On Tue, Feb 07, 2023 at 11:36:58AM +0200, Nadav Ivgi via bitcoin-dev wrote:
> > Since Taproot (more generally any kind of MAST) spends have variable size
> 
> Isn't this the case with any arbitrary script execution? Non-taproot

This is even been true for P2PKH inputs: you can double the space of your
scriptSigs by using uncompressed pubkeys instead of compressed pubkeys.

> If the goal is to only allow registering simple singlesig-encumbered UTXOs
> like P2(W)PKH, the participants could be asked to prove that their P2TR
> output commits to an unspendable script path [0].

Technically, only the last person to sign needs to prove this in advance.
Everyone else can prove it with their signatures.

This distinction could be useful to support coinjoin participants spending
complex P2TR outputs into coinjoins, a perfectly valid use-case in theory so
long as they're paying appropriate fees. Though due to how difficult it is to
validate scripts reliably outside the consensus code base, allowing this for
arbitrary scripts could lead to DoS attacks where someone takes advantage of a
bug in script execution to create an invalid transaction.

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