[Lightning-dev] Final expiry probing attack

[Joost Jager]

Hi all,

In https://github.com/lightningnetwork/lightning-rfc/pull/516,
the incorrect_or_unknown_payment_details failure message is extended with
an htlc_msat field and thereby replaces the former incorrect_payment_amount
message. The objective of this change is to prevent a probing attack that
allows an intermediate node to find out the final destination of the
payment.

Shouldn't the same change be applied to the cltv expiry?

Currently in lnd, we return a final_expiry_too_soon message if the htlc
expiry is not meeting the invoice cltv delta requirement. This can be used
for probing by using low expiry values, similar to how this was previously
possible with low amounts.

The proposed change would be: when the htlc expiry doesn't meet the invoice
cltv delta requirement, return an incorrect_or_unknown_payment_details
failure (extended with a new htlc_expiry field) instead
of final_expiry_too_soon.

Joost.
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