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

["'Antoine Poinsot' via Bitcoin Development Mailing List" <bitcoindev@googlegroups.com>]

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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.l.rubin@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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