[arksingrad]

You (Alice and Bob, where Alice is Tx/Bob is Rx) need to have agreed upon the basis in which you measure for each bit ahead of time. If you get MITM'd and they don't know the basis to measure in, then:

- They have a 50% chance of measuring in the correct basis and re-sending the proper qubit - They have a 50% change of measuring in the incorrect basis, in which case their measurement means nothing and the qubit they send is in a superposition in the correct basis, leading to a chance Bob measures the wrong value

Over a very long string, it becomes exponentially unlikely that the MITM could guess the proper basis and then re-send the proper qubit to Bob. As that binary string grows in length, it's essentially impossible to MITM with any meaningful likelihood.

[tinco]

I don't know why someone downvoted you, but the flaw people are pointing out is that the initial agreeing upon a basis is equivalent to exchanging a preshared key. If you've got a preshared key, then why go through the trouble of setting up a QKD for sending PSK's? There's bound to be good reasons, but you're comment doesn't address this.