Date: Thu, 06 Jun 2019 07:30:13 +0000
To: Anthony Towns <aj@erisian.com.au>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Subject: Re: [bitcoin-dev] OP_SECURETHEBAG (supersedes OP_CHECKOUTPUTSVERIFY)
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Good morning aj,


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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-com=
plete smart contracts, in much the same way as `OP_CHECKSIGFROMSTACK`?

Pass in the spent transaction (serialised for txid) and the spending transa=
ction (serialised for sighash) as part of the witness of the spending trans=
action.

Script verifies that the spending transaction witness value is indeed the s=
pending transaction by `OP_SHA256 <SIGHASH_ALL> OP_SWAP OP_CAT OP_CHECKTXDI=
GESTVERIFY`.
Script verifies the spent transaction witness value is indeed the spent tra=
nsaction by hashing it, then splitting up the hash with `OP_LEFT` into byte=
s, and comparing the bytes to the bytes in the input of the spending transa=
ction 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 s=
ame script.

The Script can then access a state value, for example from an `OP_RETURN` o=
utput of the spent transaction, and enforce that a correct next-state is us=
ed in the spending transaction.
If the state is too large to fit in a standard `OP_RETURN`, then the curren=
t state can be passed in as a witness and validated against a hash commitme=
nt in an `OP_RETURN` output.

I believe this is the primary reason against not pulling data from the stac=
k.

Regards,
ZmnSCPxj