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Date: Fri, 14 Mar 2025 13:20:12 +1000
From: Anthony Towns <aj@erisian.com.au>
To: Nadav Ivgi <nadav@shesek.info>
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Subject: Re: [bitcoindev] "Recursive covenant" with CTV and CSFS
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On Wed, Mar 12, 2025 at 12:02:27PM +0200, Nadav Ivgi wrote:
> > adding CSFS and discarding the CSFS private key allows
> > you to have a single commitment that can be reused indefinitely.
> With APO alone, you can use one of two constructs:
> 2. Trusted, Infinite - using a simple non-committed signature spending back
> to the same address. This has similar properties to your CTV+CSFS construct.

Right, that's

   <01 P> OP_CHECKSIG

as the scriptPubKey, "<sig ANYPREVOUT|ALL>" as the witness, committing to
that scriptPubKey (and an anchor output probably), and after generating
that signature, the private key is discarded.

> Does adding CSFS enable any additional designs?
> I think it's impossible to get Trustless, Infinite short of having full
> introspection abilities (CAT/TXHASH/Elements-like), right?

Direct introspection certainly seems like the easiest approach.
TLUV/OP_VAULT should also get you there I think, despite not providing
full introspection.

> > You could have CTV commit to two inputs, with the second input's entire
> > value being burnt to fees, but that's fairly annoying
> Yes, preparing an exact-sized utxo for fees is indeed annoying. However
> it's not much different from CPFP - an extra tx with the same overall
> number of inputs/outputs, only around 46vB less efficient[0] (assuming you
> need change[1]). So at least for some use-cases it's not terrible either.

CPFP is easier to RBF, so still superior, I think.

> Of course, the most WU-optimal construct is APO|SINGLE (implying ACP) that
> you mentioned, where no extra transaction is needed at all.

A big problem with that is that a griefer can potentially attach large
inputs or outputs to your tx and get their package relayed before yours,
making RBF attempts expensive in high-feerate scenarios, potentially
resulting in nothing being confirmed for an extended period. Could
potentially be solved by TRUC or similar rules (or pure replace-by-feerate
rules), though.

Also, for better or worse, an even more WU-optimal construction is simply
paying miners directly, out of band.

Cheers,
aj

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