Re: [bitcoin-dev] Annex Purpose Discussion: OP_ANNEX, Turing Completeness, and other considerations

From: Christian Decker <decker.christian@gmail•com>
To: ZmnSCPxj <ZmnSCPxj@protonmail•com>,
	 Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Annex Purpose Discussion: OP_ANNEX, Turing Completeness, and other considerations
Date: Sun, 6 Mar 2022 13:55:53 +0100	[thread overview]
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We'd have to be very carefully with this kind of third-party malleability,
since it'd make transaction pinning trivial without even requiring the
ability to spend one of the outputs (which current CPFP based pinning
attacks require).

Cheers,
Christian

On Sat, 5 Mar 2022, 00:33 ZmnSCPxj via bitcoin-dev, <
bitcoin-dev@lists•linuxfoundation.org> wrote:

> Good morning Jeremy,
>
> Umm `OP_ANNEX` seems boring ....
>
>
> > It seems like one good option is if we just go on and banish the
> OP_ANNEX. Maybe that solves some of this? I sort of think so. It definitely
> seems like we're not supposed to access it via script, given the quote from
> above:
> >
> > Execute the script, according to the applicable script rules[11], using
> the witness stack elements excluding the script s, the control block c, and
> the annex a if present, as initial stack.
> > If we were meant to have it, we would have not nixed it from the stack,
> no? Or would have made the opcode for it as a part of taproot...
> >
> > But recall that the annex is committed to by the signature.
> >
> > So it's only a matter of time till we see some sort of Cat and Schnorr
> Tricks III the Annex Edition that lets you use G cleverly to get the annex
> onto the stack again, and then it's like we had OP_ANNEX all along, or
> without CAT, at least something that we can detect that the value has
> changed and cause this satisfier looping issue somehow.
>
> ... Never mind I take that back.
>
> Hmmm.
>
> Actually if the Annex is supposed to be ***just*** for adding weight to
> the transaction so that we can do something like increase limits on SCRIPT
> execution, then it does *not* have to be covered by any signature.
> It would then be third-party malleable, but suppose we have a "valid"
> transaction on the mempool where the Annex weight is the minimum necessary:
>
> * If a malleated transaction has a too-low Annex, then the malleated
> transaction fails validation and the current transaction stays in the
> mempool.
> * If a malleated transaction has a higher Annex, then the malleated
> transaction has lower feerate than the current transaction and cannot evict
> it from the mempool.
>
> Regards,
> ZmnSCPxj
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