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Date: Fri, 20 Oct 2023 21:03:49 -0400
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To: Peter Todd <pete@petertodd.org>
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From: Matt Corallo <lf-lists@mattcorallo.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
 security@ariard.me,
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 <lightning-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] [Lightning-dev] Full Disclosure: CVE-2023-40231 /
 CVE-2023-40232 / CVE-2023-40233 / CVE-2023-40234 "All your mempool are
 belong to us"
Precedence: list


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