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Date: Fri, 20 Oct 2023 21:55:12 -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>,
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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 9:25 PM, Peter Todd wrote:
> On Fri, Oct 20, 2023 at 09:03:49PM -0400, Matt Corallo wrote:
>>> 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.
> 
> Quite the contrary. Schnorr signatures are 64 bytes, so in situations like
> lightning where the transaction form is deterministically derived, signing 100
> extra transactions requires just 6400 extra bytes. Even a very slow 100KB/s
> connection can transfer that in 64ms; latency will still dominate.

Lightning today isn't all that much data, but multiply it by 100 and we start racking up data enough 
that people may start to have to store a really material amount of data for larger nodes and dealing 
with that starts to be a much bigger pain then when we're talking about a GiB or twenty.

> RBF has a minimum incremental relay fee of 1sat/vByte by default. So if you use
> those 100 pre-signed transaction variants to do nothing more than sign every
> possible minimum incremental relay, you've covered a range of 1sat/vByte to
> 100sat/vByte. I believe that is sufficient to get mined for any block in
> Bitcoin's entire modern history.
> 
> CPFP meanwhile requires two transactions, and thus extra bytes. Other than edge
> cases with very large transactions in low-fee environments, there's no
> circumstance where CPFP beats RBF.

What I was referring to is that if we have the SIGHASH_SINGLE|ANYONECANPAY we can combine many HTLC 
claims into one transaction, vs pre-signing means we're stuck with a ton of individual transactions.

Matt