I think your error is that you expect that for OWF each and any f(x) computation should be hard to reverse. But it seems incorrect, f.e. modular squaring is easily reversible for x=0.

By using g(x) = x - f(x) you will have a pair of functions that are hard to revert, but their sum is identity function.

I think your error is that you expect that for OWF each and any f(x) computation should be hard to reverse. But it seems incorrect, f.e. modular squaring is easily reversible for x=0. EDIT: Wikipedia specifically says "Note that, by this definition, the function must be "hard to invert" in the average-case, rather than worst-case sense."

By using g(x) = x - f(x) you will have a pair of functions that are hard to revert, but their sum is identity function.