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Comment by benmmurphy | Hacker News Reader

i don't understand how it is not susceptible to MITM attacks. how do you know you know the thing at the other end is really who you want to talk to without using some kind of preshared key? maybe an eavesdropper can't view the conversation between you and the other end but that doesn't matter if someone can easily pretend to be the other end.

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Both standard quantum key distribution (QKD) and device-independent quantum key distribution schemes assume that the two parties share an authenticated classical channel. Without authentication, of course you cannot prevent man-in-the-middle attacks.

Authentication can be established using a shared secret (which is why purists sometimes refer to QKD as a key expansion protocol). Or, it can be established using signatures and certificate authorities. It is worth pointing out that man-in-the-middle (MITM) attacks must be made online; if you break a signature scheme a year after the QKD protocol finishes, it does not compromise the key.

It's possible to structure "quantum communication" (meaning passing qubits back and forth, basically) so that if someone in the middle reads them at all, the recipient is aware of it. It's not really possible to prevent the interception but it is possible always to detect it, at least in theory.

As far as I understand it (which isn't too far), quantum bits are physical things. So only the person who has the qubits can see the key exchange. As long as the qubits are physically safe in the hands of the person you want to talk to (admittedly a big if), there can't be a MITM attack.

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