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On 11/4/13 11:38 AM, Mike Hearn wrote:
> The Merkle branch doesn't get stored indefinitely though, whereas
> the coinbase hash does. The data stored in the coinbase [output]
> can always just be the 256-bit root hash truncated to less.
>
> I doubt the additional bytes make much difference really, so the
> additional complexity may not be worth it. But it wouldn't be an
> issue to do.

The bits make a difference if you are merged mining. You can use the
birthday attack to construct two data trees whose hash match the
(truncated) value, each containing separate aux block headers. This
allows you to double-count the bitcoin PoW for more than one aux block
on the same chain, potentially facilitating aux chain attacks.

If you want 128 bits of security for merged mined aux chains, you need
256 bits of hash in the coinbase.

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