Subject: Re: [bitcoindev] Relax OP_RETURN standardness restrictions

From: Peter Todd <[email-masked]>
Date: Sun, 20 Apr 2025 08:43:46 +0000

On Thu, Apr 17, 2025 at 06:52:34PM +0000, 'Antoine Poinsot' via Bitcoin Development Mailing List wrote:
> Hi,
> 
> Standardness rules exist for 3 mains reasons: mitigate DoS vectors, provide upgrade hooks, or as a nudge to deter some usages.
> 
> Bitcoin Core will by default only relay and mine transactions with at most a single OP_RETURN output, with a scriptPubKey no larger than 83 bytes. This standardness rule falls into the third category: it aims to mildly deter data storage while still allowing a less harmful alternative than using non-provably-unspendable outputs.
> 
> Developers are now designing constructions that work around these limitations. An example is Clementine, the recently-announced Citrea bridge, which uses unspendable Taproot outputs to store data in its "WatchtowerChallenge" transaction due to the standardness restrictions on the size of OP_RETURNs[^0]. Meanwhile, we have witnessed in recent years that the nudge is ineffective to deter storing data onchain.
> 
> Since the restrictions on the usage of OP_RETURN outputs encourage harmful practices while being ineffective in deterring unwanted usage, i propose to drop them. I suggest to start by lifting the restriction on the size of the scriptPubKey for OP_RETURN outputs, as a first minimal step to stop encouraging harmful behaviour, and to then proceed to lift the restriction on the number of OP_RETURN outputs per transactions.

I would suggest removing both limits at the same time.

While multiple OP_Return outputs are more expensive than a single one
for the same amount of total data. In some cases they're necessary for
technical reasons, e.g. if signing with SIGHASH_SINGLE.

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

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