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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 15:59:37 -0400
In-Reply-To: <26b96fb1-d916-474a-bd23-920becc3412cn@googlegroups.com>
Cc: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
To: Greg Sanders <gsanders87@gmail.com>
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Hey Greg,

The CTV commitment includes the number of inputs. I notice that the TEMPLAT=
EHASH commitment does not. What=E2=80=99s the rationale there?

Thanks,=20

Rijndael

> On Jul 9, 2025, at 2:19=E2=80=AFPM, Greg Sanders <gsanders87@gmail.com> w=
rote:
>=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 op=
erations to achieve these capabilities
>=20
> For this e-mail I would like to address the third point proactively: desi=
gn of the operations to achieve these capabilities.
>=20
> Antoine Poinsot, Steven Roose, and I have been working on a familiar, yet=
 concrete technical proposal that focuses on three well-understood capabili=
ties:
>=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_templ=
atehash/bip-templatehash-csfs-ik.md) and PR here(https://github.com/instagi=
bbs/bips/pull/1), with full motivation for this particular bundle and ratio=
nale discussing alternatives. Our main contribution is a fully specified `O=
P_TEMPLATEHASH` as a drop-in replacement for BIP119 `OP_CHECKTEMPLATEVERIFY=
`. `OP_TEMPLATEHASH` is a simpler and more modern implementation of the "ne=
xt transaction" capability. It differs in committing to the Taproot annex a=
nd 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 of=
 rebindable signatures more efficient.
> - Re-use the existing pre-computed Taproot sighash fields only instead of=
 introducing new ones (substantially simplifying the implementation and rev=
iew of the specifications).
> - Not commit to the spending transaction's scriptSigs (which are both une=
cessary and may incentivize ad-hoc uses of legacy input scripts as programs=
).
> - Not unnecessarily modify the less well-understood legacy Script.
>=20
> Another notable difference is the lack of "bare CTV" analogue, which is i=
mplemented here(https://github.com/instagibbs/bitcoin/tree/p2th) but left o=
ut 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 implementation=
 is provided here(https://github.com/instagibbs/bitcoin/pull/3). The bundle=
 itself is heavily inspired by "LNHANCE"(https://delvingbitcoin.org/t/lnhan=
ce-bips-and-implementation/376).
>=20
> We are hopeful that an opcode/implementation-focused discussion can be he=
ld
> 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 concept=
s
> is concerned.
>=20
> Best,
> Greg
>=20
> (1) https://groups.google.com/g/bitcoindev/c/-qJc1EWQzY0
>=20
> --=20
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egroups.com?utm_medium=3Demail&utm_source=3Dfooter>.

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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,<div><br></div><div>The =
CTV commitment includes the number of inputs. I notice that the TEMPLATEHAS=
H commitment does not. What=E2=80=99s the rationale there?</div><div><br></=
div><div>Thanks,&nbsp;</div><div><br></div><div>Rijndael<br id=3D"lineBreak=
AtBeginningOfMessage"><div><br><blockquote type=3D"cite"><div>On Jul 9, 202=
5, at 2:19=E2=80=AFPM, Greg Sanders &lt;gsanders87@gmail.com&gt; wrote:</di=
v><br class=3D"Apple-interchange-newline"><div>Hello all,<br><br>This is a =
bit of a follow-up from "What's a good stopping point? ... CTV/CSFS..." fro=
m [^1]<br><br>&gt; There has been several objections to this proposal, whic=
h we can group into three categories:<br>exploration of alternatives, demon=
stration of usage, and design of the operations to achieve these capabiliti=
es<br><br>For this e-mail I would like to address the third point proactive=
ly: design of the operations to achieve these capabilities.<br><br>Antoine =
Poinsot, Steven Roose, and I have been working on a familiar, yet concrete =
technical proposal that focuses on three well-understood capabilities:<br><=
br>1. "Next transaction" capability, ala BIP119<br>2. "Verify signature of =
message on stack", ala BIP348<br>3. "Push taproot internal key onto stack",=
 ala BIP349<br><br>These first two capabilities can offer radical simplific=
ations<br>to well-understood systems when combined. The third is a simple<b=
r>update that dovetails with the first two.<br><br>The BIP text is here(htt=
ps://github.com/instagibbs/bips/blob/bip_op_templatehash/bip-templatehash-c=
sfs-ik.md) and PR here(https://github.com/instagibbs/bips/pull/1), with ful=
l motivation for this particular bundle and rationale discussing alternativ=
es. Our main contribution is a fully specified `OP_TEMPLATEHASH` as a drop-=
in replacement for BIP119 `OP_CHECKTEMPLATEVERIFY`. `OP_TEMPLATEHASH` is a =
simpler and more modern implementation of the "next transaction" capability=
. It differs in committing to the Taproot annex and being otherwise Taproot=
 native, which allows us to:<div><br>- Use the `OP_SUCCESS` upgrade hooks i=
n place of legacy `OP_NOP`s and be able to push the template hash on the st=
ack making the flagship use case of rebindable signatures more efficient.<b=
r>- Re-use the existing pre-computed Taproot sighash fields only instead of=
 introducing new ones (substantially simplifying the implementation and rev=
iew of the specifications).<br>- Not commit to the spending transaction's s=
criptSigs (which are both unecessary and may incentivize ad-hoc uses of leg=
acy input scripts as programs).<br>- Not unnecessarily modify the less well=
-understood legacy Script.<br><br>Another notable difference is the lack of=
 "bare CTV" analogue, which is implemented here(https://github.com/instagib=
bs/bitcoin/tree/p2th) but left out of the bundle due to lack of demonstrate=
d utility.<br><br>The BIP for `OP_TEMPLATEHASH` is here(https://github.com/=
instagibbs/bips/blob/bip_op_templatehash/bip-templatehash.md) and a complet=
e implementation is provided here(https://github.com/instagibbs/bitcoin/pul=
l/3). The bundle itself is heavily inspired by "LNHANCE"(https://delvingbit=
coin.org/t/lnhance-bips-and-implementation/376).<br><br>We are hopeful that=
 an opcode/implementation-focused discussion can be held<br>concurrently wi=
th other efforts such as discussions as to whether<br>or not this capabilit=
y set is a good stopping point, including whether<br>this bundle is worth i=
mplementing on its own at all, as well as what<br>level of assurances we sh=
ould have as far as tooling and proof of concepts<br>is concerned.<br><br>B=
est,<br>Greg<br><br>(1) https://groups.google.com/g/bitcoindev/c/-qJc1EWQzY=
0</div><div><br class=3D"webkit-block-placeholder"></div>

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