Paper 2025/1864

Black-Box Separation Between Pseudorandom Unitaries, Pseudorandom Isometries, and Pseudorandom Function-Like States

Abstract

Pseudorandom functions (PRFs) are one of the most fundamental primitives in classical cryptography. On the other hand, in quantum cryptography, it is possible that PRFs do not exist but their quantum analogues could exist, and still enabling many applications including SKE, MACs, commitments, multiparty computations, and more. Pseudorandom unitaries (PRUs) [Ji, Liu, Song, Crypto 2018], pseudorandom isometries (PRIs) [Ananth, Gulati, Kaleoglu, Lin, Eurocrypt 2024], and pseudorandom function-like state generators (PRFSGs) [Ananth, Qian, Yuen, Crypto 2022] are major quantum analogs of PRFs. PRUs imply PRIs, and PRIs imply PRFSGs, but the converse implications remain unknown. An important open question is whether these natural quantum analogues of PRFs are equivalent. In this paper, we partially resolve this question by ruling out black-box constructions of them: \begin{enumerate} \item There are no black-box constructions of $O(\log\lambda)$-ancilla PRUs from PRFSGs. \item There are no black-box constructions of $O(\log\lambda)$-ancilla PRIs with $O(\log\lambda)$ stretch from PRFSGs. \item There are no black-box constructions of $O(\log\lambda)$-ancilla PRIs with $O(\log\lambda)$ stretch from PRIs with $\Omega(\lambda)$ stretch. \end{enumerate} Here, $O(\log\lambda)$-ancilla means that the generation algorithm uses at most $O(\log\lambda)$ ancilla qubits. PRIs with $s(\lambda)$ stretch is PRIs mapping $\lambda$ qubits to $\lambda+s(\lambda)$ qubits. To rule out the above black-box constructions, we construct a unitary oracle that separates them. For the separations, we construct an adversary based on the quantum singular value transformation, which would be independent of interest and should be useful for other oracle separations in quantum cryptography.