Subject: Re: [bitcoin-dev] CPFP Carve-Out for Fee-Prediction Issues in Contracting Applications (eg Lightning)

From: "Russell O'Connor" <[email-masked]>
Date: Fri, 30 Nov 2018 12:38:04 -0500

On Fri, Nov 30, 2018 at 9:50 AM Matt Corallo via bitcoin-dev <
[email-masked]> wrote:

> To partially-address the CPFP security model considerations, a next step
> might involve tweaking Lightning's commitment transaction to have two
> small-value outputs which are immediately spendable, one by each channel
> participant, allowing them to chain children off without allowng
> unrelated third-parties to chain children. Obviously this does not
> address the specific attack so we need a small tweak to the anti-DoS
> CPFP rules in Bitcoin Core/BIP 125:
>

It seems to me that this two-output scheme does address the specific attack
without tweaking the RBF rules of BIP 125, since you are not doing an RBF
at all.

Suppose we have a 1k-vbyte unconfirmed transaction, TX0, with outputs Z, A,
and B, where A and B are small outputs controlled by the participants Alice
and Bob respectively, with a 1ksat fee, yielding a fee rate of 1sat/vbyte.
Someone, maybe Alice, attempts to pin the transaction, maliciously or not,
by attaching a 10k-vbyte transaction, TX1, to either output Z or output A,
with a fee of 21ksats.  This brings the fee rate for the TX0-TX1 package to
2sat/vbyte, being 11k-vbyte total size with 22ksats in total fees.

Now Bob wants to CPFP to increase the effective fee rate of TX0 to
3sats/vbyte using output B.  He attaches a 1k-vbyte transaction, TX2, to
output B with a fee of 5ksats.  This ought to create a new TX0-TX2 package
with a 3sat/vbyte fee rate, being 2k-vbyte total size with 6ksats in total
fees.  TX1 has now been excluded from the package containing TX0. But TX1
hasn't been replaced, so the RBF rules from BIP125 don't apply.  TX1 is
still a valid unconfirmed transaction operating at a fee rate of
2.1sats/vbyte.

That said, I'm not an expert on how packages and package fee rates are
calculated in Bitcoin Core, so I am speculating a bit.  And, because I'm
talking with Matt, it's more likely that I'm mistaken.  AFAIK, any rules
about CPFP's behaviour in Bitcoin Core is undocumented.