International Association for Cryptologic Research

International Association
for Cryptologic Research


Qian Li


How (not) to Build Quantum PKE in Minicrypt
The seminal work by Impagliazzo and Rudich (STOC'89) demonstrated the impossibility of constructing classical public key encryption (PKE) from one-way functions (OWF) in a black-box manner. However, the question remains: can quantum PKE (QPKE) be constructed from quantumly secure OWF? A recent line of work has shown that it is indeed possible to build QPKE from OWF, but with one caveat --- they rely on quantum public keys, which cannot be authenticated and reused. In this work, we re-examine the possibility of perfect complete QPKE in the quantum random oracle model (QROM), where OWF exists. Our first main result: QPKE with classical public keys, secret keys and ciphertext, does not exist in the QROM, if the key generation only makes classical queries. Therefore, a necessary condition for constructing such QPKE from OWF is to have the key generation classically ``un-simulatable’’. Previous discussions (Austrin~et al. CRYPTO'22) on the impossibility of QPKE from OWF rely on a seemingly strong conjecture. Our work makes a significant step towards a complete and unconditional quantization of Impagliazzo and Rudich’s results. Our second main result extends to QPKE with quantum public keys. The second main result: QPKE with quantum public keys, classical secret keys and ciphertext, does not exist in the QROM, if the key generation only makes classical queries and the quantum public key is either pure or ``efficiently clonable''. The result is tight due to these existing QPKEs (Barooti et al. TCC'23, Morimae and Yamakawa QIP'24, Malavolta and Walter QIP'24). Our result further gives evidence on why existing QPKEs lose reusability. To achieve these results, we use a novel argument based on conditional mutual information and quanttum Markov chain by Fawzi and Renner (Communications in Mathematical Physics). We believe the techniques used in the work will find other usefulness in separations in quantum cryptography/complexity.
Unifying Presampling via Concentration Bounds 📺
Auxiliary-input (AI) idealized models, such as auxiliary-input random oracle model (AI-ROM) and auxiliary-input random permutation model (AI-PRM), play a critical role in assessing non-uniform security of symmetric key and hash function constructions. However, obtaining security bounds in these models is often much more challenging. The presampling technique, introduced by Unruh (CRYPTO' 07), generically reduces security proofs in the auxiliary-input models to much simpler bit-fixing models. This technique has been further optimized by Coretti, Dodis, Guo, Steinberger (EUROCRYPT' 18), and generalized by Coretti, Dodis, Guo (CRYPTO' 18), resulting in powerful tools for proving non-uniform security bounds in various idealized models. We study the possibility of leveraging the presampling technique to the quantum world. To this end, (*) We show that such leveraging will {resolve a major open problem in quantum computing, which is closely related to the famous Aaronson-Ambainis conjecture (ITCS' 11). (*) Faced with this barrier, we give a new but equivalent bit-fixing model and a simple proof of presampling techniques for arbitrary oracle distribution in the classical setting, including AI-ROM and AI-RPM. Our theorem matches the best-known security loss and unifies previous presampling techniques. (*) Finally, we leverage our new classical presampling techniques to a novel ``quantum bit-fixing'' version of presampling. It matches the optimal security loss of the classical presampling. Using our techniques, we give the first post-quantum non-uniform security for salted Merkle-Damgard hash functions and reprove the tight non-uniform security for function inversion by Chung et al. (FOCS' 20).