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

From: ZmnSCPxj <[email-masked]>
Date: Fri, 04 Mar 2022 23:33:10 +0000

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