Covenant : a often formal, solemn, and binding settlement.
This phrase has change into one of the crucial charged phrases within the Bitcoin house. They’re the perfect factor since sliced bread. They’re essentially the most harmful factor because the atom bomb. They aren’t actually going to do something to scale Bitcoin, however they’re neat.
Everybody has a very completely different perspective in the direction of them. Now we have the pro-faction, the anti-faction, the ambivalent faction. To make issues worse, covenant is frankly a really obscure time period in its description of mature and concrete proposals to the protocol that might be categorized as covenants.
The levels of distinction between the performance of various proposals which were put ahead is gigantic. A few of them create completely new design areas for what it’s attainable to construct on high of Bitcoin, whereas others strictly talking don’t add any new performance in any respect, they merely optimize issues which might be already presently attainable with a big diploma of complexity and overhead.
Let’s create a brand new definition particular to Bitcoin.
Covenant : any script that ensures some, or all, of the outputs created by a transaction spending an enter with a covenant script should match sure specified standards for the spending transaction to be consensus legitimate.
So in much less strict phrases, if a Bitcoin script presently restricts who can spend a coin by demanding an authorization proof, i.e. a cryptographic signature, or when it may be spent, i.e. after a timelock expires or the spender can present the preimage to a hash, a covenant script restricts how it may be spent, i.e. to who, how a lot to which individual, and so forth. A covenant script may even prohibit a coin in order that it should be spent to a different covenant script.
That final half is the core of what has made covenant such a contentious phrase. Many individuals have massive reservations about including a brand new technique to “lock” bitcoins that may self-propagate and guarantee future cash are restricted in a similar way. Many individuals have considerations about this getting used to wreck fungibility or institute censorship regimes.
I really feel it essential to level out that each of this stuff might be completed proper now, with no covenant script functionality, just by utilizing multisig. Any authority can refuse to permit withdrawals to be processed from exchanges except they’re to a 2-of-2 multisig the place that authority holds one key. From there they will merely refuse to signal transactions sending to addresses the place they don’t maintain a required key, and set up no matter blacklist or whitelist scheme they desired opaquely and completely off-chain.
That stated, it’s nonetheless vital for Bitcoin customers to have a grasp and understanding of the distinction of energy and suppleness between all of the completely different covenant proposals that presently exist.
There are two core issues that covenants search to allow as a way to apply restrictions to how cash are spent, introspection and ahead information carrying.
Introspection is the flexibility to examine completely different components of the transaction that’s being evaluated whereas making an attempt to spend a selected coin. So for example, if you wish to prohibit a coin in order that it must be spent to a selected handle, you’ve gotten to have the ability to evaluate the handle specified within the enter’s covenant script to the handle specified within the output of the transaction spending it. Opcodes that allow introspection are ones that give us the flexibility to match completely different components of the spending transaction towards restrictions included within the script being evaluated. The extra granular you may get with introspection regarding which explicit components of a transaction you’ll be able to look at, the extra highly effective it turns into.
Ahead information carrying is expounded to introspection, and in some ways a consequence of it, that lets you guarantee some piece of data is carried ahead and included in every new covenant script in order that it may be used within the subsequent analysis of the covenant script. That is completed through the use of introspection to limit sure components of the transaction so tightly that they need to embody the precise desired information or they’re invalid. The extra highly effective introspective functionality you’ve gotten, the extra flexibly you’ll be able to carry information ahead, and the extra flexibly you should use that information.
That is simply the primary introduction to a sequence of articles to come back over the following few weeks taking a look at all the most important covenant proposals which might be in a mature state, have obtained latest curiosity, or are conceptually critically vital sufficient that builders agree on their usefulness however not but a concrete design. This gained’t be 100% full, however it will likely be comparatively complete. A couple of of them additionally are usually not strictly covenants per se, however compose very tightly with them.
These will embody:
CHECKTEMPLATEVERIFY CHECKSIGFROMSTACK TXHASH OP_VAULT CHECKCONTRACTVERIFY CAT TWEAKVERIFY
