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The question leaves to guess what the compression function is, and how it uses rounds. That being unspecified, no, increasing the number of rounds in the last compression function is not guaranteed to prevent length-extension attack, even if the compression function is changed and rather improved by adding more rounds to it. Proof by counterexample: modify ...


1

No. Length extension attacks are not attacks against hashes as hashes. They're attacks against hashes when used for purposes that require stronger security properties. A length extension calculation on a Merkle-Damgård hash allows an someone to calculate $H(A||S)$ for a specific non-empty suffix $S$ given the knowledge of only $\mathsf{length}(A)$ and $H(A)$ ...


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The chance that any other file of any length would have the same SHA256 hash is approximately 1 in 11579208923731619542357098500868790785326998466564056403945758400791 3129639936, which is 2 to the 256th power. In short, not very likely.


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