Hey all

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.

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.
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.
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.

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

• I think TXHASH is too far from being ready at this point:
  
• 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.
• 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)
• 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 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.

That being said, I'm in no way married to the exact details of the 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:
  
• 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
• the addition of an INTERNALKEY opcode, saving an additional 33 bytes for the rebindable signature use case

Best

Steven

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’s
Elements for at least 8 years. It represents no significant
computational burden over bitcoin’s most often used opcode, OP_CHECKSIG.
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 – 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’Beirne (@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)

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