Re: Shared key?
@g0rg0r - thanks for taking the trouble to respond
If I understand you right, you are saying that the 'system' will know if the message has been intercepted. This may well be true - I can't tell from the article. But re-reading it, I know I am even more confused!
Firstly it's not clear if the quantum bit only applies to the transmission of the keys as opposed to the transmission of the subsequently encrypted message. I think it is keys only and the message can, in theory, be sent by any means.
Secondly, it didn't dawn on me until later that this is NOT a public key system. It's shared keys:
Alice then sends the K1-encrypted message and K3-encrypted checksum to Bob. Bob uses K1 to decrypt the message, and verifies it came from Alice by decrypting the checksum with K3 and recomputing it using the random number N they'd shared previously.
If Alice uses K1 and K3 to encrypt and Bob uses K1 and K3 to decrypt then this is nothing more than a fancy shared-key scheme.
It also means there is a backdoor because the control centre also knows K1, K3 and N so they can decrypt any message at any time. (Providing they can get a copy, of course.)
[Disclaimer: this is all based on the info in the article which may well be wrong. :-) ]