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Message-ID: <035f8b9c-9711-4edb-9d01-bef4a96320e1@roose.io>
Date: Tue, 17 Jun 2025 12:22:10 +0100
MIME-Version: 1.0
Subject: Re: [bitcoindev] CTV + CSFS: a letter
To: James O'Beirne <james.obeirne@gmail.com>,
 Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
References: <a86c2737-db79-4f54-9c1d-51beeb765163n@googlegroups.com>
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Hey all


I've been following the discussion and noticed TXHASH is mentioned a=20
lot. As a signer of the letter and author of the TXHASH proposal, I'd=20
like to chime in with some thoughts.

However, I'd like to first express my disappointment with the amount of=20
drama this letter has caused. It quite literally merely asks Core=20
contributors to put this proposal on their agenda with some urgency. No=20
threats, no hard words.
Given that only a handful of Core contributors had thus far participated=20
in the conversation on the proposal elsewhere, it seemed like a suitable=20
next step to communicate that we want Core contributors to provide their=20
position within this conversation.
I strongly stand against an approach involving independent activation=20
clients and I think the sentiment of this e-mail aligns with the=20
preference of having Core be involved in any deployment of protocol=20
upgrades.

On the proposal itself. I have heard some mentions of TXHASH and why we,=20
as the developer community, wouldn't go=C2=A0 straight for TXHASH.

  * I think TXHASH is too far from being ready at this point:
      o The TXHASH BIP and spec has not had the level of
        review/engagement that I had hoped for. I'm personally pretty
        happy with the result, especially since it only has had serious
        looks from myself and Rearden. But it definitely needs a lot
        more scrutiny. It's a very opinionated design (I think it's
        unavoidable for such an opcode), so there is a lot of room for
        making different and maybe better decisions on many fronts.
      o Once we have the TXHASH semantics, it would be very valuable to
        consider also introducing the same semantics as a top-level
        sighash flag system, so you can spend coins directly with a
        sighash based on TXHASH semantics. (I dubbed this TXSIGHASH and
        I recently did some simulations on how such a construction would
        look for fee sponsoring and stacking
        https://delvingbitcoin.org/t/jit-fees-with-txhash-comparing-options=
-for-sponsorring-and-stacking/1760)
      o However, this also invites some additional review of possible
        taproot changes (pay-to-tapscript, re-adding parity byte, ..)
        and will necessarily take some more time for consideration and
        design.
  * I strongly believe it would benefit development of new bitcoin use
    cases if we would have a simple version of templating and rebindable
    signatures sooner rather than later. Both BitVM and Ark are fairly
    recent ideas that stand to benefit significantly from such new
    functionality, but I think having them actually available will
    invite a lot more engagement leading to new ideas and new
    improvements. These are not use case-specific changes, but useful
    general building blocks.
  * Both CSFS and CTV are fairly unopinionated opcodes by design, when
    you think of CTV in the sense that it fixes the minimum tx fields to
    ensure txid stability.
  * I think there is both enough excitement for this proposal and there
    would be enough future excitement for a TXHASH or CCV addition that
    I don't fear upgrade churn will be a future hurdle.
  * Even after possible TXHASH activation, I think there will stay some
    demand for the simpler CTV/TEMPLATEHASH variant. It doesn't just
    save a byte on the wire, but thinking in a txid-stable model is just
    more intuitive. I believe that many advanced use cases will take
    advantage from doing things the TXHASH way (enabling stacking and
    cheaper fee sponsoring), but some developers will always favor the
    simplicity of txid stability over the benefits of adding complexity.

The above arguments convinced me that an activation based on CTV and=20
CSFS makes sense today. We have lots of developers waiting to make use=20
of the functionality it enables and we can continue development of=20
further changes meanwhile.

That being said, I'm in no way married to the exact details of the=20
proposals.

  * I am sympathetic to worries of changing legacy/v0 script. I failed
    to convince myself of any valuable usage for bare CTV.
  * I am sympathetic to the consideration of revisiting scriptSig and
    annex-related modifications to the template hash.
  * I am sympathetic to the decision to optimize better for the
    rebindable signature use cases, meaning:
      o the idea to swap CTV for a TEMPLATEHASH opcode which adds a
        1-byte VERIFY for the template use case but saves 33 bytes for
        the signature use case
      o the addition of an INTERNALKEY opcode, saving an additional 33
        bytes for the rebindable signature use case

All of the above changes I think can be decided on with minimal=20
bikeshedding and still encompass the same semantics of the original=20
proposal.


Best

Steven


On 6/9/25 12:40, James O'Beirne wrote:
> Good morning,
>
> A letter has been published advocating for the final review and
> activation of OP_CHECKTEMPLATEVERIFY (BIP-119) and OP_CHECKSIGFROMSTACK
> (BIP-348).
>
> The full text of the letter can be found at https://ctv-csfs.com. It is
> reproduced below.
>
> ---
>
> To the technical bitcoin community,
>
> We believe that the best next step for bitcoin would be to activate
> OP_CHECKTEMPLATEVERIFY (CTV, BIP-119) and OP_CHECKSIGFROMSTACK (CSFS,
> BIP-348). These opcodes enable functionality for a broad set of uses
> that will allow bitcoin to preserve and expand its role as a scarce,
> censorship-resistant store of value.
>
> While there are a few promising proposals to improve bitcoin at the
> consensus layer which may someday be deployed, we believe that CTV and
> CSFS are uniquely well reviewed, simple, and have been proven to be both
> safe and widely demanded.
>
> CTV was first formalized in BIP-119 over 5 years ago. Despite many
> attempts at refinement or replacement, it has remained the most widely
> preferred method for enforcing pregenerated transaction sequences using
> consensus. It unlocks valuable functionality for scaling solutions,
> vaults, congestion control, non-custodial mining, discreet log
> contracts, and more.
>
> CSFS is a primitive opcode that has been deployed to Blockstream=E2=80=99=
s
> Elements for at least 8 years. It represents no significant
> computational burden over bitcoin=E2=80=99s most often used opcode, OP_CH=
ECKSIG.
> It can be combined with CTV to implement ln-symmetry, a longstanding
> improvement to Lightning. It also unlocks a variety of other use cases.
>
> We respectfully ask Bitcoin Core contributors to prioritize the review
> and integration of CTV (PR #31989 or similar) and CSFS (PR #32247 or
> similar) within the next six months. We believe this timeline allows for
> rigorous final review and activation planning.
>
> This request isn't meant to suggest that these contributors dictate the
> consensus process, but rather it is an acknowledgement that before these
> opcodes can be activated, they must be implemented in the most widely
> used bitcoin client.
>
> As application and protocol developers, we are convinced of the
> significant benefits that these changes would bring to end users of
> bitcoin =E2=80=93 even if only considering their use for layer 1 security=
 and
> layer 2 scaling solutions. We are optimistic that given the limited size
> and scope of these changes in both concept and implementation, they
> represent a realistic next step in the continuing and important work of
> preserving bitcoin's unique promise.
>
> Signed,
>
> Abdel (Starkware)
> Andrew Poelstra (@apoelstra)
> Ben Carman (@benthecarman)
> Ben Kaufman (@ben-kaufman)
> Brandon Black (@reardencode)
> Brian Langel (for Five Bells)
> Buck Perley (@puckberley)
> Calle (Cashu)
> Calvin Kim (@kcalvinalvin)
> Chun Wang (f2pool)
> Christian Decker (@cdecker)
> Coinjoined Chris (Bitsurance.eu)
> Evan Kaloudis (for Zeus)
> fiatjaf (@fiatjaf)
> Floppy (@1440000bytes)
> Gary Krause (@average-gary)
> Harsha Goli (@arshbot)
> Hunter Beast (@cryptoquick)
> Jad Mubaslat (@champbronc2)
> James O=E2=80=99Beirne (@jamesob)
> Jameson Lopp (@jlopp)
> Johan Halseth (@halseth)
> Luke Childs (@lukechilds)
> Matt Black (for Atomic Finance)
> Michael Tidwell (@miketwenty1)
> Nick Hansen (for Luxor Mining)
> Nitesh (@nitesh_btc)
> nvk (@nvk)
> Owen Kemeys (for Foundation)
> Paul Sztorc (@psztorc)
> Portland.HODL (for MARA Pool)
> Rijndael (@rot13maxi)
> Rob Hamilton (@rob1ham)
> Robin Linus (@RobinLinus)
> Sanket Kanjalkar (@sanket1729)
> Sean Ryan (Anchorage)
> Seth for Privacy (for Cake Wallet)
> Simanta Gautam (Alpen Labs)
> Steven Roose (@stevenroose)
> stutxo (@stutxo)
> Talip (@otaliptus)
> mononaut (@mononautical)
> vnprc (@vnprc)
>
> --=20
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    <p>Hey all</p>
    <p><br>
    </p>
    <p>I've been following the discussion and noticed TXHASH is
      mentioned a lot. As a signer of the letter and author of the
      TXHASH proposal, I'd like to chime in with some thoughts.</p>
    <p>However, I'd like to first express my disappointment with the
      amount of drama this letter has caused. It quite literally merely
      asks Core contributors to put this proposal on their agenda with
      some urgency. No threats, no hard words.<br>
      Given that only a handful of Core contributors had thus far
      participated in the conversation on the proposal elsewhere, it
      seemed like a suitable next step to communicate that we want Core
      contributors to provide their position within this conversation.<br>
      I strongly stand against an approach involving independent
      activation clients and I think the sentiment of this e-mail aligns
      with the preference of having Core be involved in any deployment
      of protocol upgrades.<br>
    </p>
    <p>On the proposal itself. I have heard some mentions of TXHASH and
      why we, as the developer community, wouldn't go=C2=A0 straight for
      TXHASH.</p>
    <ul>
      <li>I think TXHASH is too far from being ready at this point:<br>
      </li>
      <ul>
        <li>The TXHASH BIP and spec has not had the level of
          review/engagement that I had hoped for. I'm personally pretty
          happy with the result, especially since it only has had
          serious looks from myself and Rearden. But it definitely needs
          a lot more scrutiny. It's a very opinionated design (I think
          it's unavoidable for such an opcode), so there is a lot of
          room for making different and maybe better decisions on many
          fronts.</li>
        <li>Once we have the TXHASH semantics, it would be very valuable
          to consider also introducing the same semantics as a top-level
          sighash flag system, so you can spend coins directly with a
          sighash based on TXHASH semantics. (I dubbed this TXSIGHASH
          and I recently did some simulations on how such a construction
          would look for fee sponsoring and stacking
<a class=3D"moz-txt-link-freetext" href=3D"https://delvingbitcoin.org/t/jit=
-fees-with-txhash-comparing-options-for-sponsorring-and-stacking/1760">http=
s://delvingbitcoin.org/t/jit-fees-with-txhash-comparing-options-for-sponsor=
ring-and-stacking/1760</a>)</li>
        <li>However, this also invites some additional review of
          possible taproot changes (pay-to-tapscript, re-adding parity
          byte, ..) and will necessarily take some more time for
          consideration and design.<br>
        </li>
      </ul>
      <li>I strongly believe it would benefit development of new bitcoin
        use cases if we would have a simple version of templating and
        rebindable signatures sooner rather than later. Both BitVM and
        Ark are fairly recent ideas that stand to benefit significantly
        from such new functionality, but I think having them actually
        available will invite a lot more engagement leading to new ideas
        and new improvements. These are not use case-specific changes,
        but useful general building blocks.<br>
      </li>
      <li>Both CSFS and CTV are fairly unopinionated opcodes by design,
        when you think of CTV in the sense that it fixes the minimum tx
        fields to ensure txid stability.<br>
      </li>
      <li>I think there is both enough excitement for this proposal and
        there would be enough future excitement for a TXHASH or CCV
        addition that I don't fear upgrade churn will be a future
        hurdle.</li>
      <li>Even after possible TXHASH activation, I think there will stay
        some demand for the simpler CTV/TEMPLATEHASH variant. It doesn't
        just save a byte on the wire, but thinking in a txid-stable
        model is just more intuitive. I believe that many advanced use
        cases will take advantage from doing things the TXHASH way
        (enabling stacking and cheaper fee sponsoring), but some
        developers will always favor the simplicity of txid stability
        over the benefits of adding complexity.<br>
      </li>
    </ul>
    <p>The above arguments convinced me that an activation based on CTV
      and CSFS makes sense today. We have lots of developers waiting to
      make use of the functionality it enables and we can continue
      development of further changes meanwhile. </p>
    <p>That being said, I'm in no way married to the exact details of
      the proposals.<br>
    </p>
    <ul>
      <li>I am sympathetic to worries of changing legacy/v0 script. I
        failed to convince myself of any valuable usage for bare CTV.</li>
      <li>I am sympathetic to the consideration of revisiting scriptSig
        and annex-related modifications to the template hash.<br>
      </li>
      <li>I am sympathetic to the decision to optimize better for the
        rebindable signature use cases, meaning:<br>
      </li>
      <ul>
        <li>the idea to swap CTV for a TEMPLATEHASH opcode which adds a
          1-byte VERIFY for the template use case but saves 33 bytes for
          the signature use case</li>
        <li>the addition of an INTERNALKEY opcode, saving an additional
          33 bytes for the rebindable signature use case</li>
      </ul>
    </ul>
    All of the above changes I think can be decided on with minimal
    bikeshedding and still encompass the same semantics of the original
    proposal.<br>
    <p><br>
    </p>
    <p>Best</p>
    <p>Steven<br>
    </p>
    <p><br>
    </p>
    <div class=3D"moz-cite-prefix">On 6/9/25 12:40, James O'Beirne wrote:<b=
r>
    </div>
    <blockquote type=3D"cite"
      cite=3D"mid:a86c2737-db79-4f54-9c1d-51beeb765163n@googlegroups.com">
      <meta http-equiv=3D"content-type" content=3D"text/html; charset=3DUTF=
-8">
      Good morning,<br>
      <br>
      A letter has been published advocating for the final review and<br>
      activation of OP_CHECKTEMPLATEVERIFY (BIP-119) and
      OP_CHECKSIGFROMSTACK<br>
      (BIP-348). <br>
      <br>
      The full text of the letter can be found at <a class=3D"moz-txt-link-=
freetext" href=3D"https://ctv-csfs.com">https://ctv-csfs.com</a>.
      It is<br>
      reproduced below.<br>
      <br>
      ---<br>
      <br>
      To the technical bitcoin community,<br>
      <br>
      We believe that the best next step for bitcoin would be to
      activate<br>
      OP_CHECKTEMPLATEVERIFY (CTV, BIP-119) and OP_CHECKSIGFROMSTACK
      (CSFS,<br>
      BIP-348). These opcodes enable functionality for a broad set of
      uses<br>
      that will allow bitcoin to preserve and expand its role as a
      scarce,<br>
      censorship-resistant store of value.<br>
      <br>
      While there are a few promising proposals to improve bitcoin at
      the<br>
      consensus layer which may someday be deployed, we believe that CTV
      and<br>
      CSFS are uniquely well reviewed, simple, and have been proven to
      be both<br>
      safe and widely demanded.<br>
      <br>
      CTV was first formalized in BIP-119 over 5 years ago. Despite many<br=
>
      attempts at refinement or replacement, it has remained the most
      widely<br>
      preferred method for enforcing pregenerated transaction sequences
      using<br>
      consensus. It unlocks valuable functionality for scaling
      solutions,<br>
      vaults, congestion control, non-custodial mining, discreet log<br>
      contracts, and more.<br>
      <br>
      CSFS is a primitive opcode that has been deployed to Blockstream=E2=
=80=99s<br>
      Elements for at least 8 years. It represents no significant<br>
      computational burden over bitcoin=E2=80=99s most often used opcode,
      OP_CHECKSIG.<br>
      It can be combined with CTV to implement ln-symmetry, a
      longstanding<br>
      improvement to Lightning. It also unlocks a variety of other use
      cases.<br>
      <br>
      We respectfully ask Bitcoin Core contributors to prioritize the
      review<br>
      and integration of CTV (PR #31989 or similar) and CSFS (PR #32247
      or<br>
      similar) within the next six months. We believe this timeline
      allows for<br>
      rigorous final review and activation planning.<br>
      <br>
      This request isn't meant to suggest that these contributors
      dictate the<br>
      consensus process, but rather it is an acknowledgement that before
      these<br>
      opcodes can be activated, they must be implemented in the most
      widely<br>
      used bitcoin client.<br>
      <br>
      As application and protocol developers, we are convinced of the<br>
      significant benefits that these changes would bring to end users
      of<br>
      bitcoin =E2=80=93 even if only considering their use for layer 1 secu=
rity
      and<br>
      layer 2 scaling solutions. We are optimistic that given the
      limited size<br>
      and scope of these changes in both concept and implementation,
      they<br>
      represent a realistic next step in the continuing and important
      work of<br>
      preserving bitcoin's unique promise.<br>
      <br>
      Signed, <br>
      <br>
      Abdel (Starkware)<br>
      Andrew Poelstra (@apoelstra)<br>
      Ben Carman (@benthecarman)<br>
      Ben Kaufman (@ben-kaufman)<br>
      Brandon Black (@reardencode)<br>
      Brian Langel (for Five Bells)<br>
      Buck Perley (@puckberley)<br>
      Calle (Cashu)<br>
      Calvin Kim (@kcalvinalvin)<br>
      Chun Wang (f2pool)<br>
      Christian Decker (@cdecker)<br>
      Coinjoined Chris (Bitsurance.eu)<br>
      Evan Kaloudis (for Zeus)<br>
      fiatjaf (@fiatjaf)<br>
      Floppy (@1440000bytes)<br>
      Gary Krause (@average-gary)<br>
      Harsha Goli (@arshbot)<br>
      Hunter Beast (@cryptoquick)<br>
      Jad Mubaslat (@champbronc2)<br>
      James O=E2=80=99Beirne (@jamesob)<br>
      Jameson Lopp (@jlopp)<br>
      Johan Halseth (@halseth)<br>
      Luke Childs (@lukechilds)<br>
      Matt Black (for Atomic Finance)<br>
      Michael Tidwell (@miketwenty1)<br>
      Nick Hansen (for Luxor Mining)<br>
      Nitesh (@nitesh_btc)<br>
      nvk (@nvk)<br>
      Owen Kemeys (for Foundation)<br>
      Paul Sztorc (@psztorc)<br>
      Portland.HODL (for MARA Pool)<br>
      Rijndael (@rot13maxi)<br>
      Rob Hamilton (@rob1ham)<br>
      Robin Linus (@RobinLinus)<br>
      Sanket Kanjalkar (@sanket1729)<br>
      Sean Ryan (Anchorage)<br>
      Seth for Privacy (for Cake Wallet)<br>
      Simanta Gautam (Alpen Labs)<br>
      Steven Roose (@stevenroose)<br>
      stutxo (@stutxo)<br>
      Talip (@otaliptus)<br>
      mononaut (@mononautical)<br>
      vnprc (@vnprc)<br>
      <br>
      -- <br>
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