>The hash is this tiny bit of information, and somehow is expected to be sufficient to uniquely describe some arbitrarily large amount of information It doesn'

>The hash is this tiny bit of information, and somehow is expected to be sufficient to uniquely describe some arbitrarily large amount of information It doesn't really describe the information it just indexes each piece. For that it is a very large key space. A 128 bit key could index 10^38 objects. That's a lot of slots. Provided your hash has sufficient mixing you will need a lot of shots to hit anything. >If one can truly "easily" derive a "secure" hash function Semantic security has a precise definition. We just have to make empirical assumptions about whether an input cipher is semantically secure. The hash is only secure if the cipher was but the derivation is easy.