tl;dr: I don't think CTV is ready yet (but probably close), and in any case definitely not worth reviving BIP 9 with its known flaws and vulnerability.
My review here is based solely on the BIP, with no outside context (aside from current consensus rules, of course). In particular, I have _not_ looked at the CTV code proposed for Bitcoin Core yet. >Covenants are restrictions on how a coin may be spent beyond key ownership. nit: Poorly phrased. Even simple scripts can do that already. >A few examples are described below, which should be the subject of future non-consensus standardization efforts. I would ideally like to see fully implemented BIPs for at least one of these (preferably the claimed CoinJoin improvements) before we move toward activation. >Congestion Controlled Transactions I think this use case hasn't been fully thought through yet. It seems like it would be desirable for this purpose, to allow any of the recipients to claim their portion of the payment without footing the fee for every other payment included in the batch. This is still a covenant-type solution, but one that BIP 119 cannot support as-is. (I realise this may be a known and accepted limitation, but I think it should be addressed in the BIP) >Payment Channels Why batch mere channel creation? Seems like the spending transaction should really be the channel closing. >CHECKTEMPLATEVERIFY makes it much easier to set up trustless CoinJoins than previously because participants agree on a single output which pays all participants, which will be lower fee than before. I don't see how. They still have to agree in advance on the outputs, and the total fees will logically be higher than not using CTV...? >Further Each participant doesn't need to know the totality of the outputs committed to by that output, they only have to verify their own sub-tree will pay them. I don't see any way to do this with the provided implementation. >Deployment could be done via BIP 9 VersionBits deployed through Speedy Trial. Hard NACK on this. BIP 9 at this point represents developers attempting to disregard and impose their will over community consensus, as well as an attempt to force a miner veto backdoor/vulnerability on deployment. It should never be used again. Speedy Trial implemented with BIP 8 made sense* as a possible neutral compromise between LOT=True and LOT=False (which could be deployed prior to or in parallel), but using BIP 9 would destroy this. As with Taproot, any future deployments should use BIP 8 again, until a better solution is developed. Reverting back to a known flawed and vulnerable activation method should not be done, and it would be better not to deploy CTV at all at such an expense. The fact that certain developers attempted to deploy a BIP 9 alternative activation for Taproot against community consensus, and that even managed to get released as "Bitcoin Core", makes it all the more important that the community firmly rejects any further action to force this regression. * it is my opinion a BIP 8 ST would be an okay compromise under those circumstances; others do disagree that ST is acceptable at all > This ensures that for a given known input, the TXIDs can also be known ahead of time. Otherwise, CHECKTEMPLATEVERIFY would not be usable for Batched Channel Creation constructions as the redemption TXID could be malleated and pre-signed transactions invalidated, unless the channels are built using an Eltoo-like protocol. Why is it a problem for them to use an Eltoo-like protocol? Why not just commit to the txid itself if that's the goal? >P2SH is incompatible with CHECKTEMPLATEVERIFY Maybe the CTV opcode should only be defined/enforced within witness scripts? >nLockTime should generally be fixed to 0 (in the case of a payment tree, only the *first* lock time is needed to prevent fee-sniping the root) Your "Congestion Controlled Transactions" example would only make sense with the spending transaction much later than the "root", and so could benefit from fee sniping malleability. (In fact, in that example, it would be better not to commit to locktime at all.) >In the CHECKTEMPLATEVERIFY approach, the covenants are severely restricted to simple templates. The structure of CHECKTEMPLATEVERIFY template is such that the outputs must be known exactly at the time of construction. Based on a destructuring argument, it is only possible to create templates which expand in a finite number of steps. It's not clear to me that this holds if OP_CAT or OP_SHA256STREAM get added. >For example, a exchange's hot wallet might use an address which can automatically be moved to a cold storage address after a relative timeout. Wouldn't it make more sense to just have a UTXO both cold+hot can spend, then throw away the hot key? >In contrast to previous forks, OP_CHECKTEMPLATEVERIFY will not make scripts valid for policy until the new rule is active. Policy isn't validity, and cannot be dictated by BIPs (or anyone/anything, for that matter). Luke _______________________________________________ bitcoin-dev mailing list firstname.lastname@example.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev