On Wed, Oct 17, 2018 at 07:27:32PM +0100, Michael Rogers wrote: [...]
If we decided not to use the key blinding trick, and just allowed both parties to know the private key, do you see any attacks?
[...]
If I'm understanding your proposal correctly, I believe it would leave you vulnerable to a Key Compromise Impersonation (KCI) attack.
Imagine the scenario where Alice and Bob exchange the information to establish their temporary rendezvous onion which they both know the private key to, and they agree that Bob will be the client and Alice will be the server.
But, before Bob connects to it, the adversary somehow obtains a copy of everything Bob knows (but they don't have the ability to modify data or software on his computer - they just got a copy of his secrets somehow).
Obviously the adversary could then impersonate Bob to Alice, because they know everything that Bob knows. But, perhaps less obviously, in the case that Bob is supposed to connect to Alice's temporary onion which Bob (and now the adversary) know the key to, the adversary can also now impersonate Alice to Bob (by overwriting Alice's descriptors and taking over her temporary onion service).
In this scenario, if Bob hadn't known the private key for Alice's temporary onion that he is connecting to, impersonating Alice to Bob would not have been possible.
And of course, if the adversary can successfully impersonate both parties to eachother at this phase, they can provide their own long-term keys to each of them, and establish a long-term bidirectional mitm - which, I think, would otherwise not be possible even in the event that one party's long-term key was compromised.
Bob losing control of the key material before using it (without his computer being otherwise compromised) admittedly seems like an unlikely scenario, but you asked for "any attacks", so, I think KCI is one (if I'm understanding your proposal correctly).
~leif