Subject: [bitcoindev] Re: What's a good stopping point? Making the case for the capabilities enabled by CTV+CSFS

From: Harsha Goli <[email-masked]>
Date: Tue, 24 Jun 2025 07:29:25 -0700 (PDT)

​We will start by making the case that the capabilities "commit to the 
transaction spending this output"
and "verify a BIP340 signature for an arbitrary message" are a good 
stopping point for a Bitcoin soft fork. We invite
anyone to share objections in reply to this thread so they can be addressed 
or inform a better course of action.

We can consider narrower commitment combinations once we establish that 
broad commitments are a success for bitcoin. I reject the assumption some 
have that narrower commitment combinations might have more market demand 
than broader combinations, and that such assumptions might be an adequate 
reason to move forward with alternative proposals.

Thanks,
Harsha, sometimes known as arshbot

On Monday, June 23, 2025 at 9:26:20 AM UTC-4 Antoine Poinsot wrote:

> There is excitement in the technical community about a CTV+CSFS soft fork 
> as specified in BIP119 and BIP348. We think
> this combination of opcodes offers desirable capabilities to scale Bitcoin 
> payments and is worth considering to soft
> fork into Bitcoin. 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.
>
> In an attempt to help the proposal move forward we would like to kick-off 
> a discussion about the first category:
> alternatives. We will start by making the case that the capabilities 
> "commit to the transaction spending this output"
> and "verify a BIP340 signature for an arbitrary message" are a good 
> stopping point for a Bitcoin soft fork. We invite
> anyone to share objections in reply to this thread so they can be 
> addressed or inform a better course of action.
>
> Let's keep the discussion focused on the capabilities, not the specific 
> way of designing operations to achieve them. For
> the sake of this discussion `OP_CTV` would be equivalent to 
> `OP_TEMPLATEHASH` (push the template hash on the stack) as
> the capability "commit to the transaction to spend an output". `OP_TXHASH` 
> would be separate, as a "programmable
> transaction introspection" capability.
>
> The ability to commit to the exact transaction spending an output is 
> useful to reduce interactivity in second-layer
> protocols. For instance it can reduce roundtrips[^0] in the implementation 
> of LN-Symmetry, or make receiving an Ark
> "vtxo" non-interactive[^1]. Additionally, it enables significant 
> optimizations[^2] in the implementation of Discreet Log
> Contracts.
>
> The ability to verify a signature for an arbitrary message in Tapscript 
> enables oracle attestations and a form of
> delegation. Oracle attestation for instance significantly reduce[^3] the 
> onchain footprint of BitVM. Reducing an
> application's onchain footprint benefits all Bitcoin users by easing block 
> space competition, and it's especially
> important for applications that generate very large transactions, which 
> could otherwise increase pressure toward mining
> centralization[^4].
>
> Together, these two features enable a third capability: rebindable 
> transaction signatures. Rebindable signatures make
> possible a new type of payment channels, LN-Symmetry ("Eltoo"), whose 
> simplicity makes practical advanced constructs
> such as multiparty channels. They also enable further interactivity 
> reduction in second layer protocols, as illustrated
> by the Ark variant "Erk"[^5] or the dramatic simplification[^6] they bring 
> to upgrading today's Lightning (i.e. without
> switching to LN-Symmetry) from HTLCs to PTLCs.
>
> These capabilities are simple and modular. They are well understood and 
> present a low risk of enabling unwanted
> behaviour. They do not increase the cost of validation, have low 
> implementation complexity and are unlikely to become
> redundant, even if more powerful capabilities are added in the future. 
> These capabilities improve existing
> tried-and-proven protocols used daily by Bitcoin users, like the Lightning 
> Network. They also make new ones possible
> either at all or through realistic interactivity requirements. The new 
> enabled protocols take a similar approach to
> existing Bitcoin scaling solutions, only with different tradeoffs not 
> previously available. We can therefore reasonably
> expect they won't introduce new systemic incentives, while broadening the 
> range of supported use cases.
>
> The first alternative approach to address is doing nothing. Doing nothing 
> is *the* valid schelling point in a system
> where consensus changes must be agreed on by a supermajority of the 
> economic activity, and ideally by all stakeholders
> in the system. Unless there is a critical vulnerability being fixed, the 
> onus is on the proposer to overcome the various
> valid objections. Further, a number of smart contracts have been deployed 
> on Bitcoin and more are incoming, with or
> without consensus changes. No softforks on the horizon are known to 
> generate asymptotic scaling, and what's more,
> on-chain demand has not been high except on infrequent intervals.
>
> As said prior, we believe the capabilities of CTV+CSFS reach an 
> appropriate bar for consideration for activation. While
> it will not achieve asymptotic scaling, it will enable significant 
> reduction in complexity in already-deployed systems,
> and is worth deploying for their specific benefits. Regardless, it's 
> important to emphasize it again: the onus is on the
> proposer to overcome objections.
>
> Other alternative approaches to scaling Bitcoin payments have been 
> proposed such as with validation rollups[^7], enabled
> by the ability to verify a zero-knowledge proof directly in Bitcoin 
> Script. These rollups are trustless and could
> effectuate a modest factor throughput increase under realistic assumptions 
> and transaction load. This approach, used on
> altcoins but new to Bitcoin, has yet to reach consensus among the 
> technical community and Bitcoin users more broadly.
> Relative immaturity of many of the employed crypto-systems make designing 
> a ZKP-specific primitive a difficult task.
> Further, trustless composibility with interactive protocols like LN to 
> achieve further scaling are speculative at the
> time. Nonetheless, the capabilities that enable this alternative approach 
> to scaling are neither exclusionary nor
> redundant with those proposed here.
>
> It makes sense to focus first on capabilities improving the 
> tried-and-proven approach, as the newer approach
> (and the capabilities enabling it) may come with different tradeoffs.
>
> Yet another alternative is a set of more powerful capabilities, enabling 
> the use cases that "commit to next transaction"
> and "verify a BIP340 signature for an arbitrary message" enable and more. 
> For instance replacing "commit to the exact
> transaction which must spend this output" with "programmable introspection 
> on the spending transaction's fields" has
> been considered. However this approach increases implementation complexity 
> and broadens the risk surface[^8], which
> warrants a compelling demonstration that arbitrary transaction 
> introspection does enable important use cases not
> achievable with more minimal capabilities.
>
> Finally, a more radical alternative is to focus efforts to make Bitcoin 
> smart contracts more capable with a sane
> re-design of its scripting language. Similarly to the alternative of more 
> powerful Bitcoin Script capabilities, it
> remains to be shown that more expressivity is both safe and desirable. 
> Furthermore, it is unclear that such an
> undertaking would be achievable with the (very) limited engineering 
> resources currently allocated to extending Bitcoin
> scripting capabilities.
>
> In conclusion, we believe the bundle of capabilities "commitment to the 
> transaction spending an output" and "BIP340
> signature verification of arbitrary message" to be the best direction for 
> Bitcoin to take. These are well-understood,
> simple capabilities, substantially improving an existing well-understood 
> approach to scaling Bitcoin payments. This
> direction does not preclude research into more advanced capabilities, 
> though questions remain about their overall
> desirability.
>
> Antoine Poinsot & Greg Sanders
>
> [^0]: https://delvingbitcoin.org/t/ln-symmetry-project-recap/359
> [^1]: https://delvingbitcoin.org/t/the-ark-case-for-ctv/1528
> [^2]: 
> https://gnusha.org/pi/bitcoindev/[email-masked] 
> <https://gnusha.org/pi/bitcoindev/[email-masked]>
> [^3]: 
> https://delvingbitcoin.org/t/how-ctv-csfs-improves-bitvm-bridges/1591
> [^4]: 
> https://delvingbitcoin.org/t/non-confiscatory-transaction-weight-limit/1732/8
> [^5]: 
> https://delvingbitcoin.org/t/evolving-the-ark-protocol-using-ctv-and-csfs/1602
> [^6]: 
> https://delvingbitcoin.org/t/ctv-csfs-can-we-reach-consensus-on-a-first-step-towards-covenants/1509/18
> [^7]: https://github.com/john-light/validity-rollups
> [^8]: https://bluematt.bitcoin.ninja/2024/04/16/stop-calling-it-mev
>

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