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Is sha256, when using an input of 32 bytes, a perfect hash function?

ua flag

As the title says, is sha256 collision-free if the input is 32-byte long (same size as its output)?

In case it's not, what is the most efficient and non-reversible collision-free function? (it doesn't even need to be a hash function).

fgrieu avatar
ng flag
Duplicate of [this](https://crypto.stackexchange.com/q/301/555), and of [this](https://crypto.stackexchange.com/q/75458/555), itself marked as a duplicate of [this](https://crypto.stackexchange.com/q/68431/555). Summary: we believe that no, but exhibiting a mathematical proof in either direction is infeasible.
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