Instantiating the Hash-then-evaluate paradigm: Strengthening PRFs, PCFs, and OPRFs
We instantiate the hash-then-evaluate paradigm for pseudorandom functions (PRFs), textsf{PRF}(k,x):=textsf{wPRF}(k,textsf{RO}(x)), which builds a PRF textsf{PRF} from a weak PRF textsf{wPRF} via a public pre-processing random oracle textsf{RO}. In applications to secure multiparty computation (MPC), only the low-complexity textsf{wPRF} performs secret-depending operations. Our construction replaces textsf{RO} by f(k_textsf{H},textsf{elf}(x)), where f is a non-adaptive PRF and the key k_textsf{H} is public and thus known to the distinguishing adversary. We show that, perhaps surprisingly, several existing weak PRF candidates are plausibly also secure when their inputs are generated by f(k_textsf{H},textsf{elf}(.)). Firstly, analogous cryptanalysis applies (because pseudorandomness of f implies good statistical properties) and/or secondly an attack against the weak PRF with such pseudorandom inputs generated by f would imply surprising results such as key agreement from the hardness of the high-noise version of the Learning Parity with Noise (LPN) when implementing both textsf{wPRF} and f from this assumption. Our simple transformation of replacing textsf{RO}(cdot ) public pre-processing by f(k_textsf{H},textsf{elf}(x)) public pre-processing applies to the entire family of PRF-style functions. Specifically, we obtain results for oblivious PRFs, which are a core building block for password-based authenticated key exchange (PAKE) and private set intersection (PSI) protocols, and we also obtain results for pseudorandom correlation functions (PCF), which are a key tool for silent oblivious transfer (OT) extension.
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