I can't seem to find an answer to this. For instance, I have this question:

Show that, if $P=NP$, there aren't any pseudo-random generators (even with amplification factor $n+1$).

My gut tells me this is because, in a world where $P=NP$, pretty much any process can be efficiently inverted so there aren't one-way functions (which pseudo-random generators rely on).


Amplification is equivalent to "stretch", the number of (seemingly) random bits that your algorithm generates given the truly random seed.

Let $G:\{0,1\}^*\rightarrow\{0,1\}^*$ be a PRG that maps strings of length $n$ to strings of length $l(n)$, then $l(n)$ is said to be the stretch function of $G$. If $l(n)>n$ and $l$ is injective, then you have $\forall n :|Im(G)\cap\{0,1\}^n|\le 2^{n-1}$. You can use this to construct a language $L\in NP$, such that $L\in P$ allows you to break $G$ (construct a distinguisher with success probability $\ge\frac{1}{2})$. Note that you can relax the injectivity requirement by ignoring some of the output (i.e. given a PRG with stretch $l(n)>n$, you can construct a PRG with stretch $l'(n)=n+1$).

  • $\begingroup$ I'm having trouble understanding this expression: $\forall n :|Im(G)\cap\{0,1\}^n|\le 2^{n-1}$. What is $Im$ in this context? $\endgroup$ – Pedro Costa Jun 17 '20 at 9:49
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    $\begingroup$ The image of $G$ $\endgroup$ – Ariel Jun 17 '20 at 10:01
  • $\begingroup$ Then isn't $Im(G)\cap\{0,1\}^n = \emptyset$ since $Im(G)$ has outputs with size $n+1$ (considering the example I gave) and $\{0,1\}^n$ has outputs with size $n$. So, let's say, isn't $000$ considered different from $00$? $\endgroup$ – Pedro Costa Jun 17 '20 at 11:13
  • $\begingroup$ $Im(G)$ is the image of $G$ as a function from $\{0,1\}^*$ to $\{0,1\}^*$, you were talking about $Im\left(G|_{\{0,1\}^n}\right)$ $\endgroup$ – Ariel Jun 17 '20 at 11:14
  • $\begingroup$ But you said $G$ maps strings of length $n$ to strings of length $l(n)$. So in that case, won't $Im(G)$ have outputs with size $l(n)$? $\endgroup$ – Pedro Costa Jun 17 '20 at 11:17

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