Right, a fairly obvious resolution. My question is why is that not 
sufficient - especially given that a similar (context free) check is 
required for duplicated tx malleability? We'd just be swapping one trivial 
check (first input not null) for another (tx size not 64 bytes).

Best,
Eric
On Tuesday, June 18, 2024 at 7:46:28 AM UTC-4 Antoine Poinsot wrote:

> Hi Eric,
>
> It is. This is what is implemented in Bitcoin Core, see this snippet 
> <https://github.com/bitcoin/bitcoin/blob/41544b8f96dbc9c6b8998acd6522200d67cdc16d/src/validation.cpp#L4547-L4552> 
> and section 4.1 of the document you reference:
>
> Another check that was also being done in CheckBlock() relates to the 
> coinbase transaction: if the first transaction in a block fails the 
> required structure of a coinbase – one input, with previous output hash 
> of all zeros and index of all ones – then the block will fail validation. 
> The side effect of this test being in CheckBlock() was that even though 
> the block malleability discussed in section 3.1 was unknown, we were 
> effectively protected against it – as described above, it would take at 
> least 224 bits of work to produce a malleated block that passed the 
> coinbase check.
>
>
> Best,
> Antoine
> On Tuesday, June 18th, 2024 at 12:15 AM, Eric Voskuil <er...@voskuil.org> 
> wrote:
>
> Hi Antoine,
>
> Regarding potential malleability pertaining to blocks with only 64 byte 
> transactions, why is not a deserialization phase check for the coinbase 
> input as a null point not sufficient mitigation (computational 
> infeasibility) for any implementation that desires to perform permanent 
> invalidity marking?
>
> Best,
> Eric
>
> ref: Weaknesses in Bitcoin’s Merkle Root Construction 
> <https://lists.linuxfoundation.org/pipermail/bitcoin-dev/attachments/20190225/a27d8837/attachment-0001.pdf>
>
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>
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