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I am looking into building a merkle tree algorithm and I need to know if SHA256 is resistant to second preimage attack.

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SHA-256 has no known collision attack and has no known second pre-image attack and has no known pre-image attack.

There was a claim Has SHA256 been broken by Treadwell Stanton DuPont? but is has already debunked.

Currently, we have generic attacks: $2^{256}$ pre-image resistance, $2^{256}$ secondary pre-image resistance and $2^{128}$ collision resistance. Collision resistance is lower due to the birthday attack. The academical attacks are on reduced rounds, therefore not practical, yet.

Note per comment: It is not SHA-256 or any other Cryptographic hash's weakness. It is a problem with hash trees that enables to find secondary pre-images and it can be mitigated with domain separation as mentioned in rfc6992. You can also see this problem in this post;

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  • $\begingroup$ Thank you. Furthermore do I have to take into account that leaf nodes and the internal nodes have different hashes like for example it is done for sha-1?As explained here crypto.stackexchange.com/questions/2097/… $\endgroup$ – Hoistas Sep 19 at 6:34
  • $\begingroup$ are you asking adding 1 and 0 for domain separation? $\endgroup$ – kelalaka Sep 19 at 6:57
  • $\begingroup$ Yes I am asking about adding data to make the leafs and the nodes different. Do we have to do that for sha256? $\endgroup$ – Hoistas Sep 19 at 9:47
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    $\begingroup$ That is a different question and you can find the answer here $\endgroup$ – kelalaka Sep 19 at 9:51

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