Cryptography Reference
In-Depth Information
3. Broadcast their encrypted messages to everyone in the group.
It doesn't matter what order this happens.
4. When everyone has received the messages of everyone else,
everyone begins sending their keys,
k
i
, out to the group.
5. Everyone decrypts all of the packets, checks tomake sure that
B
is at the beginning of each packet, and finally sums the answers
to reveal the message.
These bit commitment protocols make it nearly impossible for
someone to cheat. If there were no
stuck at the beginning of the
answers that were encrypted, a sophisticated user might be able to
find two different keys that reveal different answers. If he wanted to
tell the group that he was reporting a match, then hemight show one
key. If he wanted to reveal the other, then he could send out another
key. This might be possible if the encrypted packet was only one
bit long. But it would be near impossible if each encrypted packet
began with the same bitstring,
B
.Findingsuchapairofkeyswould
be highly unlikely, which is why the bitstring
B
B
should be chosen as
late as practical.
The combination of these two functions makes it easy to imple-
ment a DC network using asynchronous communications. There is
no need for people to announce their answers in synchrony. Nor is
there any reason for people to be adjacent to each other when they
flip the coin.
11.3.1 Cheating DC Nets
There are a wide variety of ways for people to subvert the DC net-
works, but there are adequate defenses tomany of the approaches. If
people conspire to work together and reveal their information about
bits to others around the table, then there is nothing that can be
done to stop tracing. In these situations, anonymous remailers can
be more secure because they're as secure as their strongest link.
Another major problemmight be jamming. Someone on the net-
work could just broadcast extra messages from time to time and thus
disrupt the message of someone who is legitimately broadcasting. If,
for instance, a message is emerging from the network, a malicious
member of the group could start broadcasting at the same time and
destroy the rest of the transmission. Unfortunately, the nature of DC
networks means that the identity of this person is hidden.
If social problems become important, then it is possible to re-
veal who is disrupting the network by getting everyone on the net-
work to reveal their coin flips. When this is done, it is possible to
