Just to be clear, while OP_CHECKTXDIGESTVERIFY would enable this style of covenants if it pulled data from the stack, the OP_SECURETHEBAG probably cannot create covenants even if it were to pull the data from the stack unless some OP_TWEEKPUBKEY operation is added to Script because the "commitment of the script itself" isn't part of the OP_SECURETHEBAG.

So with regards to OP_SECURETHEBAG, I am also "not really seeing any reason to complicate the spec to ensure the digest is precommitted as part of the opcode."

On Thu, Jun 6, 2019 at 3:33 AM ZmnSCPxj via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
Good morning aj,


Sent with ProtonMail Secure Email.

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Wednesday, June 5, 2019 5:30 PM, Anthony Towns via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

> On Fri, May 31, 2019 at 10:35:45PM -0700, Jeremy via bitcoin-dev wrote:
>
> > OP_CHECKOUTPUTSHASHVERIFY is retracted in favor of OP_SECURETHEBAG*.
>
> I think you could generalise that slightly and make it fit in
> with the existing opcode naming by calling it something like
> "OP_CHECKTXDIGESTVERIFY" and pull a 33-byte value from the stack,
> consisting of a sha256 hash and a sighash-byte, and adding a new sighash
> value corresponding to the set of info you want to include in the hash,
> which I think sounds a bit like "SIGHASH_EXACTLY_ONE_INPUT | SIGHASH_ALL"
>
> FWIW, I'm not really seeing any reason to complicate the spec to ensure
> the digest is precommitted as part of the opcode.
>

I believe in combination with `OP_LEFT` and `OP_CAT` this allows Turing-complete smart contracts, in much the same way as `OP_CHECKSIGFROMSTACK`?

Pass in the spent transaction (serialised for txid) and the spending transaction (serialised for sighash) as part of the witness of the spending transaction.

Script verifies that the spending transaction witness value is indeed the spending transaction by `OP_SHA256 <SIGHASH_ALL> OP_SWAP OP_CAT OP_CHECKTXDIGESTVERIFY`.
Script verifies the spent transaction witness value is indeed the spent transaction by hashing it, then splitting up the hash with `OP_LEFT` into bytes, and comparing the bytes to the bytes in the input of the spending transaction witness value (txid being the bytes in reversed order).

Then the Script can extract a commitment of itself by extracting the output of the spent transaction.
This lets the Script check that the spending transaction also pays to the same script.

The Script can then access a state value, for example from an `OP_RETURN` output of the spent transaction, and enforce that a correct next-state is used in the spending transaction.
If the state is too large to fit in a standard `OP_RETURN`, then the current state can be passed in as a witness and validated against a hash commitment in an `OP_RETURN` output.

I believe this is the primary reason against not pulling data from the stack.

Regards,
ZmnSCPxj
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