Hi Jeremy, Thanks for taking the time to lay out your objection here. I’m glad we now have a clear statement of it.
As AJ points out, your examples are all anyone-can-spend, so BIP 54 is correct here. But it's fair that it could be clearer with regard to witness-stripped serialized size. Thanks for pointing this, i'll push an update. What's more important is your claim that invalidating 64-byte transactions could complicate working with advanced scripting features that may eventually be added to Bitcoin. If that's the case i agree it should *at least* be discussed in BIP 54's rationale section. However, every time this was brought up, no one could come up with an example that barely resembles today's Bitcoin. Of course one can come up with hypothetical constructions of a very different Bitcoin in which a 64-byte transaction could be useful. But that is not the goal. The goal is to find a script for a 64-byte transaction that is *plausible* Bitcoin would adopt in the future. And as long as we are going to have a model where coins commit to their spending conditions, a 64-byte transaction is always either burning the spent coin's value or letting anyone spend it, regardless of the introspection or covenanty features that get added to these spending conditions. Therefore i do not currently see any plausible Bitcoin scripting upgrade that not only could not be designed to accommodate the 64-byte witness-stripped size exception, but also whose usage could even result in a 64-byte transaction in the first place. Antoine On Friday, May 1st, 2026 at 5:15 PM, jeremy <[email protected]> wrote: > For fun, let's start with a pop-quiz: > > Select all that apply: There can exist a transaction of ___ bytes serialized > size that BIP-0054's 64-byte restriction invalidates: > > A) 64 Bytes > B) 0 Bytes > C) 1.5MB > D) 32 BytesE) 5MB > > > The answer is A, 64 Bytes, and -- perhaps surprisingly -- C, 1.5MB. > > Why is this the case? > > BIP-0054 uses the term 64-byte transaction, but defines it as follows: > > Transactions whose witness-stripped serialized size is exactly 64 bytes are > > invalid. > > In a [personally run] straw-poll of devs at a recent conference, no-one knew > this precise edge condition or that the transactions could have a meaningful > witness. For clarity, the restriction on bytes is on > INVALID_TX_NONWITNESS_SIZE, not on the size with Witness. > Therefore, it is more accurate to refer to this in all sentences throughout > the BIP as: > > > > transactions with exactly 64 bytes of non-witness data, > > due to the propensity for confusion. > > BIP-0054 also makes a comment that the transactions it invalidates are > essentially useless: > > > 64-byte transactions can only contain a scriptPubKey that lets anyone spend > > the funds, or one that burns them. > > > This is not strictly correct. Here are a few examples of current and future > uses for 64-byte transactions: > > Current Uses: > - A transaction that donates to a future miner from a segwit (any version) > output via a spend to something like <512> OP_CSV (-> push2 bytes 512 csv -> > 0x02 0x00 0x02 0xb2) > - That same output which is used as a connector output for things that should > be claimed by a miner at a future time > - Pay-to-Anchor / ephemeral anchor outputs -- while typically p2a is for txns > you want to add a subsidy ability, a 64-byte txn could be used to shim a > keyed anchor to a p2a output after a certain delay. > > Future Uses: > - Future work which might use output scripts for e.g. Transaction Sponsor > encodings > - Future covenants work which encodes time-of-creation run scripts that e.g. > quine an input; possibly in conjunction with sponsors > - Future where we have expensive reusable PQ or Contract public keys that are > posted once and referred to by index > > > While, in a sense, current uses are much more concerning than future uses, > with introspection opcodes, it might create substantive additional complexity > to ensure that there is always a valid way to add a padding byte without > upsetting a state machine. > > As there are now documented use cases for 64-byte transactions that this > proposal makes more difficult to do, I recommend replacing the text in the > BIP that says > > > 64-byte transactions can only contain a scriptPubKey that lets anyone spend > > the funds, or one that burns them. > > With something like: > > There are documented use cases for 64-byte transactions that this proposal > > makes more difficult to cleanly do, but we do not believe these use cases > > will ever be valuable or worth protecting. > > > Or a more accurate reflection of the BIP-0054 authors' opinion. > > Jeremy > > > -- > 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 [email protected]. > To view this discussion visit > https://groups.google.com/d/msgid/bitcoindev/123e5545-2eda-4eca-9532-4f4cea2b83ecn%40googlegroups.com. -- You received this message because you are subscribed to the Google Groups "Bitcoin Development Mailing List" group. 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