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To: Jeremy Rubin <jeremy.l.rubin@gmail.com>
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Cc: Bitcoin development mailing list <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] OP_SECURETHEBAG (supersedes OP_CHECKOUTPUTSVERIFY)
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Good morning Jeremy,

While `OP_SECURETHEBAG` commits to the desired output script of the spendin=
g TX, what is being referred to here is the ability to verify the output sc=
ript being spent, i.e. the script that actually contains the `OP_SECURETHEB=
AG`.
By this, we are able to create a contract that ensures that it is paid agai=
n (covenants), which in combination with a little more introspection of TX =
data, allows us to verify the execution of steps of a Turing-complete progr=
am.

It is surprisingly easy to make a language inadvertently Turing-complete, w=
hich is basically the argument here,
That is, with just a little more power and some additional operations that =
would appear reasonable to add by themselves (`OP_CAT`, `OP_LEFT`, `OP_TWEA=
KPUBKEY`) on top of some form of requiring a particular output script, it i=
s possible to validate the execution of Turing-complete programs on the Bit=
coin blockchain.

Thus, with quining (a script which gets the text of its own code as part of=
 the static data it has), `OP_TWEAKPUBKEY`, and a `OP_SECURETHEBAG` that ge=
ts its argument from the stack, it will be possible to make Turing-complete=
 Bitcoin SCRIPT.

I would mildly suggest that we might very well want to consider creating a =
well-designed way of injecting Turing-completeness into Bitcoin SCRIPT (req=
uiring it to be behind a Taproot, so that bugs in Turing-complete code at l=
east have a chance to be bugfixed by agreement of the Taproot signing set) =
since we might eventually find ourselves introducing it inadvertently later=
 in any case.

Regards,
ZmnSCPxj


Sent with ProtonMail Secure Email.

=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90 Original Me=
ssage =E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90
On Sunday, June 23, 2019 6:41 AM, Jeremy Rubin <jeremy.l.rubin@gmail.com> w=
rote:

> Can you clarify this comment?
>
> We do in fact commit to the script and scriptsig itself (not the witness =
stack) in OP_SECURETHEBAG (unless I'm missing what you meant)?
>
> On Thu, Jun 20, 2019, 10:59 AM Russell O'Connor via bitcoin-dev <bitcoin-=
dev@lists.linuxfoundation.org> wrote:
>
> > 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 un=
less some OP_TWEEKPUBKEY operation is added to Script because the "commitme=
nt of the script itself" isn't part of the OP_SECURETHEBAG.
> >
> > So with regards to OP_SECURETHEBAG, I am also "not really seeing any re=
ason 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@li=
sts.linuxfoundation.org> wrote:
> >
> > > Good morning aj,
> > >
> > > Sent with ProtonMail Secure Email.
> > >
> > > =E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90 Origi=
nal Message =E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=
=90
> > > On Wednesday, June 5, 2019 5:30 PM, Anthony Towns via bitcoin-dev <bi=
tcoin-dev@lists.linuxfoundation.org> wrote:
> > >
> > > > On Fri, May 31, 2019 at 10:35:45PM -0700, Jeremy via bitcoin-dev wr=
ote:
> > > >
> > > > > OP_CHECKOUTPUTSHASHVERIFY is retracted in favor of OP_SECURETHEBA=
G*.
> > > >
> > > > 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 si=
ghash
> > > > value corresponding to the set of info you want to include in the h=
ash,
> > > > which I think sounds a bit like "SIGHASH_EXACTLY_ONE_INPUT | SIGHAS=
H_ALL"
> > > >
> > > > FWIW, I'm not really seeing any reason to complicate the spec to en=
sure
> > > > the digest is precommitted as part of the opcode.
> > > >
> > >
> > > I believe in combination with `OP_LEFT` and `OP_CAT` this allows Turi=
ng-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_CHE=
CKTXDIGESTVERIFY`.
> > > Script verifies the spent transaction witness value is indeed the spe=
nt transaction by hashing it, then splitting up the hash with `OP_LEFT` int=
o 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_RET=
URN` 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 co=
mmitment in an `OP_RETURN` output.
> > >
> > > I believe this is the primary reason against not pulling data from th=
e stack.
> > >
> > > Regards,
> > > ZmnSCPxj
> > > _______________________________________________
> > > bitcoin-dev mailing list
> > > bitcoin-dev@lists.linuxfoundation.org
> > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
> >
> > _______________________________________________
> > bitcoin-dev mailing list
> > bitcoin-dev@lists.linuxfoundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev