At 10:10 AM 1/20/01 -0800, [EMAIL PROTECTED] wrote:

>This analysis will focus on one particular kind of attack.  Eve will make
>measurements of the photon polarization angle as it travels through the
>network and attempt to deduce information about the signals being sent
>by the participants.

This appears to be a correct analysis of this particular attack.  However, 
this is not Eve's strongest attack.  So let's move the focus.

A much better strategy for Eve is to _not_ make so many 
measurements.  Rather, she should preserve the photon in all its analog, 
quantum-mechanical glory and recirculate it back to Bob, bypassing the 
other participants in the ring.

Then Bob, in blissful ignorance, will decrypt his own signal.  We have 
reduced the problem to the trivial case of the one-person ring;  in such a 
ring it is obvious whether Bob sent a message or not.

The contrast with the conventional Dining Cryptographer's ring is 
illuminating:  In the DC ring, Bob depends on somebody else (indeed 
everybody else) to undo the transformations that he applies, so that if Eve 
attempts to spoof, short-circuit, or partition the ring, the results will 
be cryptologically random.

The SC net appears to have a problem at the algorithm level (not at the 
physics level), namely it doesn't involve the other participants in the 
right way.  It is too easy for Eve to simulate the other 
participants.  This could be patched up by adding macroscopic (i.e. 
non-quantum) authentication protocols, but the cost of doing this would 
probably be comparable to the cost of implementing the classical DC 
network.  So it's not clear what is the advantage of the SC network.


One could imagine a hybrid scheme:
   1) The participants exchange keys, as in the conventional DC net, and
   2) The participants process the signal by rotating the polarization, or 
shifting the quantum phase, or other unconventional, non-Boolean 
   3) They could recirculate the signal C>1 times if desired.

Right now this seems like a solution in search of a problem;  that is, I 
don't know any problems for which the solution requires ideas (2) and (3), 
but they seem like interesting ideas that should be good for something.

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