcaster if the nodes of the network keep their coin flips pri-
vate. Nor can anyone determine the recipient if the messages are
encrypted.
The major limitation to DC nets is the high cost of information
traffic. Everymember of the networkmust flip coins with their neigh-
bor and then broadcast this information to the group. This can be
done in blocks, but it is still a significant cost. n people mean that

network bandwidth increases by a factor of 2 n .

The Disguise DC nets offer an ideal way to obscure the source of a
transmission. If this transmission is encrypted, then only the
intended recipient should be able to read it.

How Secure Is It? The system is secure if all of the information about
the coin flips is kept secret. Otherwise, the group can track
down the sender by revealing all of this information.

Further Reading