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Date: Wed, 9 Jul 2025 11:19:22 -0700 (PDT)
From: Greg Sanders <gsanders87@gmail.com>
To: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
Message-Id: <26b96fb1-d916-474a-bd23-920becc3412cn@googlegroups.com>
Subject: [bitcoindev] A Taproot-native (re-)bindable transaction bundle proposal
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Hello all,
This is a bit of a follow-up from "What's a good stopping point? ...
CTV/CSFS..." from [^1]
> 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
operations to achieve these capabilities
For this e-mail I would like to address the third point proactively: design
of the operations to achieve these capabilities.
Antoine Poinsot, Steven Roose, and I have been working on a familiar, yet
concrete technical proposal that focuses on three well-understood
capabilities:
1. "Next transaction" capability, ala BIP119
2. "Verify signature of message on stack", ala BIP348
3. "Push taproot internal key onto stack", ala BIP349
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.
The BIP text is
here(https://github.com/instagibbs/bips/blob/bip_op_templatehash/bip-templatehash-csfs-ik.md)
and PR here(https://github.com/instagibbs/bips/pull/1), with full
motivation for this particular bundle and rationale discussing
alternatives. 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:
- 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
review of the specifications).
- Not commit to the spending transaction's scriptSigs (which are both
unecessary and may incentivize ad-hoc uses of legacy input scripts as
programs).
- Not unnecessarily modify the less well-understood legacy Script.
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.
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/lnhance-bips-and-implementation/376).
We are hopeful that an opcode/implementation-focused discussion can be held
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 concepts
is concerned.
Best,
Greg
(1) https://groups.google.com/g/bitcoindev/c/-qJc1EWQzY0
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Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Hello all,<br /><br />This is a bit of a follow-up from "What's a good stop=
ping point? ... CTV/CSFS..." from [^1]<br /><br />> There has been sever=
al objections to this proposal, which we can group into three categories:<b=
r />exploration of alternatives, demonstration of usage, and design of the =
operations to achieve these capabilities<br /><br />For this e-mail I would=
like to address the third point proactively: design of the operations to a=
chieve these capabilities.<br /><br />Antoine Poinsot, Steven Roose, and I =
have been working on a familiar, yet concrete technical proposal that focus=
es 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 simplifications<br />to we=
ll-understood systems when combined. The third is a simple<br />update that=
dovetails with the first two.<br /><br />The BIP text is here(https://gith=
ub.com/instagibbs/bips/blob/bip_op_templatehash/bip-templatehash-csfs-ik.md=
) and PR here(https://github.com/instagibbs/bips/pull/1), with full motivat=
ion for this particular bundle and rationale discussing alternatives. Our m=
ain contribution is a fully specified `OP_TEMPLATEHASH` as a drop-in replac=
ement for BIP119 `OP_CHECKTEMPLATEVERIFY`. `OP_TEMPLATEHASH` is a simpler a=
nd more modern implementation of the "next transaction" capability. It diff=
ers in committing to the Taproot annex and being otherwise Taproot native, =
which allows us to:<div><br />- Use the `OP_SUCCESS` upgrade hooks in place=
of legacy `OP_NOP`s and be able to push the template hash on the stack mak=
ing the flagship use case of rebindable signatures more efficient.<br />- R=
e-use the existing pre-computed Taproot sighash fields only instead of intr=
oducing new ones (substantially simplifying the implementation and review o=
f the specifications).<br />- Not commit to the spending transaction's scri=
ptSigs (which are both unecessary and may incentivize ad-hoc uses of legacy=
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/insta=
gibbs/bitcoin/tree/p2th) but left out of the bundle due to lack of demonstr=
ated utility.<br /><br />The BIP for `OP_TEMPLATEHASH` is here(https://gith=
ub.com/instagibbs/bips/blob/bip_op_templatehash/bip-templatehash.md) and a =
complete implementation is provided here(https://github.com/instagibbs/bitc=
oin/pull/3). The bundle itself is heavily inspired by "LNHANCE"(https://del=
vingbitcoin.org/t/lnhance-bips-and-implementation/376).<br /><br />We are h=
opeful that an opcode/implementation-focused discussion can be held<br />co=
ncurrently with other efforts such as discussions as to whether<br />or not=
this capability set is a good stopping point, including whether<br />this =
bundle is worth implementing on its own at all, as well as what<br />level =
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.googl=
e.com/g/bitcoindev/c/-qJc1EWQzY0</div>
<p></p>
-- <br />
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To view this discussion visit <a href=3D"https://groups.google.com/d/msgid/=
bitcoindev/26b96fb1-d916-474a-bd23-920becc3412cn%40googlegroups.com?utm_med=
ium=3Demail&utm_source=3Dfooter">https://groups.google.com/d/msgid/bitcoind=
ev/26b96fb1-d916-474a-bd23-920becc3412cn%40googlegroups.com</a>.<br />
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