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

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

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They aren't really doing "additional checks" in the way you're framing it. 
There's just a new rule applied to existing proofs, no additional data 
compared to before. That new rule is *something* like  given proof : 
([(Hash, LEFT | RIGHT) | ODDNODE ], Transaction)

then as you hash you just forbid that any LEFT | RIGHT or LEFT | LEFT is 
allowed to be parseable as a transaction.

This forces that if a 64-byte transaction is present, it must be a leaf.

This prevents the other 64 byte transaction issue because you can't embed a 
txid sub-tree inside the 64-byte transaction to lie to an SPV user.

My estimate is that this case happens naturally like 1 in 2 million blocks. 
On Tuesday, June 2, 2026 at 8:40:05 AM UTC-4 Greg Sanders wrote:

> Hi Jeremy,
>
> Why does the SPV verifier need to do additional checks? SPV implies it's 
> simply trusting the heaviest chain. Clearly they could validate it, but I'm 
> not seeing necessity in the security model.
>
> Greg
>
> On Monday, June 1, 2026 at 4:22:28 PM UTC-4 jeremy 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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