Can quantum computers break quantum cryptography?

Question

I happened to learn that encryption by quantum cryptography would be impossible to break as it's state changes the instant of an eavesdropping event(by non-quantum systems) occurs. But I'm not sure if it's still held in the case of the eavesdropper using a quantum computer(not sure how that works either) as well. Couldn't find the answer on Google so felt the community could shed some light on this.

it seems the question I'm asking maybe pointless as quantum computers are computers capable of factoring large numbers efficiently. So I guess quantum cryptography is unbreakable after all...or is it? — Allen, Sep 10 '18 at 16:46
Quantum computers use quantum physics to accelerate certain calculations like factoring. Quantum cryptography uses quantum physics to create a channel to transmit information in such a way that eavesdropping causes the information to change which then can be detected. — j.p., Sep 11 '18 at 06:02

score 6 · Accepted Answer · answered Sep 10 '18 at 23:18

6

I happened to learn that encryption by quantum cryptography would be impossible to break as it's state changes the instant an eavesdropping event (by non-quantum systems) occurs.

Actually, the QKD system doesn't assume that the eavesdropper is nonquantum; he is allowed to attempt to generate an entangled state with the qubit being transmitted. However, even the act of generating such an entangled state would alter the received bit (with a probability that is bounded away from zero), hence any such eavesdropper is detectable, even if the eavesdropper is allowed a quantum computer.

[Insert standard skepticism about whether QKD is a practical system, compared to the alternatives]

answered Sep 10 '18 at 23:18

poncho

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[Also insert standard reference to fgrieu's detailed discussion on the topic] – SEJPM Sep 11 '18 at 08:50
There is a man in the middle attack for the Quantum key exchange. – kelalaka Sep 12 '18 at 06:00
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@kelalaka: how is there a MITM attack (assuming that the attacker cannot fake the authenticated parts of the exchange)? Or, are you talking about a side channel attack (where the attacker gets information beyond the actual qubit)? – poncho Sep 12 '18 at 10:12

Can quantum computers break quantum cryptography?

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