Good morning aj,

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

> On Fri, May 31, 2019 at 10:35:45PM -0700, Jeremy via bitcoin-dev wrote:
> 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 

Script verifies that the spending transaction witness value is indeed the 
spending transaction by `OP_SHA256 <SIGHASH_ALL> OP_SWAP OP_CAT 
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 

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.

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