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Shinobi’s Strawman is a weekly sequence the place our Technical Editor Shinobi challenges the Bitcoin neighborhood, aiming to fire up dialog round heated technical debates.
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Final week I revealed a brief immediate seeking to hear some readers considering on completely different covenant proposals. The design house of covenants has quickly developed within the final two years or so, going from a mere two proposals (CTV and APO) to virtually a dozen completely different proposals for very fundamental performance that may provide massive optimizations to current use instances equivalent to chilly storage schemes or off-chain protocols just like the Lightning community. It is quite a bit to motive by on the a part of the communities on this house attempting to evaluate what proposals they need to or shouldn’t assist. Particularly when the explanations for supporting or not supporting any particular proposal could possibly be something from not wanting or viewing as harmful issues it allows, to effectivity variations between one proposal and one other that implement the identical performance.
I feel whereas the developer and extra technical consumer communities are transferring nearer to consensus on what’s or is just not fascinating, or which proposals allow particular use instances extra effectively, the bigger communities of energetic customers are rather more unsure or undecided. Taking that under consideration I’m going to interrupt up the primary response right here to handle in items
REARDEN
Rearden, proposer of Template Key, despatched this in over Twitter:
BIP119/CTV/OP_CHECKTEMPLATEVERIFY is much and away probably the most totally explored, properly reasoned, prepared for activation covenant proposal. It alone allows a class of scaling enhancements to bitcoin (of which Ark is an instance). What makes CTV a blindingly apparent addition to bitcoin is that it composes superbly with different proposed modifications, as seen in OP_VAULT. CTV is a constructing block for bitcoin like OP_CHECKSIG, most likely extra basic than CLTV or CSV.
OP_VAULT additionally contains 2 covenant opcodes past CTV: one which restricts spends to carrying by the identical taptree with solely the chosen leaf modified in a selected manner (OP_UNVAULT); and one other which restricts spends to a selected output script (handle) (OP_VAULT_RECOVER). Whereas these are launched within the OP_VAULT proposal, they’re additionally designed to be composable and will allow a broader ecosystem of self-custody enhancements than OP_VAULT.
OP_VAULT has truly developed fairly a bit because the unique proposal. Initially it was a really fundamental two half proposal, OP_VAULT and OP_UNVAULT. Every would settle for three items of knowledge as enter for validation, with OP_UNVAULT requiring two of these items of knowledge be the identical as an OP_VAULT enter being spent. OP_VAULT, when making a vault, requires the hash of a restoration key in chilly storage, a time lock delay size, and the hash of a key used to signal the transaction spending from the vault. The ensuing UTXO locked to OP_UNVAULT that may be created would require having the identical time lock worth because the OP_VAULT enter, the identical hash of the restoration key, after which a hash requiring the eventual withdrawal transaction to match that actual hash (this half is basically CTV baked into the OP_UNVAULT). So this proposal very merely achieved supporting vaults, but additionally form of carried out CTV, simply with the requirement that you just create an OP_VAULT output, and should spend from it into an OP_UNVAULT output to have the ability to use it for CTV functions. This may at all times be stoppable by sweeping it to a restoration handle earlier than the timelock expired and the “CTV” path was spendable.
The newest model of OP_VAULT has been modified fairly closely, and really in some methods appears to be like wholly completely different in undertaking the identical factor. Firstly, there isn’t any OP_UNVAULT. That’s changed with a separate OP_VAULT_RECOVER opcode. Secondly, OP_VAULT is now fully performed in tapscript. This implies each OP_VAULT restriction on spending from the vault, as a substitute of being its personal naked script in a UTXO, is now performed as a taptree path. There are plenty of information values it takes as arguments, however the three issues it’s in the end implementing is that this: information describing a brand new script to exchange the taptree path being spent with, which is the primary output being spent to, and the quantity that has to return into the vault, which is the second output being spent to. The script of the primary output, the funds being set as much as withdraw from the vault, should be the very same taptree in its entirety, aside from the one leaf being spent (which is changed). This new leaf makes use of CTV itself to decide to a time locked transaction proscribing the place the withdrawal will ultimately go. Each different path besides the one spent that exists within the vault tree nonetheless exists on this output. Together with a path utilizing OP_VAULT_RECOVER, which accurately simply specifies the restoration chilly storage path, and which output within the spending transaction has to match that script. It additionally enforces that the complete enter quantity has to go to that output. The second output within the transaction beneath dialogue simply precisely replicates the taptree being spent from, and requires the outlined quantity to place again into the vault.
Not solely does this refactored model of OP_VAULT reap the benefits of CTV itself, in order that it may be used by itself with out unneeded inefficiency, however using taproot for OP_VAULT permits for extra flexibility in vault building. Completely different paths can enable much less and fewer to be re-vaulted, however with longer time locks, for instance. Utilizing CTV as a substitute of baking that into OP_UNVAULT within the earlier proposal additionally opens the door to utilizing one thing apart from CTV to fill that function in a vault scheme if it turns into out there in future.
I feel Rearden is fully proper that each of those proposals have very clear and apparent use instances with little or no draw back, and the present state of every proposal has gotten to a degree that there isn’t any useless redundancy between them, and each complement one another fairly properly.
BIP118/APO/SIGHASH_ANYPREVOUT is a really path dependent resolution to the issue of lowering the burden of working lightning nodes and watchtowers. It began with the easy thought of letting an enter not decide to its prevout level; however you may’t add a brand new sighash flag with out a new key model, so we get a brand new key model; you may’t skip solely this enter’s prevout as a result of that might result in quadratic hashing work in validation, so we get implied ANYONECANPAY; the ensuing tx dimension is massive, except we add a magic key for the taproot inner key; probably &c. The result’s a change that’s _sold_ as “simply including a sighash flag” however truly provides a brand new key model, then reuses many of the current sighash, and unintentionally provides a model of inefficient covenant by pre-signed output scripts.
My Template Key draft goals to resolve most of APO’s path dependent decisions by taking a step again and what will be performed as soon as a brand new key model is required. Template Key takes a contemporary take a look at signature hashing modes, and abandons the prevailing ones in favor of a brand new set with higher flexibility and particularly supposed to be usable with both signature opcodes or CTV (most of them any manner). I am 100% constructive that I missed some essential issues within the design of Template Key, however I feel it is directionally extra right than APO.
All that stated, I’d not stand in the way in which of a neighborhood effort to activate APO; it is not evil, simply could possibly be higher.
Once more, I am just about in settlement with Rearden right here. Your complete challenge of APO creating a brand new sighash flag with completely different semantics is it isn’t one thing you are able to do in a backwards appropriate manner in the event you simply attempt to apply it to the whole lot, so solely a brand new key model would have the ability to truly use it. There have been fairly a couple of completely different tweaks and design modifications/realizations over time, and in the end all of it has been aimed on the single new use of sighash flags: having a signature that may be reattached to any enter versioned proper utilizing the identical script and holding the identical worth quantity.
If we’re going to be extending the sighash flag system, there may be extra helpful issues that could possibly be performed except for simply APO. I have never actually digested his Template Key proposal in depth, however one good profit of getting extra flexibility in what inputs and outputs a signature does or does not decide to is making it simpler to vary output quantities later in multiparty protocols to extend charges. Some protocols might do that with extra sighash choices with out everybody having to coordinate to do it.
In the end I feel APO is certainly a desired performance, however there may be nonetheless room in my view to have a look at extra environment friendly and/or extra versatile methods to perform extra than simply APO in a single go.
Different 2 sats: Just like you, I initially had a intestine adverse response to covenants, and particularly recursive ones; however on the finish of the day every _recipient_ of bitcoin chooses their receiving output script (handle) and might select to completely lock them in some silly manner if they need. That is already doable, covenants simply make it doable in new and inventive ways in which additionally allow helpful/sensible issues. Even when we _are_ involved about recursion, the one proposal presently near able to activate that allows it’s APO. A lot has been made concerning the potential for APO to allow recursion and Spookchains. As I stated in my Template Key draft someplace nonetheless, these are an educational curiosity solely; as they require trusted setup with deleted keys, and might nonetheless solely recurse inside a totally pre-mapped set of states.
My solely concern at this level with recursive covenants is not the potential for presidency blacklists, or censoring management, however enabling issues that distort the general system incentives by introducing an excessive amount of complicated performance. For the attraction that’s the third time, I once more agree with Rearden. Not one of the concrete covenant proposals revealed so far allow sufficient complexity to convincingly open the door to any severe incentive distortions.
ANON
An anon on Telegram despatched this:
Sup Shinobi,
A couple of years in the past when Jeremy was proposing OP_CTV activation, the vast majority of the louder voices on Bitcoin Twitter got here collectively to shout idioms of ossification and typically stop any form of constructive discourse from progressing the activation makes an attempt. So far as I can inform, the vast majority of adverse emotions in direction of covenants had been primarily based mostly on two elements; the concern of restriction on future coin spends by massive regulating our bodies, and using speedy trial. I by no means actually understood the primary half, as covenants are opt-in by nature, and any spends out of a covenant take away any ahead dealing with restrictions on transactions. The concern of a authorities someway implementing and requiring everybody to affix a covenant and prohibit future funds appears unfounded, to not point out this similar scenario might kind of be replicated in a intelligent multisignature scheme. As for the speedy trial issues, I suppose I perceive that barely, however solely from a meta-stance, and never particularly to any parts current in OP_CTV itself.
Now that the mud has settled, do you’ve any sympathy for the fears of the covenant deniers of yesterday? Is there any benefit to their social rejection? Why did OP_CTV obtain such a stage of technical consensus however fail to realize the identical stage of social acceptance?
Greatest,
Confused Covenant Cultist
To a level after all I do, it is a very wholesome signal for energetic Bitcoiners to be by default skeptical of proposals or modifications that they don’t absolutely perceive, both in how they work or what they’re for. I feel a very good a part of the response when Jeremy was discussing activation went far past that, with quite a few folks actively in dangerous religion persevering with to “voice issues” after they’d been defined and definitively demonstrated to be baseless. A variety of that although, such as you stated, intertwines with issues and points over utilizing Speedy Trial as an activation mechanism once more.
To place it bluntly, I simply suppose in a big half it was merely folks not liking Jeremy on a private stage, or at the least the way in which that he engaged on the subject of CTV and its activation publicly. It is unhappy, and positively undeserved, however I see the entire incident as a case of individuals not having an issue with the message (in the event that they understood it), simply the messenger.
Till subsequent week, ciao.
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