# Inverting One-Way Functions

One of the conditions that a one-way function has to satisfy is the following:

$$Pr[A(f(x))\in f^{-1}(f(x))] \leq negl(n)$$

Now, suppose that we have the following function that's not one way: $$f(x) = x + 123$$ We know this isn't one-way since we can easily invert; that is: $$f^{-1}(x) = x - 123$$ But from the perspective of the adversary, if we have f(1) = 124, it would still essentially "brute-force" its way in order to find the inverse, right? In other words, the adversary would input random x's into f(x) until it finds a solution of 124 rather than formulating an inverse function (like a human normally would).

Is my understanding correct?

• An intelligent adversary would notice the trivial pattern after two or three tries. Brute force is what you use when you have no better cryptanalytic tool at your disposal. Oct 9, 2015 at 23:42
• I see. But then, if we actually have a one-way function, the adversary would find it hard to invert regardless of whether or not the adversary is intelligent, which in that case, just entering random points would be its best bet. Right? And this is why the adversary should invert it with only a negligible probability in this case? Oct 9, 2015 at 23:50
• The adversary doesn't have to just evaluate $f$ on various inputs; it knows the function $f$. Since it knows that $f(x)=x+123$, given $y=f(x)$ for unknown $x$ it can just compute $y-123$ to invert. Oct 10, 2015 at 0:25
• According to Kerckhoff's principle, one always assumes that the adversary knows the function f. Also, he may do anything which helps him findinng a solution.
– user27950
Oct 10, 2015 at 7:20

More fundamentally, although the word “adversary” naturally suggests it must be a sentient being such as a human, remember that an adversary is in fact just an algorithm. The algorithm $A$ which on input $x$ returns $x-123$ inverts your function with probability $1$, demonstrating that your function is not one-way. How a human attacker would actually be able to "come up" with this algorithm is irrelevant, what matters is that such an algorithm exists.