alt Hacker News> QKD is an ongoing field of research where new issues are routinely being discovered.
This always bothers me a bit. QKD is on a very solid theoretical footing — if you have an authenticated classical communication channel and an actual quantum communication channel that sends actual qubits that are genuinely only in the basis you think they’re in, then it’s secure, full stop. It’s been proven for decades.
But this is hard (hint: a commercially useful quantum computer does not exist yet), so people fudge it with optical techniques that approximate, poorly, what is needed. And the result is not secure.
Replies
QKD is interesting from the PoV of perfect secrecy. But AFAIK with e.g. BB84, the basis orientation communication (used to detect OTP delivery eavesdropping) is done with Wegman-Carter (unconditionally secure) authentication using... a pre-shared key.
So if you're only interested in computational security that is post-quantum, why not pre-share a symmetric key for some AEAD scheme? You'll get forward secrecy with hash ratchet and neither provides future secrecy in principle.
Neither solves the bootstrap and QKD requires a really, really expensive and complex infrastructure just to provide perfect secrecy which we're fine without.
I wouldn't call it "solid theoretical footing". The rough sketch of QKD is
1. BB84 key exchange requires an authenticated channel. typically you do this with a 2. Carter-Wegman MAC, which is information-theoretically secure, but requires shared randomness that cannot be reused.
Successful protocol execution refreshes randomness (you can net gain from it), so you can communicate back and forth continuously when everything is working. An MiTM who simulates a network failure though can expend some of your pre-shared randomness (without it being refreshed). If they do this enough, they can exhaust your shared randomness, and bring down the link until you exchange more shared randomness somehow out of band. if you want to maintain information theoretic security, this might involve e.g. a courier with a USB or whatever (or a carrier pigeon, who knows).
This is still "secure", but is also a significant issue any QKD (even "real" QKD) has that classical cryptography does not have, and has always made me question the "solid" story for QKD.