Subject: Re: [bitcoindev] Re: Standard Unstructured Annex

From: Peter Todd <[email-masked]>
Date: Mon, 24 Mar 2025 16:17:46 +0000

On Thu, Mar 20, 2025 at 03:47:16PM -0700, Antoine Riard wrote:
> Hi Peter,
> 
> See also that can be relevant for taproot annex support:
> https://github.com/bitcoin/bips/pull/1381

Thanks.

> > 1) All non-empty annexes start with the byte 0x00, to distinguish them
> > from consensus-relevant annexes. This ensures that any use of the
> > annex will not conflict with future soft-forks that may assign
> > meaning to the annex.
> 
> So IIUC, it would be 1-byte: 0x00 | <random_data payload>.

Correct.

When annex data finally does get a consensus meaning any encoding scheme
starting with a non-zero byte will be compatible. Most likely we'll get
some tag-length-value encoding scheme.

Applications already using annexes who want to also take advantage of
new consensus features will of course have to upgrade their encoding
schemes to match. But I think that's fine.

> > 2) All inputs have an annex. This ensures that use of the annex is
> > opt-in, preventing transaction pinning attacks in multi-party
> > protocols. This requirement may be relaxed in the future, eg to allow
> > spends of keyless outputs, and/or if RBF for witness-only
> > replacements is implemented.
> 
> I think it's good to start with all inputs have an annex. It avoids
> the kind of issue, like what if the annex size is inflated to downgrade
> the feerate of the multi-party transaction (e.g to have a coinjoin
> stucking in network mempools).

Glad to hear you agree.

> One thing that might be missed, without having looked to the code, is
> potentially a policy transaction-relay rule to limit the max size of the
> annex, to avoid the same concern than above. There shouldn't be max
> limit for now, as normally the annex is not standard at all as a taproot
> data field.

Libre Relay has no limit on OP_Return output size; I'm not going to
artificially limit annex usage either. The requirement to opt-in to
annex usage should be sufficient.

There is a possibility of a multi-party, annex-using, protocol where
someone does a pinning attack by re-signing their transaction with a
bigger annex. But witness-RBF in combination with replace-by-fee-rate
will fix this, so I'm not concerned. No such protocols actually exist
yet anyway, so we can figure that out later.

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

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