What is the proof that the existence of one-way-functions implies $P \neq NP$? I am not an expert in this field, but I have read that the existence of one-way functions implies $P \neq NP$. Since there seem to be so many different definitions of one-way-functions and I have not seen a proof  I wanted to ask if someone knows where to read this, or knows the proof. 
Please if you give a proof, also give a precise definition of which version of "one-way-function" you use. The proof can of course be detailed, the detailed, the better! :) 
Thanks!
$P \ne NP$ if and only if worst-case one-way functions exist.

Reference:

Alan L. Selman. A survey of one-way functions in complexity theory. Mathematical systems theory, 25(3):203–221, 1992.