On 10/20/23 8:15 PM, Peter Todd wrote:
On Fri, Oct 20, 2023 at 05:05:48PM -0400, Matt Corallo wrote:
Sadly this only is really viable for pre-anchor channels. With anchor
channels the attack can be performed by either side of the closure, as the
HTLCs are now, at max, only signed SIGHASH_SINGLE|ANYONECANPAY, allowing you
to add more inputs and perform this attack even as the broadcaster.
I don't think its really viable to walk that change back to fix this, as it
also fixed plenty of other issues with channel usability and important
edge-cases.
What are anchor outputs used for other than increasing fees?
Because if we've pre-signed the full fee range, there is simply no need for
anchor outputs. Under any circumstance we can broadcast a transaction with a
sufficiently high fee to get mined.
Indeed, that is what anchor outputs are for. Removing the pre-set feerate solved a number of issues
with edge-cases and helped address the fee-inflation attack. Now, just using pre-signed transactions
doesn't have to re-introduce those issues - as long as the broadcaster gets to pick which of the
possible transactions they broadcast its just another transaction of theirs.
Still, I'm generally really dubious of the multiple pre-signed transaction thing, (a) it would mean
more fee overhead (not the end of the world for a force-closure, but it sucks to have all these
individual transactions rolling around and be unable to batch), but more importantly (b) its a bunch
of overhead to keep track of a ton of variants across a sufficiently granular set of feerates for it
to not result in substantially overspending on fees.
Like I mentioned in the previous mail, this is really a policy bug - we're talking about a
transaction pattern that might well happen where miners aren't getting the optimal value in
transaction fees (potentially by a good bit). This needs to be fixed at the policy/Bitcoin Core
layer, not in the lightning world (as much as its pretty resource-intensive to fix in the policy
domain, I think).
Matt
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