Is this man in middle atack possible in quantum cryptography?

Question

Lets consider there is channel between Alice and Bob, sequered with quantum cryptography. To put it simple, Alice is left, Bob is right and the quantum states are 'up' ~ U and 'down' ~ D. Alice and Bob generates pair of bounded particles ,always one u one d but you dont know wtigth one flyes left or right , what fly always in both direction. Also they can capture particles. If captured, a particle is destroyed.

Alice captures the particle what flyes left and sends free what flyes to the right, Bob does the mirrored. If anyone captures the particle, he would know that other particle of the pair is.

Lets consider the MiM attacker Eve, what can do like Alice and Bob do.

Eve captures the next particle send by Alice, let it be U. So she knows, that the particle what Alice captured locally was D. But the particle U is destroyed, Bob will know that way, that there is something wrong with the channel.

Important : Eve then generates own particles, while capturing the left one. If the captured is D, Eve knows that the pair is U and it might be send further to Bob. In the model that there are only 2 states, the probability of producing the right pair is 50% , therefore Eve in average need to be capable to generate and send particles twice faster that Alice and Bob do, that generally fair possible practicly.

Alice can not distinguish between cases whether a sent particle was captured by Bob or Eve, neither Bob can distinguish, because the quantum features of particles match.

Later Eve can, as usually in MiM take over a channel and start to send false data to them.

Is there something wrong it this szenario or generaly quantum cryptography is a big talk about nothing?

Answer 1

Is there something wrong it this scenario?

Obviously, yes. Any such QKD system would require an authenticated channel back from Bob to Alice; that's to make sure that Bob is the one that's actually receiving the quantum signal, and not a man-in-the-middle.

More fundamentally, your system doesn't actually take advantage of any Quantum Effects; by measuring a particle, you can deduce its entire quantum state (or, at least, as much of the state that the protocol cares about); as such, it can be entirely described with classical physics.

There are actually several proposed QC systems; the simplest is probably the original BB84 system. This system has the photon being in one of four quantum states, and takes advantage of the fact that a measurement will give only 1 bit (and this is the crucial quantum effect); using the authenticated channel, Alice and Bob can coordinate which of Bob's measurements gave answers consistent with what Alice sent; because the attacker cannot measure things without losing information, any attempt by him to measure and retransmit will introduce a measurable error rate.

[Or] quantum cryptography is a big talk about nothing?

A number of people (including myself) are quite skeptical about quantum cryptography; however that doesn't have with us not thinking it doesn't work. Instead, we look at the limitations (need for special quantum channel, cost of equipment, low data rate, low range, vulnerability to side channels), and ask "what advantage does it have compared with classical cryptography (either symmetric or asymmetric) to justify dealing with these limitations".