Subject: Re: [bitcoindev] A Taproot-native (re-)bindable transaction bundle proposal

From: Rijndael <[email-masked]>
Date: Wed, 9 Jul 2025 16:05:41 -0400

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. 

Rijndael

> On Jul 9, 2025, at 3:59 PM, Rijndael <[email-masked]> wrote:
> 
> Hey Greg,
> 
> The CTV commitment includes the number of inputs. I notice that the TEMPLATEHASH commitment does not. What’s the rationale there?
> 
> Thanks, 
> 
> Rijndael
> 
>> On Jul 9, 2025, at 2:19 PM, Greg Sanders <[email-masked]> wrote:
>> 
>> 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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