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From: Rijndael <rot13maxi@gmail.com>
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Subject: Re: [bitcoindev] A Taproot-native (re-)bindable transaction bundle proposal
Date: Wed, 9 Jul 2025 16:05:41 -0400
In-Reply-To: <682337FB-F451-459F-8B4C-56E61C49FA4F@gmail.com>
Cc: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
To: Greg Sanders <gsanders87@gmail.com>
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Hey Greg (again),

I think I answered my own question: the TEMPLATEHASH commitment is a subset=
 of the bip341 common signature message, which does not commit to number of=
 inputs so this doesnt either. The sequences commitment _should_ implicitly=
 fix the number of inputs.=20

Rijndael


> On Jul 9, 2025, at 3:59=E2=80=AFPM, Rijndael <rot13maxi@gmail.com> wrote:
>=20
> Hey Greg,
>=20
> The CTV commitment includes the number of inputs. I notice that the TEMPL=
ATEHASH commitment does not. What=E2=80=99s the rationale there?
>=20
> Thanks,=20
>=20
> Rijndael
>=20
>> On Jul 9, 2025, at 2:19=E2=80=AFPM, Greg Sanders <gsanders87@gmail.com> =
wrote:
>>=20
>> Hello all,
>>=20
>> This is a bit of a follow-up from "What's a good stopping point? ... CTV=
/CSFS..." from [^1]
>>=20
>> > There has been several objections to this proposal, which we can group=
 into three categories:
>> exploration of alternatives, demonstration of usage, and design of the o=
perations to achieve these capabilities
>>=20
>> For this e-mail I would like to address the third point proactively: des=
ign of the operations to achieve these capabilities.
>>=20
>> Antoine Poinsot, Steven Roose, and I have been working on a familiar, ye=
t concrete technical proposal that focuses on three well-understood capabil=
ities:
>>=20
>> 1. "Next transaction" capability, ala BIP119
>> 2. "Verify signature of message on stack", ala BIP348
>> 3. "Push taproot internal key onto stack", ala BIP349
>>=20
>> These first two capabilities can offer radical simplifications
>> to well-understood systems when combined. The third is a simple
>> update that dovetails with the first two.
>>=20
>> The BIP text is here(https://github.com/instagibbs/bips/blob/bip_op_temp=
latehash/bip-templatehash-csfs-ik.md) and PR here(https://github.com/instag=
ibbs/bips/pull/1), with full motivation for this particular bundle and rati=
onale discussing alternatives. Our main contribution is a fully specified `=
OP_TEMPLATEHASH` as a drop-in replacement for BIP119 `OP_CHECKTEMPLATEVERIF=
Y`. `OP_TEMPLATEHASH` is a simpler and more modern implementation of the "n=
ext transaction" capability. It differs in committing to the Taproot annex =
and being otherwise Taproot native, which allows us to:
>>=20
>> - Use the `OP_SUCCESS` upgrade hooks in place of legacy `OP_NOP`s and be=
 able to push the template hash on the stack making the flagship use case o=
f rebindable signatures more efficient.
>> - Re-use the existing pre-computed Taproot sighash fields only instead o=
f introducing new ones (substantially simplifying the implementation and re=
view of the specifications).
>> - Not commit to the spending transaction's scriptSigs (which are both un=
ecessary and may incentivize ad-hoc uses of legacy input scripts as program=
s).
>> - Not unnecessarily modify the less well-understood legacy Script.
>>=20
>> Another notable difference is the lack of "bare CTV" analogue, which is =
implemented here(https://github.com/instagibbs/bitcoin/tree/p2th) but left =
out of the bundle due to lack of demonstrated utility.
>>=20
>> The BIP for `OP_TEMPLATEHASH` is here(https://github.com/instagibbs/bips=
/blob/bip_op_templatehash/bip-templatehash.md) and a complete implementatio=
n is provided here(https://github.com/instagibbs/bitcoin/pull/3). The bundl=
e itself is heavily inspired by "LNHANCE"(https://delvingbitcoin.org/t/lnha=
nce-bips-and-implementation/376).
>>=20
>> We are hopeful that an opcode/implementation-focused discussion can be h=
eld
>> concurrently with other efforts such as discussions as to whether
>> or not this capability set is a good stopping point, including whether
>> this bundle is worth implementing on its own at all, as well as what
>> level of assurances we should have as far as tooling and proof of concep=
ts
>> is concerned.
>>=20
>> Best,
>> Greg
>>=20
>> (1) https://groups.google.com/g/bitcoindev/c/-qJc1EWQzY0
>>=20
>> --=20
>> You received this message because you are subscribed to the Google Group=
s "Bitcoin Development Mailing List" group.
>> To unsubscribe from this group and stop receiving emails from it, send a=
n email to bitcoindev+unsubscribe@googlegroups.com <mailto:bitcoindev+unsub=
scribe@googlegroups.com>.
>> To view this discussion visit https://groups.google.com/d/msgid/bitcoind=
ev/26b96fb1-d916-474a-bd23-920becc3412cn%40googlegroups.com <https://groups=
.google.com/d/msgid/bitcoindev/26b96fb1-d916-474a-bd23-920becc3412cn%40goog=
legroups.com?utm_medium=3Demail&utm_source=3Dfooter>.
>=20

--=20
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<html><head><meta http-equiv=3D"content-type" content=3D"text/html; charset=
=3Dutf-8"></head><body style=3D"overflow-wrap: break-word; -webkit-nbsp-mod=
e: space; line-break: after-white-space;">Hey Greg (again),<div><br></div><=
div>I think I answered my own question: the TEMPLATEHASH commitment is a su=
bset of the bip341 common signature message, which does not commit to numbe=
r of inputs so this doesnt either. The sequences commitment _should_ implic=
itly fix the number of inputs.&nbsp;</div><div><br></div><div>Rijndael</div=
><div><br></div><div><div><br><blockquote type=3D"cite"><div>On Jul 9, 2025=
, at 3:59=E2=80=AFPM, Rijndael &lt;rot13maxi@gmail.com&gt; wrote:</div><br =
class=3D"Apple-interchange-newline"><div><meta http-equiv=3D"content-type" =
content=3D"text/html; charset=3Dutf-8"><div style=3D"overflow-wrap: break-w=
ord; -webkit-nbsp-mode: space; line-break: after-white-space;">Hey Greg,<di=
v><br></div><div>The CTV commitment includes the number of inputs. I notice=
 that the TEMPLATEHASH commitment does not. What=E2=80=99s the rationale th=
ere?</div><div><br></div><div>Thanks,&nbsp;</div><div><br></div><div>Rijnda=
el<br id=3D"lineBreakAtBeginningOfMessage"><div><br><blockquote type=3D"cit=
e"><div>On Jul 9, 2025, at 2:19=E2=80=AFPM, Greg Sanders &lt;gsanders87@gma=
il.com&gt; wrote:</div><br class=3D"Apple-interchange-newline"><div>Hello a=
ll,<br><br>This is a bit of a follow-up from "What's a good stopping point?=
 ... CTV/CSFS..." from [^1]<br><br>&gt; There has been several objections t=
o this proposal, which we can group into three categories:<br>exploration o=
f alternatives, demonstration of usage, and design of the operations to ach=
ieve these capabilities<br><br>For this e-mail I would like to address the =
third point proactively: design of the operations to achieve these capabili=
ties.<br><br>Antoine Poinsot, Steven Roose, and I have been working on a fa=
miliar, yet concrete technical proposal that focuses on three well-understo=
od capabilities:<br><br>1. "Next transaction" capability, ala BIP119<br>2. =
"Verify signature of message on stack", ala BIP348<br>3. "Push taproot inte=
rnal key onto stack", ala BIP349<br><br>These first two capabilities can of=
fer radical simplifications<br>to well-understood systems when combined. Th=
e third is a simple<br>update that dovetails with the first two.<br><br>The=
 BIP text is here(https://github.com/instagibbs/bips/blob/bip_op_templateha=
sh/bip-templatehash-csfs-ik.md) and PR here(https://github.com/instagibbs/b=
ips/pull/1), with full motivation for this particular bundle and rationale =
discussing alternatives. Our main contribution is a fully specified `OP_TEM=
PLATEHASH` as a drop-in replacement for BIP119 `OP_CHECKTEMPLATEVERIFY`. `O=
P_TEMPLATEHASH` is a simpler and more modern implementation of the "next tr=
ansaction" capability. It differs in committing to the Taproot annex and be=
ing otherwise Taproot native, which allows us to:<div><br>- Use the `OP_SUC=
CESS` upgrade hooks in place of legacy `OP_NOP`s and be able to push the te=
mplate hash on the stack making the flagship use case of rebindable signatu=
res more efficient.<br>- Re-use the existing pre-computed Taproot sighash f=
ields only instead of introducing new ones (substantially simplifying the i=
mplementation and review of the specifications).<br>- Not commit to the spe=
nding transaction's scriptSigs (which are both unecessary and may incentivi=
ze ad-hoc uses of legacy input scripts as programs).<br>- Not unnecessarily=
 modify the less well-understood legacy Script.<br><br>Another notable diff=
erence is the lack of "bare CTV" analogue, which is implemented here(https:=
//github.com/instagibbs/bitcoin/tree/p2th) but left out of the bundle due t=
o lack of demonstrated utility.<br><br>The BIP for `OP_TEMPLATEHASH` is her=
e(https://github.com/instagibbs/bips/blob/bip_op_templatehash/bip-templateh=
ash.md) and a complete implementation is provided here(https://github.com/i=
nstagibbs/bitcoin/pull/3). The bundle itself is heavily inspired by "LNHANC=
E"(https://delvingbitcoin.org/t/lnhance-bips-and-implementation/376).<br><b=
r>We are hopeful that an opcode/implementation-focused discussion can be he=
ld<br>concurrently with other efforts such as discussions as to whether<br>=
or not this capability set is a good stopping point, including whether<br>t=
his bundle is worth implementing on its own at all, as well as what<br>leve=
l of assurances we should have as far as tooling and proof of concepts<br>i=
s concerned.<br><br>Best,<br>Greg<br><br>(1) https://groups.google.com/g/bi=
tcoindev/c/-qJc1EWQzY0</div><div><br class=3D"webkit-block-placeholder"></d=
iv>

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