On 31/12/18 00:50, Punk wrote:

On Sun, 30 Dec 2018 22:21:06 +0000 (UTC) jim bell wrote:

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ZME Science: Yes, a quantum internet is possible, new study shows. https://www.zmescience.com/research/technology/quantum-internet-possible-261...

So what happens when a 'quantum' link used to send keys is tapped? Does the key still get through?

Yes, but. Initially it was thought that cloning the signal (as required for tapping it) is impossible due to the no-cloning theorem, part of quantum physics. Therein would lie the untappable nature of Quantum links. Physics itself would protect you. However it was later realised that the no-cloning theorem only says you can't clone a quantum state perfectly - it doesn't say you can't clone it at all. There are several techniques which can be used for partial cloning, some such as the well-known 1->2UQC have hard theoretical limits of 5/6 perfection - ie 5 photons in 6 can be cloned. Others have higher limits. I think there is even one which doesn't have any theoretical limits apart from "not absolutely perfect", but these are harder to implement. While it is possible to design a quantum key exchange machine to defeat 1->2UQC cloning attacks by taking advantage of the statistical limitations of the 1->2UQC cloning process, as far as I am aware (admittedly not very far) none of the machines in existence do so. Plus, a secure authenticated side-channel is required to prevent MITM attacks. Quantum cryptography is not very useful for anything (except maybe getting research funding). Happy New Year to all! Peter Fairbrother

On 12/31/18, Steven Schear wrote:

Retrorefltors are very useful. They are hard to beat for covert comms. Unfortunately, optical ones are limited by weather. Much more versatile, if possibly less covert, are radio types. Ultra lights the size of the Echo are now easily deployed via inflatable and UV regidification.

Retros point back, perhaps not so much useful for direct comms outside their relative dispersion return fields. Planar mirrors, lenses, over horizon, ionosphere, relays, lofted nuke comms, etc are different. https://en.wikipedia.org/wiki/Retroreflector https://en.wikipedia.org/wiki/List_of_passive_satellites