John Denker writes:
> 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.

Yes, that's a very strong attack.  I don't think I am going to be able
to come up with any straightforward fixes against it.  It's back to the
drawing board on this one...

> 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 
> transformations.
>    3) They could recirculate the signal C>1 times if desired.

Another such hybrid idea would be to use quantum key exchange to initially
share random strings between each pair of participants in step 1, then to
run a regular DC net.  You can trivially use the regular DC net algorithm
with a photon rather than a conventional data packet - where you would
toggle the bit in the data packet, you rotate the photon polarization
90 degrees.  This provides no more and no less security than a DC net
at probably much higher cost, so as you say it is hardly worthwhile on
its own.

Other ideas I plan to pursue include hybrid schemes where quantum key
exchange runs simultaneously with the photon-based DC net algorithm to
perhaps provide slightly more efficiency than using two different phases.
And I'm still hopeful that some variant on quantum key exchange can work
for the information flow required in the SC net.

The thing that makes quantum key exchange work is that the eavesdropper
sometimes guesses wrong about what basis to use, and the protocol then
amplifies her resulting gaps in knowledge.  This is harder for a SC net
because if Eve gets even partial information about who is transmitting,
we can't make her forget it.

I'll keep working on it.  Thanks again to John and the others who have
offered helpful criticism and suggestions.

Hal Finney

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