[Cryptography] QM giveth, QM taketh away

[Bill Frantz]

On 2/12/21 at 9:39 PM, leichter at lrw.com (Jerry Leichter) wrote:

>Quantum key distribution (theoretically) provides a way for 
>Alice and Bob to share a random bitstream with strong 
>randomness and privacy guarantees.  It doesn't give a way to 
>transmit a message as such - but once they have that shared 
>bitstream, then can use it as a one-time pad.
>
>Note that the actual physical realization of QKD has proven to 
>be much trickier than the neat theoretical examples.  Still, it 
>seems to be getting there - if this is what you want.  Keep in 
>mind that QKD (and encryption using XOR with a one-time pad) 
>have theoretically perfect security properties, but provide no 
>authentication (so as with raw DH, you can end up conducting a 
>secure communication but with no way of knowing who you are 
>actually communicating with).

If I remember correctly, and the quantum techniques haven't 
changed, QKD requires sending entangled states between the two 
parties and examining how they settle when the entanglement is 
broken. The last I looked this required a physical or line of 
sight link.

If all this is vaguely true, then protecting the link is a 
viable way of providing a reasonable level of authentication for 
the other end.

I vaguely remember hearing about a communication link in the 
Washington DC area that ran beside a freeway. To protect the 
link against various "tapping in" attacks ended with the link 
costing more per mile than the freeway beside it. Protecting a 
quantum link might take this level of effort.

Cheers - Bill

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