The fast hash for internal nodes needs to use an IV that is not the standard SHA-256 IV. Instead needs to use some other fixed value, which should itself be the SHA-256 hash of some fixed string (e.g. the string "BIP ???" or "Fash SHA-256").
As it stands, I believe someone can claim a leaf node as an internal node by creating a proof that provides a phony right-hand branch claiming to have hash 0x80000..0000100 (which is really the padding value for the second half of a double SHA-256 hash). (I was schooled by Peter Todd by a similar issue in the past.) On Wed, Sep 6, 2017 at 8:38 PM, Mark Friedenbach via bitcoin-dev < bitcoin-dev@lists.linuxfoundation.org> wrote: > Fast Merkle Trees > BIP: https://gist.github.com/maaku/41b0054de0731321d23e9da90ba4ee0a > Code: https://github.com/maaku/bitcoin/tree/fast-merkle-tree >
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