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

From: Rijndael <[email-masked]>
Date: Wed, 9 Jul 2025 15:59:37 -0400

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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