John Denker, <[EMAIL PROTECTED]>, writes:
> At 10:35 PM 1/15/01 -0800, [EMAIL PROTECTED] wrote:
> >Here is a rough idea for a quantum-cryptography variant on the DC Net,
> >the Dining Cryptographers Net invented by David Chaum.
> Hmmm.  This seems like a mistake in the physics.  If the attacker, Eve, 
> knows that a photon has either vertical (0 degrees) or horizontal (90 
> degrees) polarization, she can measure it at any point in the ring without 
> destroying any information, and therefore without risk of detection.

Yes, I see that John is right.  A conceptually simple method is to measure
the photon using a polarizer.  If the photon is absorbed, the eavesdropper
knows its polarization state and can simply emit a new photon with the
required state.  In either case the measurement is not detected.

The version with random orientations should still be somewhat resistant
to such measurements.  Eve would not know how to orient her measuring
apparatus and so would likely perturb the photon.  The effect would
largely be to introduce noise into the output, which should be detectable
by the participants at some level.


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