Re: [bitcoindev] Prohibit Merkle Internal Node Preimages That Encode Minimal 64-Byte Transactions

[Murch <murch@murch.one>]

Hi Jeremy,

As AJ pointed out in "[BIP-0054] 64-Byte Transactions and Potential 
Legitimate Uses", forbidding 64-byte stripped transactions would at most 
incur that transaction creators have to pad a to-be-invented use for an 
anyone-can-spend output script with an `OP_NOP` if they hit the 64-byte 
case, or if the uncertainty whether the case will be hit is the issue, 
to circumvent it per paying for one extra byte by always padding.

Could you please explain why potentially needing to add a 1-byte padding 
for an "esoteric use case" is a bigger burden for the ecosystem than 
saddling every node with checking thousands of hashes during block 
validation on whether they match a valid transaction pattern?

As far as I’m following this conversation so far, this proposal’s 
trade-offs don’t feel convincing to me.

Cheers,
Murch

On 2026-06-09 09:37, jeremy wrote:
> I also want to make an opinionated point here, separate from the 
> technical message above:
> 
> These are esoteric use cases. This is precisely why it's important to 
> get right. The edge of understanding is where future protocols end up 
> with exploitable vulnerabilities.
> On Tuesday, June 9, 2026 at 12:30:31 PM UTC-4 jeremy wrote:
> 
>     Well, it's definitely possible to grind s.t. you have a higher up
>     node that _looks_ like a transaction.  It would be grinding an
>     interior node that has the desired shape on the left (5 bytes) and
>     on the right (depends what you're grinding for, but it's mostly the
>     length fields and script that need to be specific).
> 
>     You can then in theory have a proof system that is tricked into
>     thinking a UTXO exists that does not actually exist, and it might
>     think it's spendable in a given block.
> 
>     E.g., if the script is <H> CLTV, this would be provably not spent
>     until block H.
> 
>     And the SPV proof would be admitted.
> 
> 
>     then you'd have proof of tx + proof based on script of not until H time.
> 
> 
>     What /could/ something like this be used for?
> 
> 
>     Let's say some sort of "proof of mutex of miner claim" -- many
>     things can be offered to a miner conditioned on them using a
>     particular height locked UTXO to claim it.
> 
>     Since they can only spend the utxo one way, they can only pick one
>     of the offers and the others can expire. E.g.:
> 
>     Faked Output X: H w/ <H> CLTV
> 
>     TR{Nums, {<H+1> CLTV <PK> CHECKSIG, <H> CLTV <MuSig Federation>
>     CHECKSIG}} + Presigned via MuSig spending X using SIGHASH_NONE to
>     allow late binding of the miner's preferred destination (or not
>     SIGHASH_NONE and an OP_TRUE) ;
> 
>     or if GETINPUT opcode existed:
>     TR{Nums, {<H+1> CLTV <PK> CHECKSIG, <H> CLTV 1 GETINPUT <X> EQUAL}}
> 
>     ----------
> 
>     As you note, it won't be spending a known output, because of the
>     hash boundaries. But I can still think of protocol applications
>     where creating a false connector could be harmful.
> 
> 
>     ------------
> 
> 
>     The rule I propose mitigates this issue because it would be
>     disallowed to have any deserializable txn as an interior node.
>     On Friday, June 5, 2026 at 5:39:49 PM UTC-4 Antoine Poinsot wrote:
> 
>         Jeremy,
> 
>         Good point. I think this could be more straightforwardly
>         restated like so. "Invalidating 64-byte transactions only fixes
>         malleability in one direction: confusing a leaf for a node.
>         However malleability in the other direction may also be
>         exploited by grinding an inner node to trick an SPV verifier
>         that accepts proofs for 64-byte transactions."
> 
>         So you are correct that BIP 54 should only claim that
>         invalidating 64-byte transactions addresses the issue with no
>         change to SPV verifiers for those SPV verifiers that expect
>         proofs for transactions that cannot be 64-byte (i.e. checking
>         for deposits to any interesting scripts, or more generally any
>         transaction through which value actually flows).
> 
>         In fact this is also true for any SPV verifier that expects
>         proofs for transactions which could be 64-byte, as long as it is
>         computationally infeasible to grind those transactions.
>         Interestingly, this is true of the connector output example you
>         gave! If 64-byte transactions were invalid and a miner wanted to
>         attack a protocol by faking an SPV proof that a specific
>         connector output was spent, it would need to grind over 256 bits
>         for any inner node to match that prevout. So really the
>         attacking miner would only ever be able to fake a proof for a
>         64-byte transaction that anybody else would be able to create
>         anyways.
> 
>         So i think the point stands that preventing malleability only in
>         one direction (by invalidating 64-byte transactions) is
>         sufficient, and does not require SPV verifiers to do anything.
>         Do you have a counter-example?
> 
>         Antoine
>         On Monday, June 1st, 2026 at 4:22 PM, jeremy
>         <jeremy....@gmail.com> wrote:
>>         Antoine,
>>
>>         Rejecting nodes with any valid tx in path, without this rule,
>>         is problematic, because it _can_ be possible for an attacking
>>         miner to engineer that scenario by grinding one TXID leaf to
>>         mask a subtree, which could have major consequences. Third
>>         party malleability vulnerability to deposit / withdrawal
>>         masking is a serious bug. Worth thinking that through very
>>         carefully before recommending these mitigations. Do you have
>>         an end-to-end working example of such a mitigation that
>>         doesn't have these issues?
>>
>>         > This is incorrect for any bridge, wallet, or deposit system
>>         that does not receive funds to a script that either burns the
>>         funds or that anyone can spend.
>>
>>         The problem is that from the perspective of a wide variety of
>>         layer 2 protocols, you actually do want to be able to simply
>>         close out a UTXO and prove a UTXO is spent.
>>
>>         In the current L2 protocol design space, value doesn't always
>>         flow directly along the output, the UTXO may be being used as
>>         a connector input, and the spend of that output may be making
>>         a different output available after a timeout and excluding an
>>         alternative spend.
>>
>>         Best,
>>
>>         Jeremy
>>
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