Re: Quantum cryptography tackles video

Jan Panteltje wrote: 
On a sunny day (Wed, 11 May 2005 12:31:25 -0400) it happened Baugh
<baconbaugh@xxxxxxxxxxx> wrote in <Grqge.3571$sV7.2740@xxxxxxxx>:
>
Baugh:
You are mistaken. The point of quantum crypt. is that should the man
in the middle read the signal he will affect it. You can't anonymously tap a quantum signal. 

Wrong, the way it works is that they generate a key, use OTP (one time pad),
and xor the data with it.
The key is send via the fiber, as polarized photon pairs, so if A measures
one polarization, B must have the other.
HOWEVER one can buy these units commercially, cut the fiber, decode, fix the
data, then re-encode (generate a new pair of photons).
The ONLY thing protecting you against that attack is a strict protocol, NOT
the quantum stuff.
Look it up, sci.crypt, about 2 weeks ago.


Pardon, I didn't read the article. I did attend a couple of seminars on the issue. I don't know if this is a case of what I was explained but
the point of the method is that the pair is entangled.

This means given two sets of polarization measurements, vert vs horiz.
and l-cir vs r-circ, All corresponding pair mesurements will be correlated. However if you intercept one of the pair and measure one
of the polarization modes (say linear) you cannot use that to replicate the that half of an entangled pair. You can reproduce the linear value
but the circular polarization is now no-longer correlated with that
of the other of the pair.

The critical point is that what you can measure and then reproduce (tapping the line) is mode dependent (circular vs linear polarization, or x-spin vs y-spin vs z-spin), but the entanglement correlates all modes (or anti-correlates which comes to the same thing). This stronger correlation is at the cost of any actual control of which the values are. Thus one side doesn't actually send the other a specific signal,
they both observe the same modes of an entangled pair and get private
access to the random outcomes.

You could only tap the line if you knew ahead of time which modes the two listners would choose to observe. By using a true random number generator and the values of previous bits to decide the next they can establish a protocol only if you aren't listening to the majority of the bits. Their failure is their detection of a tapped line. Their success assures that you've not been listening to enough of the signal to get the full key. This is a bit simplified but that is the essence of the method.

Regards,
James Baugh


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