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[185.70.43.22]) by gmr-mx.google.com with ESMTPS id 5b1f17b1804b1-4535e984052si2755385e9.1.2025.06.23.06.14.34 for (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 23 Jun 2025 06:14:34 -0700 (PDT) Received-SPF: pass (google.com: domain of darosior@protonmail.com designates 185.70.43.22 as permitted sender) client-ip=185.70.43.22; Date: Mon, 23 Jun 2025 13:14:29 +0000 To: Bitcoin Development Mailing List From: "'Antoine Poinsot' via Bitcoin Development Mailing List" Subject: [bitcoindev] What's a good stopping point? Making the case for the capabilities enabled by CTV+CSFS Message-ID: Feedback-ID: 7060259:user:proton X-Pm-Message-ID: 1956d62f2b83cfd4ce8096c8fd1146d244375825 MIME-Version: 1.0 Content-Type: text/plain; charset="UTF-8" X-Original-Sender: darosior@protonmail.com X-Original-Authentication-Results: gmr-mx.google.com; dkim=pass header.i=@protonmail.com header.s=protonmail3 header.b=xdUCYcIg; spf=pass (google.com: domain of darosior@protonmail.com designates 185.70.43.22 as permitted sender) smtp.mailfrom=darosior@protonmail.com; dmarc=pass (p=QUARANTINE sp=QUARANTINE dis=NONE) header.from=protonmail.com X-Original-From: Antoine Poinsot Reply-To: Antoine Poinsot Precedence: list Mailing-list: list bitcoindev@googlegroups.com; contact bitcoindev+owners@googlegroups.com List-ID: X-Google-Group-Id: 786775582512 List-Post: , List-Help: , List-Archive: , List-Unsubscribe: , X-Spam-Score: -1.0 (-) 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/CAH5Bsr2vxL3FWXnJTszMQj83jTVdRvvuVpimEfY7JpFCyP1AZA@mail.gmail.com [^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 -- You received this message because you are subscribed to the Google Groups "Bitcoin Development Mailing List" group. To unsubscribe from this group and stop receiving emails from it, send an email to bitcoindev+unsubscribe@googlegroups.com. 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