[bitcoin-dev] [Mempool spam] Should we as developers reject non-standard Taproot transactions from full nodes?

Mon May 8 12:33:40 UTC 2023

Hi Ali

I'd point you to Andrew Poelstra's post from January 2023 [0] and a Bitcoin StackExchange answer I recently posted [1].

> Considering that miners are largely the entities at fault for allowing the system to be abused like this, the harmony of Bitcoin transactions is being disrupted right now.

Miners are as far as I understand including high fee rate, consensus compatible transactions in mined blocks as the system has been set up for them to do. As I say in that StackExchange answer if you don't like it:

"There are theoretically two options: a consensus change or a policy change. A consensus change disallowing a certain kind of transaction that is acceptable under current consensus rules would need a soft fork and hence would be extremely difficult to pull off assuming that it was a considered a good idea by the broader community. Embedding arbitrary data in transactions would still be possible after this hypothetical soft fork and so its effectiveness would be limited. A default policy change (or custom policy option) would attempt to prevent a certain kind of transaction from propagating across the network without needing a consensus change. However, it would still be possible to submit these kinds of consensus compatible transactions directly to miners bypassing the P2P network."

> I know that some people will have their criticisms about this, absolutists/libertarians/maximum-freedom advocates, which is fine, but we need to find a solution for this that fits everyone's common ground. We indirectly allowed this to happen, which previously wasn't possible before. So we also have a responsibility to do something to ensure that this kind of congestion can never happen again using Taproot.

It isn't a philosophy or ideology consideration, it is a purely technical one. Congestion can happen using Taproot addresses or pre-Taproot addresses. There is fixed, limited block space and variable demand for that block space. You may not like how that block space is being used but if transactions are consensus compatible and paying fees at the market rate the system is working how it should.

> to curtail the loophole in BIP 342 (which defines the validation rules for Taproot scripts) which has allowed these unintended consequences?

There were technical reasons for the design decisions in BIP 342. As Andrew says in his post [0]:

"If we ban "useless data" then it would be easy for would-be data storers
to instead embed their data inside "useful" data such as dummy
signatures or public keys. Doing so would incur a ~2x cost to them, but
if 2x is enough to disincentivize storage, then there's no need to have
this discussion because they will will be forced to stop due to fee
market competition anyway. (And if not, it means there is little demand
for Bitcoin blockspace, so what's the problem with paying miners to fill
it with data that validators don't even need to perform real computation
on?).

But if we were to ban "useful" data, for example, saying that a witness
can't have more than 20 signatures in it, then we are into the same
problem we had pre-Taproot: that it is effectively impossible construct
signing policies in a general and composeable way, because any software
that does so will need to account for multiple independent limits. We
deliberately replaced such limits with "you need to pay 50 weight for
each signature" to makes this sort of analysis tractable."

I personally get the desire to "do something". Fee spikes aren't fun especially for some Lightning use cases and many of us don't like how people are using the limited block space currently. But a game of whack-a-mole with blunt tools such as policy rules and especially consensus rules is ineffective at best and harmful at worst. You may not like this use case but assuming you embark on a game of whack-a-mole what's to stop a group of people popping up in a year declaring their opposition to your use case and trying to prevent your use case? Consensus rules are set and the rest is left to the market.

Thanks

Michael

[0]: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2023-January/021372.html
[1]: https://bitcoin.stackexchange.com/questions/118197/ddos-attack-via-brc-20-ordinals-on-bitcoin

--
Michael Folkson
Email: michaelfolkson at [protonmail.com](http://protonmail.com/)
GPG: A2CF5D71603C92010659818D2A75D601B23FEE0F

Learn about Bitcoin: https://www.youtube.com/@portofbitcoin

------- Original Message -------
On Sunday, May 7th, 2023 at 18:22, Ali Sherief via bitcoin-dev <bitcoin-dev at lists.linuxfoundation.org> wrote:

> Hi guys,
>
> I think everyone on this list knows what has happened to the Bitcoin mempool during the past 96 hours. Due to side projects such as BRC-20 having such a high volume, real bitcoin transactions are being priced out and that is what is causing the massive congestion that has arguable not been seen since December 2017. I do not count the March 2021 congestion because that was only with 1-5sat/vbyte.
>
> Such justifiably worthless ("worthless" is not even my word - that's how its creator described them[1]) tokens threaten the smooth and normal use of the Bitcoin network as a peer-to-pear digital currency, as it was intended to be used as.
>
> If the volume does not die down over the next few weeks, should we take an action? The bitcoin network is a triumvirate of developers, miners, and users. Considering that miners are largely the entities at fault for allowing the system to be abused like this, the harmony of Bitcoin transactions is being disrupted right now. Although this community has a strong history of not putting its fingers into pies unless absolutely necessary - an example being during the block size wars and Segwit - should similar action be taken now, in the form of i) BIPs and/or ii) commits into the Bitcoin Core codebase, to curtail the loophole in BIP 342 (which defines the validation rules for Taproot scripts) which has allowed these unintended consequences?
>
> An alternative would be to enforce this "censorship" at the node level and introduce a run-time option to instantly prune all non-standard Taproot transactions. This will be easier to implement, but won't hit the road until minimum next release.
>
> I know that some people will have their criticisms about this, absolutists/libertarians/maximum-freedom advocates, which is fine, but we need to find a solution for this that fits everyone's common ground. We indirectly allowed this to happen, which previously wasn't possible before. So we also have a responsibility to do something to ensure that this kind of congestion can never happen again using Taproot.
>
> -Ali
>
> ---
>
> [1]: [https://www.coindesk.com/consensus-magazine/2023/05/05/pump-the-brcs-the-promise-and-peril-of-bitcoin-backed-tokens/](https://www.coindesk.com/consensus-magazine/2023/05/05/pump-the-brcs-the-promise-and-peril-of-bitcoin-backed-tokens/?outputType=amp)
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