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Comment by kbolino

4 days ago

But once you've made a function that "cannot be used to produce the material described by the hash", you've also made a very good pseudo-randomizer. In fact, if a cryptographic hash function cannot be trusted for its ability to produce apparent randomness, then it cannot be trusted for its "intended purposes". You get both properties or neither.

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kbolino

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austin-cheney  4 days ago

This is broken logic.

There is an untested assumption that hashes achieve randomness because they appear to be a random collection of characters. Hash sequences are completely reproducible given a set of input, and that is by definition not random.

I think you are confusing loss of prediction as randomness. Never in mathematics or logic is that line of thinking correct. This can be described by equivocation, fallacy of composition, inductive fallacy, and more.

  • Tyyps  4 days ago

    I think you are mixing the function itself and it's output, if for a given input to the function the output is uniformly random, then this is a way to derive randomness. The fact that the function itself is deterministic tells you nothing about the distribution of it's output.

    • GTP  4 days ago

      You started your comment with a correct distinction, but got the wrong conclusion. Asking if somthing is random is actually a question about the process used to obtain some value and not the value itself. If I ask you if 42 is a random number, can you actually answer? I can get that number with an intrinsically random process based on some quantum effect, or I can say it from the top of my head because I just read a very famous book. You can indeed use an hash function to extract randomness, but, to be precise, we are talking about pseudo-randomness. The crucial difference here is that, if I'm measuring photons' polarization to get a random number, then an attacker repeating the same process will (very likely) obtain a different number. If I'm instead hashing some data, an attacker passing the same data through the same hash function will get the same result. Another example: if I hash the digits of pi, I will get a sequence that will pass statistical tests for randomness. But an attacker that knows how I am generating such sequence can easily reproduce it.

  • jadamson  4 days ago

    > There is an untested assumption that hashes achieve randomness because they appear to be a random collection of characters.

    lol, no. Cryptographic hash functions are specifically designed to achieve this property.

    > Never in mathematics or logic

    Let's not get ahead of ourselves. Start with English - what does "pseudo" mean?

    > This can be described by equivocation, fallacy of composition, inductive fallacy, and more.

    For example, what is a pseudo-intellectual?

    • austin-cheney  4 days ago

      > lol, no. Cryptographic hash functions are specifically designed to achieve this property.

      That completely ignores the definition of the word random.

      What I find most interesting about this thread of comments is that the article explains the failure of using hashes as a means of randomness and despite that failure people are eager to ignore what hashes are otherwise used for to invent oppositional arguments. it's weird.

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