[karmakaze]

Got it. I see the distinction. Undone as in find an input that produces a specific hash vs finding the input that originally made the hash. And we rely on not being able to do either efficiently.

From Wikipedia:

> It is not sufficient to make a function "lossy" (not one-to-one) to have a one-way function. In particular, the function that outputs the string of n zeros on any input of length n is not a one-way function because it is easy to come up with an input that will result in the same output. More precisely: For such a function that simply outputs a string of zeroes, an algorithm F that just outputs any string of length n on input f(x) will "find" a proper preimage of the output, even if it is not the input which was originally used to find the output string.