Do you agree that:
1) If there exists a shared secret, quantum encryption can provide protection, now and in the future, against MITM attacks or passive interception.
2) Streams of entangled particles can generate shared secrets where none previously existed.
DS
I think the flaw here is in authentication. Given a quantum encrypted physical link where neither side is assured of the others authenticity - a link in the "initial" state, there's no way that nodes A and B can tell the difference between:
1) A <--------------------> B
and
2) A <-----> C <------> B
In the first situation A and B generate a shared secret. In the second, A and C generate a shared secret and C and B generate a shared secret. A never knew who B was and doesn't know who B is now. There's only A <--> C . If C is a node controlled by a malicious party (a man in the middle) then your security is compromised. In order to avoid this you must establish a shared secret key in some way between A and B without C knowing - and you must be certain that it's really only A and B that are privy to this key. I don't think this is any different than public key systems in this limitation though. This is why web browsers are distributed with a set of root certificates, isn't it?
In practice I think this will mean that you do some initial verification, perhaps exchange a physical card with a shared key on it between sites, and then you're up and running secure. If the link security is broken, you must re-secure the link and start over. If you attempt to reuse the same key despite multiple intrusion detections you'll give the attacker the chance to deduce the key. It seems the danger here is that detected errors could be a physical problem on the line or it could be an attacker selectively cutting in attempting to guess bits of the key - it's important to have an error free line to ensure security.
Andrew
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