# Help with pseudorandom functions

I started a new lecture this year about crypto and I've had a few problems in the first few weeks with the exercises we got.

I would appreciate a "guide" on how to solve problems like the following one because I feel like it will haunt me later in the semester if I don't figure it out now. Please don't simply post a solution, I need to figure this one out myself.

Let $F : \{0, 1\}^n × \{0, 1\}^n → \{0, 1\}^n$ be a pseudorandom function. Prove or disprove that this is also a secure pseudorandom function: $$F'(x) := F(x ⊕ 1^n)$$

• Don't you have a textbook? Lecture notes? Have you not seen some such problems in class? – fkraiem Nov 16 '15 at 0:07

## 1 Answer

Two suggestions:

1. Does it simplify the problem for you if you omit n? (or only consider the n=1 case)

2. Suppose F(x⊕1n) would not be a pseudorandom function, what does that tell you about the pseudorandomness of the original function F?