Covenant : a often formal, solemn, and binding settlement. 

This phrase has turn out to be one of the crucial charged phrases within the Bitcoin area. They’re the most effective factor since sliced bread. They’re probably the most harmful factor for the reason that atom bomb. They aren’t actually going to do something to scale Bitcoin, however they’re neat. 

Everybody has a totally totally different angle in direction of them. We have now 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 will be categorised as covenants. 

The levels of distinction between the performance of various proposals which were put ahead is big. A few of them create fully new design areas for what it’s potential to construct on prime of Bitcoin, whereas others strictly talking don’t add any new performance in any respect, they merely optimize issues which can be already at present potential 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 must match sure specified standards for the spending transaction to be consensus legitimate. 

So in much less strict phrases, if a Bitcoin script at present 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 particular person, and many others. A covenant script may even limit a coin in order that it have to 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 strategy to “lock” bitcoins that may self-propagate and guarantee future cash are restricted in a similar way. Many individuals have issues about this getting used to wreck fungibility or institute censorship regimes. 

I really feel it essential to level out that each of these items could be achieved 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 fully off-chain. 

That mentioned, it’s nonetheless necessary for Bitcoin customers to have a grasp and understanding of the distinction of energy and suppleness between all of the totally different covenant proposals that at present exist. 

There are two core issues that covenants search to allow with a purpose to apply restrictions to how cash are spent, introspection and ahead information carrying. 

Introspection is the power to examine totally different elements of the transaction that’s being evaluated whereas attempting to spend a selected coin. So as an illustration, if you wish to limit a coin in order that it needs to be spent to a selected handle, you could have to have the ability to examine 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 power to match totally different elements of the spending transaction towards restrictions included within the script being evaluated. The extra granular you may get with introspection regarding which explicit elements of a transaction you’ll be able to study, 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 achieved through the use of introspection to limit sure elements of the transaction so tightly that they have to embrace the precise desired information or they’re invalid. The extra highly effective introspective functionality you could have, the extra flexibly you’ll be able to carry information ahead, and the extra flexibly you need to use that information. 

That is simply the primary introduction to a sequence of articles to come back over the following few weeks all the foremost covenant proposals which can be in a mature state, have obtained current curiosity, or are conceptually critically necessary sufficient that builders agree on their usefulness however not but a concrete design. This received’t be 100% full, however it is going to be comparatively complete. Just a few of them additionally will not be strictly covenants per se, however compose very tightly with them. 

These will embrace: 

1. CHECKTEMPLATEVERIFY 
2. CHECKSIGFROMSTACK 
3. TXHASH
4. OP_VAULT
5. CHECKCONTRACTVERIFY
6. CAT
7. TWEAKVERIFY