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I want to understand QKD better and know about entangled photons.

In this clip, they explain QKD:

https://www.youtube.com/watch?v=cWpqlgF7uEA

They explain that the photons always are correlated. If the photons have been eavesdropped, Alice and Bob have different keys received.

However, if Eve listens to the key and has a copy of Bob's receiver, could Eve listen to the conversation and detect the right key and modifies their own photons to send further to Bob without notice? Would QKD break down if you have the receiver box of Bob?

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  • $\begingroup$ Eve must pick a "basis" when she eavesdrop. She can learn nothing else about that qubit other than Yes / No on that basis; everything else is lost. She can attempt to make a similar qubit, but she only knows one fact about it. If Alice and Bob happen to pick the same basis, and it's different from the one Eve picked, there's a good chance that they'll get different answers. Eve has no control over that other basis. And then they'll know there's an eavesdropper. $\endgroup$ Nov 9, 2022 at 7:20

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The problem you present is the man-in-the-middle attack. What if Eve supplants Bob using the same hardware? She would get a shared key Alice-Eve. She could do the same with Bob, acting like Alice. She would have a shared key Eve-Bob.

From Laudenbach paper Continuous-Variable Quantum Key Distribution with Gaussian Modulation - The Theory of Practical Implementations:

Quantum key distribution is a method to generate a secret key between two distant parties, Alice and Bob, based on transmitting non-orthogonal quantum states. After the transmission and measurement of these quantum states, Alice and Bob exchange classical messages and perform post-processing to generate a secure key. In order to prevent a man-in-the-middle attack, Alice and Bob need to authenticate these classical messages in advance (so strictly speaking QKD is a key-growing protocol).

So, Alice and Bob need an initial shared secret key to make sure they are talking to each other and there is no Eve in the middle. This is how you solve the problem you have set out.

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