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From: Jeremy <jlrubin@mit.edu>
Date: Mon, 24 Jun 2019 11:07:20 -0700
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Subject: Re: [bitcoin-dev] OP_SECURETHEBAG (supersedes OP_CHECKOUTPUTSVERIFY)
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Do you think the following hypothesis is more or less true:

H: There is no set of pure extensions* to script E such that enabling E and
OP_SECURETHEBAG as proposed enables recursive covenants, but E alone does
not enable recursive covenants?

* Of course there are things that specifically are specifically designed to
switch on if OP_SECURETHEBAG, so pure means normal things like OP_CAT that
are a function of the arguments on the stack or hashed txn data.

This is the main draw of the design I proposed, it should be highly
improbable or impossible to accidentally introduce more behavior than
intended with a new opcode.

I think that given that H is not true for the stack reading version of the
opcode, we should avoid doing it unless strongly motivated, so as to permit
more flexibility for which opcodes we can add in the future without
introducing recursion unless it is explicitly intended.



On Mon, Jun 24, 2019, 7:35 AM Russell O'Connor <roconnor@blockstream.io>
wrote:

> OP_SECURETHEBAG doesn't include the script being executed (i.e the
> scriptPubKey specified in the output that is redeemed by this input) in i=
ts
> hash like ordinary signatures do
> <https://github.com/bitcoin/bitcoin/blob/master/src/script/interpreter.cp=
p#L1271>.
> Of course, this ScriptPubKey is indirectly committed to through the input=
's
> prevoutpoint.  However Script isn't able to reconstruct this script being
> executed from the prevoutpoint in tapscript without an implementation of
> public key tweeking in Bitcoin Script.
>
> On Sun, Jun 23, 2019 at 2:41 AM Jeremy Rubin <jeremy.l.rubin@gmail.com>
> wrote:
>
>> 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 unless some OP_TWEEKPUBKEY operation is added to Script because t=
he
>>> "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 p=
art
>>> 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.
>>>>
>>>> =E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90=E2=80=90 Origin=
al 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 <
>>>> 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 spe=
nding
>>>> 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 spen=
t
>>>> transaction by hashing it, then splitting up the hash with `OP_LEFT` i=
nto
>>>> bytes, and comparing the bytes to the bytes in the input of the spendi=
ng
>>>> 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 correc=
t
>>>> 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 ha=
sh
>>>> commitment in an `OP_RETURN` output.
>>>>
>>>> I believe this is the primary reason against not pulling data from the
>>>> 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
>>>
>>

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Content-Type: text/html; charset="UTF-8"
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<div dir=3D"auto"><div dir=3D"auto">Do you think the following hypothesis i=
s more or less true:</div><div dir=3D"auto"><br></div><div dir=3D"auto">H: =
There is no set of pure extensions* to script E such that enabling E and OP=
_SECURETHEBAG as proposed enables recursive covenants, but E alone does not=
 enable recursive covenants?</div><div dir=3D"auto">=C2=A0</div>* Of course=
 there are things that specifically are specifically designed to switch on =
if OP_SECURETHEBAG, so pure means normal things like OP_CAT that are a func=
tion of the arguments on the stack or hashed txn data.=C2=A0<div dir=3D"aut=
o"><br></div><div dir=3D"auto">This is the main draw of the design I propos=
ed, it should be highly improbable or impossible to accidentally introduce =
more behavior than intended with a new opcode.</div><div dir=3D"auto"><br><=
/div><div dir=3D"auto">I think that given that H is not true for the stack =
reading version of the opcode, we should avoid doing it unless strongly mot=
ivated, so as to permit more flexibility for which opcodes we can add in th=
e future without introducing recursion unless it is explicitly intended.=C2=
=A0</div><div dir=3D"auto"><br></div><div dir=3D"auto"><br><br><div class=
=3D"gmail_quote" dir=3D"auto"><div dir=3D"ltr" class=3D"gmail_attr">On Mon,=
 Jun 24, 2019, 7:35 AM Russell O&#39;Connor &lt;<a href=3D"mailto:roconnor@=
blockstream.io">roconnor@blockstream.io</a>&gt; wrote:<br></div><blockquote=
 class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc soli=
d;padding-left:1ex"><div dir=3D"ltr"><div dir=3D"ltr"><span class=3D"m_2250=
439416008956349gmail-m_7434839216018036533m_-216577082455711303m_2651264282=
820171888gmail-im"><span class=3D"m_2250439416008956349gmail-m_743483921601=
8036533m_-216577082455711303m_2651264282820171888gmail-im">OP_SECURETHEBAG =
doesn&#39;t include the script being executed (i.e the scriptPubKey specifi=
ed in the output that is redeemed by this input) in its hash like <a href=
=3D"https://github.com/bitcoin/bitcoin/blob/master/src/script/interpreter.c=
pp#L1271" target=3D"_blank" rel=3D"noreferrer">ordinary signatures do</a>.=
=C2=A0 Of course, this ScriptPubKey is indirectly committed to through the =
input&#39;s prevoutpoint.=C2=A0 However Script isn&#39;t able to reconstruc=
t this script being executed from the <span class=3D"m_2250439416008956349g=
mail-m_7434839216018036533m_-216577082455711303m_2651264282820171888gmail-i=
m"><span class=3D"m_2250439416008956349gmail-m_7434839216018036533m_-216577=
082455711303m_2651264282820171888gmail-im">prevoutpoint in tapscript withou=
t an implementation of public key tweeking in Bitcoin Script.<br></span></s=
pan></span></span></div><br><div class=3D"gmail_quote"><div dir=3D"ltr" cla=
ss=3D"gmail_attr">On Sun, Jun 23, 2019 at 2:41 AM Jeremy Rubin &lt;<a href=
=3D"mailto:jeremy.l.rubin@gmail.com" target=3D"_blank" rel=3D"noreferrer">j=
eremy.l.rubin@gmail.com</a>&gt; wrote:<br></div><blockquote class=3D"gmail_=
quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,=
204);padding-left:1ex"><div dir=3D"ltr"><div dir=3D"auto"><div>Can you clar=
ify this comment?</div><div><br></div><div><div style=3D"font-family:arial,=
helvetica,sans-serif;font-size:small;color:rgb(0,0,0)">We do in fact commit=
 to the script and scriptsig itself (not the witness stack) in OP_SECURETHE=
BAG (unless I&#39;m missing what you meant)?</div><br><div class=3D"gmail_q=
uote"><div dir=3D"ltr" class=3D"gmail_attr">On Thu, Jun 20, 2019, 10:59 AM =
Russell O&#39;Connor via bitcoin-dev &lt;<a href=3D"mailto:bitcoin-dev@list=
s.linuxfoundation.org" target=3D"_blank" rel=3D"noreferrer">bitcoin-dev@lis=
ts.linuxfoundation.org</a>&gt; wrote:<br></div><blockquote class=3D"gmail_q=
uote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,2=
04);padding-left:1ex"><div dir=3D"ltr"><div>Just to be clear, while <span c=
lass=3D"m_2250439416008956349gmail-m_7434839216018036533m_-2165770824557113=
03m_2651264282820171888gmail-im">OP_CHECKTXDIGESTVERIFY would enable this s=
tyle of covenants if it pulled data from the stack, the <span class=3D"m_22=
50439416008956349gmail-m_7434839216018036533m_-216577082455711303m_26512642=
82820171888gmail-im">OP_SECURETHEBAG probably cannot create covenants even =
if it were to pull the data from the stack unless some OP_TWEEKPUBKEY opera=
tion is added to Script because the &quot;commitment of the script itself&q=
uot; isn&#39;t part of the OP_SECURETHEBAG.</span></span></div><div><span c=
lass=3D"m_2250439416008956349gmail-m_7434839216018036533m_-2165770824557113=
03m_2651264282820171888gmail-im"><span class=3D"m_2250439416008956349gmail-=
m_7434839216018036533m_-216577082455711303m_2651264282820171888gmail-im"><b=
r></span></span></div><div><span class=3D"m_2250439416008956349gmail-m_7434=
839216018036533m_-216577082455711303m_2651264282820171888gmail-im"><span cl=
ass=3D"m_2250439416008956349gmail-m_7434839216018036533m_-21657708245571130=
3m_2651264282820171888gmail-im">So with regards to OP_SECURETHEBAG, I am al=
so<span class=3D"m_2250439416008956349gmail-m_7434839216018036533m_-2165770=
82455711303m_2651264282820171888gmail-im"> &quot;not really seeing any reas=
on to complicate the spec to ensure the digest is precommitted as part of t=
he opcode.&quot;</span></span></span></div><div><span class=3D"m_2250439416=
008956349gmail-m_7434839216018036533m_-216577082455711303m_2651264282820171=
888gmail-im"><span class=3D"m_2250439416008956349gmail-m_743483921601803653=
3m_-216577082455711303m_2651264282820171888gmail-im"></span></span></div></=
div><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On=
 Thu, Jun 6, 2019 at 3:33 AM ZmnSCPxj via bitcoin-dev &lt;<a href=3D"mailto=
:bitcoin-dev@lists.linuxfoundation.org" rel=3D"noreferrer noreferrer" targe=
t=3D"_blank">bitcoin-dev@lists.linuxfoundation.org</a>&gt; wrote:<br></div>=
<blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-=
left:1px solid rgb(204,204,204);padding-left:1ex">Good morning aj,<br>
<br>
<br>
Sent with ProtonMail Secure Email.<br>
<br>
=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<br>
On Wednesday, June 5, 2019 5:30 PM, Anthony Towns via bitcoin-dev &lt;<a hr=
ef=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" rel=3D"noreferrer noref=
errer" target=3D"_blank">bitcoin-dev@lists.linuxfoundation.org</a>&gt; wrot=
e:<br>
<br>
&gt; On Fri, May 31, 2019 at 10:35:45PM -0700, Jeremy via bitcoin-dev wrote=
:<br>
&gt;<br>
&gt; &gt; OP_CHECKOUTPUTSHASHVERIFY is retracted in favor of OP_SECURETHEBA=
G*.<br>
&gt;<br>
&gt; I think you could generalise that slightly and make it fit in<br>
&gt; with the existing opcode naming by calling it something like<br>
&gt; &quot;OP_CHECKTXDIGESTVERIFY&quot; and pull a 33-byte value from the s=
tack,<br>
&gt; consisting of a sha256 hash and a sighash-byte, and adding a new sigha=
sh<br>
&gt; value corresponding to the set of info you want to include in the hash=
,<br>
&gt; which I think sounds a bit like &quot;SIGHASH_EXACTLY_ONE_INPUT | SIGH=
ASH_ALL&quot;<br>
&gt;<br>
&gt; FWIW, I&#39;m not really seeing any reason to complicate the spec to e=
nsure<br>
&gt; the digest is precommitted as part of the opcode.<br>
&gt;<br>
<br>
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`?<br>
<br>
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.<br>
<br>
Script verifies that the spending transaction witness value is indeed the s=
pending transaction by `OP_SHA256 &lt;SIGHASH_ALL&gt; OP_SWAP OP_CAT OP_CHE=
CKTXDIGESTVERIFY`.<br>
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).<br>
<br>
Then the Script can extract a commitment of itself by extracting the output=
 of the spent transaction.<br>
This lets the Script check that the spending transaction also pays to the s=
ame script.<br>
<br>
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.<br>
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.<br>
<br>
I believe this is the primary reason against not pulling data from the stac=
k.<br>
<br>
Regards,<br>
ZmnSCPxj<br>
_______________________________________________<br>
bitcoin-dev mailing list<br>
<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" rel=3D"noreferrer =
noreferrer" target=3D"_blank">bitcoin-dev@lists.linuxfoundation.org</a><br>
<a href=3D"https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev" =
rel=3D"noreferrer noreferrer noreferrer" target=3D"_blank">https://lists.li=
nuxfoundation.org/mailman/listinfo/bitcoin-dev</a><br>
</blockquote></div>
_______________________________________________<br>
bitcoin-dev mailing list<br>
<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" rel=3D"noreferrer =
noreferrer" target=3D"_blank">bitcoin-dev@lists.linuxfoundation.org</a><br>
<a href=3D"https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev" =
rel=3D"noreferrer noreferrer noreferrer" target=3D"_blank">https://lists.li=
nuxfoundation.org/mailman/listinfo/bitcoin-dev</a><br>
</blockquote></div></div></div>
</div>
</blockquote></div></div>
</blockquote></div></div></div>

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