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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