Hi y'all, I'm not sure how good defenses are on implementations other than lnd, but all implementations *should* be keeping a Sphinx reply cache of the past shared secrets they know of . If a node comes across an identical shared secret of that in the cache, then they should reject that packet. Otherwise, it's possible for an adversary to inject a stale packet back into the network in order to observe the propagation of the packet through the network. This is referred to as a "replay" attack, and is a de-anonymization vector.
Typically mix nets enforce some sort of session lifetime identifier to allow nodes to garbage collect their old shared secrets state, otherwise it grows indefinitely. As our messages are actually payments with a clear expiration date (the absolute CLTV), we can use this as the lifetime of a payment circuit session. The sphinx packet construction allows some optional plaintext data to be authenticated along side the packet. In the current protocol we use this to bing the payment hash along with the packet. The rationale is that in order for me to accept the packet, the attacker must use the _same_ payment hash. If the pre-image has already been revealed, then the "victim" can instantly pull the payment, attaching a cost to a replay attempt. However, since the CLTV isn't also authenticated, then it's possible to attempt to inject a new HTLC with a fresher CLTV. If the node isn't keeping around all pre-images, then they might forward this since it passes the regular expiry tests. If we instead extend the associated data payload to cover the CLTV as well, then this binds the adversary to using the same CLTV details. As a result, the "victim" node will reject the HTLC since it has already expired. Continuing down this line, if we progressively add more payment details, for example the HTLC amount, then this forces the adversary to commit the same amount as the original HTLC, potentially making the probing vector more expensive (as they're likely to lose the funds on attempt). If this were to be deployed, then we can do it by using a new packet version in the Sphinx packet. Nodes that come across this new version (signalled by a global feature bit) would then know to include the extra information in the AD for their MAC check. While we're at it, we should also actually *commit* to the packet version. Right now nodes can swap out the version to anything they want, potentially causing another node to reject the packet. This should also be added to the AD to ensure the packet can't be modified without another node detecting it. Longer term, we may end up with _all_ payment details in the Sphinx packet. The only thing outside in the update_add_htlc message would be link level details such as the HTLC ID. Thoughts? : https://github.com/lightningnetwork/lightning-onion/blob/master/replaylog.go -- Laolu
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