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Can I use neural network to identify a hash function like sha256? I mean, generate a bunch of hash codes by different hash functions and also labels. Then, feed these codes into a neural network. Will it work?

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No. The output of a cryptographic hash function (like SHA256) is indistinguishable from random data. The only feature that'd be useful to a neural network would be the length of the hash -- and you don't need a neural network to tell you that a 256-bit value could be SHA256.

  • Is there any requirement that a secure hash function is indistinguishable from random? That seems intuitive to me for a good hash function as it leads to a more even distribution for preventing collisions, but does it lead from any of the usual requirements of hash functions (pre-image, second pre-image, and collision resistance)? – Steve Jan 9 at 3:49
  • @Steve Effectively, yes. Standard measures of collision resistance assume that all hash outputs are equally probable. For a hash function to be distinguishable from random data, some hashes would have to be more probable than others, or even entirely impossible. – duskwuff Jan 9 at 4:03

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