NAME
Crypt::Dining - The Dining Cryptographers' Protocol
SYNOPSIS
my $dc = new Crypt::Dining(
LocalAddr => '123.45.6.7',
Peers => [ '123.45.6.8', ... ],
);
my $answer = $dc->round;
my $answer = $dc->round("hello");
DESCRIPTION
The dining cryptographers' protocol is documented in Bruce Schneier's
book as a kind of "cryptographic ouija board". It works as follows:
A number of cryptographers are dining at a circular table. At the end of
the meal, the waiter is summoned and asked for the bill. He replies,
"Thank you, sir. The bill has been paid." The cryptographers now have
the problem of working out whether someone at the table paid the bill,
or whether the NSA has paid it as some sort of veiled threat. The
protocol proceeds.
Each cryptographer flips a coin, and shows the result ONLY to the
participant on his RIGHT. Each cryptographer then compares his coin with
that on his LEFT, and raises his hand if they show different faces. If
any participant paid the bill, he "cheats" and does the opposite, that
is, he raises his hand if the coins show the same face. Now, the hands
are counted. An odd number means that someone at the table paid the
bill. An even number means that the NSA paid.
ASSUMPTIONS AND IMPLEMENTATION
At most one person "cheats" at any time, otherwise the message is
scrambled. Detecting scrambling is only possible with multi-bit messages
containing a checksum.
The comparison operator described above is the XOR operator on
single-bit values. If the protocol is performed with multi-bit messages,
then the XOR is still used.
WIKIPEDIA DESCRIPTION
The following description is copied from
and is
redistributed under the GNU Free Documentation License. It is a very
slightly different protocol to that implemented here, but the result is
the same.
The dining cryptographers protocol is a method of anonymous
communication. It offers untraceability of both the sender and the
recipient.
The method is as follows: two or more cryptographers arrange themselves
around a circular dinner table, with menus hiding the interaction of
each pair of adjacent cryptographers from the rest. Each adjacent pair
picks a random number in private. Then each cryptographer announces
publicly the difference between the number on his right and the number
on his left, adding a message if he wants to transmit one. All
cryptographers then add up the publicly announced numbers. If the sum is
0, no one sent a message. If the sum is a valid message, one
cryptographer transmitted a message. If the sum is invalid, more than
one cryptographer tried to transmit a message; they wait a random time
and try again.
BUGS
If the send_*() and recv_*() methods are overridden to use TCP sockets
with very large messages, deadlock may occur around the ring unless
something intelligent is done with select().
SEE ALSO
,
Crypt::Chimera - another cryptographic curiosity.
COPYRIGHT
Copyright (c) 2005 Shevek. All rights reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.