Subject: Re: [bitcoin-dev] Standardisation of an unstructured taproot annex

From: Antoine Riard <[email-masked]>
Date: Sat, 10 Jun 2023 01:23:36 +0100

Hi Joost,

Thanks for detailing why a '0' as free-form, without any additional
constraints offers valuable engineering properties as of today.

>From a taproot annex design perspective, I think this could be very
valuable if you have a list of unstructured data use-cases you're thinking
about ? As raised on the BIP proposal, those unstructured data use-cases
could use annex tags with the benefit to combine multiple "types" of
unstructured data in a single annex payload. As you're raising smaller bits
of unstructured data might not afford the overhead though my answer with
this observation would be to move this traffic towards some L2 systems ? In
my mind, the default of adding a version byte for the usage of unstructured
data comes with the downside of having future consensus enabled use-cases
encumbering by the extended witness economic cost.

About the annex payload extension attack described by Greg. If my
understanding of this transaction-relay jamming griefing issue is correct,
we can have an annex tag in the future where the signer is committing to
the total weight of the transaction, or even the max per-input annex size ?
This should prevent a coinjoin or aggregated commitment transaction
counterparty to inflate its annex space to downgrade the overall
transaction feerate, I guess. And I think this could benefit unstructured
data use-cases too.

Best,
Antoine

Le ven. 2 juin 2023 à 22:18, Joost Jager via bitcoin-dev <
[email-masked]> a écrit :

> Hi,
>
> As it stands, the taproot annex is consensus valid but non-standard. The
> conversations around standardization seem to be leaning towards the
> adoption of a flexible Type-Length-Value (TLV) format [1]. There's no doubt
> that this approach has considerable potential. However, settling on an
> exact format may require a significant amount of time.
>
> In the interim, the benefits of making the annex available in a
> non-structured form are both evident and immediate. By allowing developers
> to utilize the taproot annex without delay, we can take advantage of its
> features today, without the need to wait for the finalization of a more
> lengthy standardization process.
>
> With this in view, I am proposing that we define any annex that begins
> with '0' as free-form, without any additional constraints. This strategy
> offers several distinct benefits:
>
> Immediate utilization: This opens the door for developers to make use of
> the taproot annex for a variety of applications straight away, thus
> eliminating the need to wait for the implementation of TLV or any other
> structured format.
>
> Future flexibility: Assigning '0'-beginning annexes as free-form keeps our
> options open for future developments and structure improvements. As we
> forge ahead in determining the best way to standardize the annex, this
> strategy ensures we do not limit ourselves by setting its structure in
> stone prematurely.
>
> Chainspace efficiency: Non-structured data may require fewer bytes
> compared to a probable TLV format, which would necessitate the encoding of
> length even when there's only a single field.
>
> In conclusion, adopting this approach will immediately broaden the
> utilization scope of the taproot annex while preserving the possibility of
> transitioning to a more structured format in the future. I believe this is
> a pragmatic and efficient route, one that can yield substantial benefits in
> both the short and long term.
>
> Joost
>
> [1] https://github.com/bitcoin/bips/pull/1381
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