That is the primary article in a sequence deep diving into particular person covenant proposals which have reached some extent of maturity meriting an in depth breakdown.
CHECKTEMPLATEVERIFY (CTV), put ahead by Jeremy Rubin with BIP 119, is probably the most mature and absolutely fleshed out covenant proposal, not solely out of the proposals we shall be overlaying, however out of the entire covenant proposals of their entirety. As I discussed within the introduction article to this sequence, there are lots of considerations within the ecosystem relating to covenants which can be too versatile enabling issues that wind up having very detrimental penalties for Bitcoin.
CTV was designed particularly to constrain its capabilities tightly sufficient to keep away from any of these considerations. To first perceive how CTV capabilities, we have to perceive the person elements of a Bitcoin transaction.
This can be a very excessive stage view of a Bitcoin transaction. It has inputs, or unspent cash (UTXOs), and outputs, the brand new unspent cash that the transaction will create when it’s confirmed in a block. There are much more items we’ll undergo, however that is the best stage view of a transaction’s construction.
Each transaction additionally has a model quantity subject for the entire transaction, indicating applicability of recent variations of guidelines or options. There may be additionally the marker and the flag, that are set to particular values to point the transaction makes use of Segwit. After that is the enter depend, the variety of inputs within the transaction. Then come the precise inputs.
Every enter comprises a TXID of the transaction that created the unspent coin being spent, a VOUT which marks what output in that transaction is being spent, the scale of the ScriptSig, and the ScriptSig, which is the unlocking script proving the enter being spent is permitted by its locking script guidelines, and eventually a Sequence quantity which is used to make sure the enter being spent is following relative timelock guidelines. i.e. the enter has existed for a sure variety of blocks or size of time since its creation.
The output depend is the following piece of information, the variety of outputs within the transaction. After this comes the precise outputs, which include an quantity of satoshis assigned to that output, the ScriptPubKey measurement, and the precise ScriptPubKey, which is the locking script for that output. Lastly the nLocktime subject applies a timelock worth in timestamp or block top that applies to the complete transaction.
Every Segwit transaction additionally comprises a Witness part, the place every enter has a corresponding witness containing a Stack Objects depend, what number of issues shall be placed on the script stack, a Dimension subject for every merchandise, and the precise information Merchandise to go on the stack.
How CTV Works
CTV is an opcode that allows probably the most fundamental type of introspection and ahead information finishing up of all of the covenant proposals. It permits a script to take a pre-defined 32 byte hash and examine that in opposition to a hash of a lot of the fields of the spending transaction. If the hash derived from the precise spending transaction doesn’t match the pre-defined hash, the transaction is invalid.
The fields it commits to are:
nVersion nLocktime Enter depend A hash of all of the nSequence fields Output depend A hash of all of the outputs Enter index (the place the enter has within the transaction, 1st enter, 2nd, and so on.)
These are all of the fields dedicated to by the CTV hash, of their entirety, and with no capacity to choose and select. That is the diploma of introspection CTV allows, “does the hash of those fields within the spending transaction match the hash within the locking script of the enter being spent,” that’s it. The hash commits to primarily the complete transaction besides the precise inputs. There’s a purpose the hash doesn’t embrace the inputs. With a view to lock an output to a 32 byte hash with CTV, it’s essential to know the hash of the transaction that you’re making certain is the one means for it to be spent. The enter locked with CTV being spent should embrace this hash to be able to be verified in opposition to CTV. That necessitates having the hash of that transaction earlier than you create the entire transaction. That’s not potential.
It’s also possible to nest CTV scripts, i.e. have an preliminary CTV script decide to a transaction with outputs that additionally embrace CTV scripts. That is what permits CTV to “carry ahead” information. All it carries ahead in apply although is no matter information is contained within the chain of transactions. You are able to do this in idea to an infinite depth, however you might be restricted in apply to a finite depth as a result of the nesting have to be generated backwards ranging from the top. It’s because every stage, or “hop,” should have the hash of the transaction transferring to the following one, in any other case you may’t create the locking script within the first place. When you don’t already know the following transaction, you may’t generate the earlier one.
What Is CTV Helpful For
CTV lets you limit an output in order that it could solely be spent, in keeping with consensus guidelines, by a precise pre-defined transaction. A few of you is perhaps asking what the large deal is, we will already pre-sign transactions. If the extent of introspection is so restricted that it could solely accomplish one thing we will already just do pre-signing, what’s the worth add?
First, pre-signed transactions at all times depart open the potential for the keyholder(s) signing new transactions and spending these cash differently. You must belief that the keyholder is not going to do that, or will delete the important thing wanted to signal with (which you additionally should belief them on). CTV removes that belief solely. As soon as the spending transaction is outlined and the output locked to that CTV hash is created, there isn’t any risk of being spent one other means, enforced by consensus.
At present the one means round that belief is to be concerned in pre-signing transactions your self utilizing multisig. Then you definitely may be fully sure that until you select to signal one your self, no different legitimate transaction spending a coin differently may be created. The issue is the extra individuals are concerned, the harder and unreliable coordinating everybody to pre-sign a transaction on the similar time turns into. Previous small sizes it turns into a completely impractical downside to unravel reliably.
CTV offers a means for individuals to know a set of transactions is dedicated with out everybody having to get on-line on the similar time to signal them. It drastically simplifies the coordination course of by permitting everybody to get the wanted data to anybody else each time they will, and as soon as that individual has everybody’s data they will create the chain of CTV transactions with out anybody else’s involvement, and everybody can confirm and be sure that the right end result is the one potential one.
That’s extremely invaluable by itself, however CTV may allow much more invaluable issues together with different opcodes, which we’ll see within the subsequent article.
Closing Ideas
CTV is a tightly restricted covenant that allows a level of introspection and ahead information carrying that’s so restricted it doesn’t exceed the precise performance of something that may be accomplished with pre-signed transactions. The worth proposition isn’t in enabling new performance in its personal proper, however drastically bettering the effectivity, scalability, and safety ensures of what may be constructed presently utilizing pre-signed transactions. This alone is a large profit to nearly each presently deployed protocol utilizing pre-signed transactions.
Listed below are among the initiatives demonstrating how totally fleshed out and explored this explicit covenant is in comparison with the others:
A fundamental fee pool instance by stutxo. A CTV vault implementation by James O’Beirne, who went on to suggest OP_VAULT (which nonetheless makes use of CTV). A proof-of-concept port of the pre-signed transaction primarily based Ark implementation from Second by Steven Roose to make use of CTV as a substitute. The Sapio Language by Jeremy Rubin himself, the next stage language for constructing contracts with CTV (additionally supporting the usage of pre-signed transactions as a substitute). Timeout Bushes, a proposal for a really fundamental coinpool design by John Regulation. Quite a few different potential protocols, similar to optimized Discreet Log Contracts (DLCs), non-interactive Lightning channels one occasion might open with out the opposite, and even decentralized methods for miners to pool collectively.
CTV is an extremely mature proposal at this level, with a excessive worth add, and no threat of enabling something driving the considerations round covenants. This could not solely be very severely thought of, however in my private opinion ought to have been activated years in the past.
