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To prevent second pre-image attacks, the Merkle Tree implementations I've looked at add a 0 prefix to the leaves and a 1 prefix to all other nodes as described at the bottom of this link:

https://flawed.net.nz/2018/02/21/attacking-merkle-trees-with-a-second-preimage-attack/

If we assume the Merkle Tree is a Complete Binary Tree, where all the leaves are on the same level and packed to the left, do we need to hash the leaves with a 0 prefix? Can we instead prefix all interior nodes with the tree depth as the article appears to hint at and not prefix the leaves? Am I missing something?

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  • $\begingroup$ As long as you don't hash the output of a previous hash in the tree in such a way that you can simulate another node then this should be alright I think. If you store the intermediate hashes then you could have a small issue with domain separation, but I could argue that simply adding a zero or a one would not really avoid that issue either. Hopefully someone can have a more authoritative answer though; this is mainly well informed intuition. $\endgroup$
    – Maarten Bodewes
    Sep 14, 2022 at 9:50
  • $\begingroup$ If they are adding only to leaves they are obviously wrong there crypto.stackexchange.com/a/71313/18298 $\endgroup$
    – kelalaka
    Sep 14, 2022 at 11:34

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