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

[jeremy <jeremy.l.rubin@gmail.com>]

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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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>>
>>
>>

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