Efficient Post-Quantum OPRF from TFHE Key Transformation with Application to OPAQUE

We present a highly efficient post-quantum Randomized Oblivious Pseudorandom Function (OPRF) construction based on a novel TFHE key transformation technique. Unlike prior lattice-based OPRF schemes that rely on complex homomorphic PRF evaluation, our approach leverages the algebraic structure of TFHE to enable direct ciphertext transformation, resulting in significant performance improvements. Our protocol achieves strong security under the Learning With Errors (LWE) assumption and provides session unlinkability, making it well-suited for privacy-preserving applications. We demonstrate the practical efficiency of our construction with a Rust implementation, achieving substantial speedups over existing schemes. Furthermore, we show how our Randomized OPRF can be adapted for use in the OPAQUE password-authenticated key exchange protocol by employing a singularization technique to ensure deterministic outputs when required. This work highlights a new, practical path for post-quantum OPRFs with strong privacy and efficiency guarantees.