Feedback-ID: i525146e8:Fastmail
Date: Sat, 21 Oct 2023 01:25:10 +0000
From: Peter Todd <pete@petertodd.org>
To: Matt Corallo <lf-lists@mattcorallo.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
 security@ariard.me,
 "lightning-dev\\\\\\\\\\\\\\\\@lists.linuxfoundation.org"
 <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"
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On Fri, Oct 20, 2023 at 09:03:49PM -0400, Matt Corallo wrote:
> > What are anchor outputs used for other than increasing fees?
> >=20
> > 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 w=
ith a
> > sufficiently high fee to get mined.
>=20
>=20
> 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 wh=
ich
> of the possible transactions they broadcast its just another transaction =
of
> theirs.
>=20
> 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 bu=
nch
> of overhead to keep track of a ton of variants across a sufficiently
> granular set of feerates for it to not result in substantially overspendi=
ng
> 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.

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.

--=20
https://petertodd.org 'peter'[:-1]@petertodd.org

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