Thanks Nick! I endorse Nick's response, with two additions:
On Thu, Jun 01, 2023 at 09:07:17AM -0400, Nick Mathewson wrote:
Onion key rotation limits the time range in which this kind of attack is useful: it will only work for as long as the onion key is listed in a live directory.
For bridges it is a little bit different, because bridges don't have an onion key listed in any public (consensus style) directory document that clients get. Rather, the client connects to the bridge directly and fetches a full timestamped descriptor from the bridge, which is signed by the bridge's identity key, and which includes the onion key that the client should use.
So if you have broken an old (rotated) onion key for a bridge, the proper attack involves MITMing the connection to the bridge, breaking or stealing the bridge's identity key too, and crafting a new descriptor that lists the old onion key.
Whereas if the bridge never rotates the onion key, then you would be able to successfully attack the CREATE cell that the client sends to the bridge -- but only if you could see it, which would involve MITMing the connection to the bridge and also being able to convince the client that you are the bridge, which I think implies having or breaking the identity key too. Doesn't seem so bad.
(Now, any attacker who can steal your onion key can probably also steal your identity key too, if you don't keep that offline, and use it to impersonate you for even longer. The advantage of using a stolen onion key is that it's much harder to detect; all the attacks I can think of that use a stolen identity key involve, whereas the onion-key-theft attack occurs when you are already in a perfect position to be a MITM.)
"...involve publishing a new signed document which others could notice" maybe?
Though for the bridge case, the attack could be more subtle, in that you could provide a specially signed descriptor only to your victim user, who would then learn the special onion key from that descriptor, use it, and never know that other users received a different descriptor.
An attack like that isn't so bad though, because we still have the second hop and third hop in the circuit, producing their own forward-secret session keys with their own properly rotated onion keys, and having the protections that Nick describes.
--Roger