Cryptographers achieve perfect secrecy with imperfect devices

Cryptographers achieve perfect secrecy with imperfect devices
quantamagazine.org

> Quantum Theory Demonstrated: Observation Affects Reality

https://www.sciencedaily.com/releases/1998/02/980227055013.h...

Would like to see this applied in a cryptographic system (if it hasn't already been done). The cryptosystem gets destroyed upon observation or an attempt to measure it is in place. Caveat being: the secret(s) gets destroyed upon someone attempting to crack them.

My naive security architect view is, I get the impression the people doing quantum engineering and those working as cryptographers have a very narrow overlap.

Reading about quantum key agreement/distribution (QKD), it's different from cryptographic functions where you have things like convolution and substitution functions, or using hashing to to distribute the information over a very large finite field. The security in quantum appears inseparable from the encoding and implementation itself, which implies there is a proof and description of entropy-in vs. entropy-out that I haven't gone deep enough to locate and read yet.

I'd speculate there would be some interesting unification of the Shannon information entropy of a plaintext with physical state thermodynamic entropy of the "enciphered" Qbits, given transmitting quantum keys like this sounds more like encoding bits into a physical quantum function than a keyed "encryption" scheme for data for the purposes of security.

To do the data exchange, it's not encrypted to a key per se, but it sounds more like a lookup table (LUT) over a field of qbit state probabilities. Doing certification of the security of such a system is different from a normal NIST algorithm candidate because it reads like you could write and evaluate the encoding scheme as a proof.

> Strange as it may sound, interference can only occur when no one is watching.

This is not true. The correct statement is that interference only manifest itself in isolated systems, i.e. systems that are not entangled with anything outside of themselves. Systems that are being "measured" or "watched" are entangled, but they are not the only such systems. Any entanglement outside of the system under consideration is enough to stop interference effects from manifesting themselves.

The reason I say "outside of the system under consideration" is because the ability to observe interference depends on where you draw the line between the system you are measuring and everything else. For example, in an EPR pair you can choose to either consider a single member of the pair, in which case you cannot observe any interference, or you can consider the pair as a whole, in which case you can.

One point to mention — which I feel quite strongly about, and I think my collaborators do as well – is that sweeping generalisations like "perfect security" are really not the point, and, if anything, have mostly done the field a disservice. Such statements do make for catchy headlines, and while there is a solid technical meaning attached to them (information-theoretic security), to a wider audience they might suggest that QKD replaces the need for careful security engineering, which is definitely not the case: if your processing nodes, say, leak out the generated key material via a classical side channel, no amount of theoretical security guarantees will save you!

Rather, device-independent quantum key distribution allows you to scale back the assumptions on your implementation to a well-motivated, minimal set. To me, this is already intriguing enough without the need for hyperbole!