Covenants are in Bitcoin already, almost?
In Bitcoin, covenants refers to restricting how an output is spent: this is from the legal term where conditions on property persist beyond sale. This is usually defined to exclude the “obvious” common requirement that the spending transaction be signed by a given key.
But to step back, there are logically three things OP_CHECKSIG (and friends) do:
- Assemble parts of the spending transaction (which parts depends on the SIGHASH flags).
- Hash that.
- Validates the hash has been signed by the given key.
For covenants, you really just want the first part: the ability to introspect so you can check whatever feature of the transaction you care about (this is the basis for OP_TX, which takes a bitmap telling it what about the spending transaction to push onto the stack so you can test it). But if you only care about equality, you can get away with something that does the first two things (this insight was the basis for Russell O’Connor’s OP_TXHASH, like OP_TX but always hashes before putting on the stack), and just check the hash is what you expected.
But, Burak points out that if you only care about equality, and are happy to specify all the fields that are covered by signatures (with the various SIGHASH variants), you can simply put a pubkey with pre-made signature and an OP_CHECKSIG into the output script! That constrains the transaction’s fields to hash to whatever that OP_CHECKSIG expects.
This, of course, is overkill: you don’t actually care about the signature operation, you’re just using it test hashes to create covenants. But unfortunately (as pointed out when I posted on Twitter, all excited!), it doesn’t quite work yet, because your signature has to commit to the script which contains the signature: a circular dependency.
But BIP-118 (a.k.a. ANYPREVOUT) proposes new SIGHASH flags which allow you not to commit to the input script, so this covenant-via-signature is possible, and Rearden Code has a tweak which makes this more powerful. I also suspect that you can probably use a simple
0x1 as the pubkey in some cases, since BIP-118 defines that to mean the taproot internal key, saving 32 bytes.