# Homomorphic Evaluation of Pseudo Random Function

I'm looking for a special Pseudo Random Function (PRF) $F_k()$ which follows the below.

For fully homomorphic encryption (FHE), $\mathit{FHE}(F_k(x)) = F_k(\mathit{FHE}(x))$.

Or is there any homomorphically evaluable doubling pseudorandom number generator algorithm? (because Goldreich-Goldwasser-Micali PRF construction is based on the doubling pseudorandom number generator.)

Thanks.

• Do mean encryption of x by FHE(x) ? – Hilder Vitor Lima Pereira Dec 3 '17 at 21:25
• @HilderVitorLimaPereira Yes, I meant it. – takita Dec 4 '17 at 2:41
• I assume FHE has to have some key, say $\kappa$. Suppose the family $\{F_k\}_k$ has the property that for any $\kappa$ and $k$, $\mathit{FHE}_\kappa(F_k(x)) = F_k(\mathit{FHE}_\kappa(x))$. Now I can trivially distinguish $F_k$ for uniform random $k$ from a uniform random function $G$. If this is the property you mean, then the answer is no, this is a contradiction in terms. So you'll need to be a little more specific if that's not the property you want! – Squeamish Ossifrage Dec 4 '17 at 17:28
• What does "=" mean in the context of your question? Do you mean the ciphertexts are equal or that the ciphertexts are encryptions of the same plaintext? – mikeazo Dec 4 '17 at 21:05
• @mikeazo I meant the latter. – takita Dec 5 '17 at 6:56

Fortunately, there is open-source code which evaluates LowMC homomorphically with HElib. To evaluate the cipher homomorphically is to compute $\mathsf{Enc}_{\mathsf{k}}(\mathsf{LowMC}(x))$ from $\mathsf{Enc}_\mathsf{k}(x)$.
So you can plug in the PRF parameters given by Grassi et al. (for 128 bit PRF security) in the script provided by Albrecht and there you have it: LowMC becomes a PRF and your question is solved - compute $\mathsf{Enc}_\mathsf{k}(\mathsf{PRF}(x))$ from $\mathsf{Enc}_\mathsf{k}(x)$. And I think this is the state of the art for computing PRFs in HE land.