Zihan Hu

Publications

1. LWE with Quantum Amplitudes: Algorithm, Hardness, and Oblivious Sampling

   Yilei Chen, Zihan Hu, Qipeng Liu, Han Luo, and Yaxin Tu

   Crypto 2025

   Abs eprint

   The learning with errors problem (LWE) is one of the most important building blocks for post-quantum cryptography. To better understand the quantum hardness of LWE, it is crucial to explore quantum variants of LWE. To this end, Chen, Liu, and Zhandry [Eurocrypt 2022] defined S|LWE> and C|LWE> problems by encoding the error of LWE samples into quantum amplitudes, and showed efficient quantum algorithms for a few interesting amplitudes. However, algorithms or hardness results of the most interesting amplitude, Gaussian, were not addressed before.

   In this paper, we show new algorithms, hardness results and applications for S|LWE> and C|LWE> with real Gaussian, Gaussian with linear or quadratic phase terms, and other related amplitudes. Let n be the dimension, q be the modulus of LWE samples. Our main results are

   1. There is a 2^{O(\sqrt{n \log q})}-time algorithm for S|LWE⟩ with Gaussian amplitude with known phase, given 2^{O(\sqrt{n \log q})} many quantum samples. The algorithm is modified from Kuperberg’s sieve, and in fact works for more general amplitudes as long as the amplitudes and phases are completely known.

   2. There is a polynomial time quantum algorithm for solving S|LWE> and C|LWE> for Gaussian with quadratic phase amplitudes, where the sample complexity is as small as \tilde{O}(n). As an application, we give a quantum oblivious LWE sampler where the core quantum sampler requires only quasi-linear sample complexity. This improves upon the previous oblivious LWE sampler given by Debris-Alazard, Fallahpour, Stehlé [STOC 2024], whose core quantum sampler requires \tilde{O}(nr) sample complexity, where r is the standard deviation of the error.

   3. There exist polynomial time quantum reductions from standard LWE or worst-case GapSVP to S|LWE> with Gaussian amplitude with small unknown phase, and arbitrarily many samples. Compared to the first two items, the appearance of the unknown phase term places a barrier in designing efficient quantum algorithm for solving standard LWE via S|LWE>.

2. Quantum Key-Revocable Dual-Regev Encryption, Revisited

   Prabhanjan Ananth, Zihan Hu, and Zikuan Huang

   TCC 2024

   Abs eprintSlides

   Quantum information can be used to achieve novel cryptographic primitives that are impossible to achieve classically. A recent work by Ananth, Poremba, Vaikuntanathan (TCC 2023) focuses on equipping the dual-Regev encryption scheme, introduced by Gentry, Peikert, Vaikuntanathan (STOC 2008), with key revocation capabilities using quantum information. They further showed that the key-revocable dual-Regev scheme implies the existence of fully homomorphic encryption and pseudorandom functions, with both of them also equipped with key revocation capabilities. Unfortunately, they were only able to prove the security of their schemes based on new conjectures and left open the problem of basing the security of key revocable dual-Regev encryption on well-studied assumptions.

   In this work, we resolve this open problem. Assuming polynomial hardness of learning with errors (over sub-exponential modulus), we show that key-revocable dual-Regev encryption is secure. As a consequence, for the first time, we achieve the following results:

   1. Key-revocable public-key encryption and key-revocable fully-homomorphic encryption satisfying classical revocation security and based on polynomial hardness of learning with errors. Prior works either did not achieve classical revocation or were based on sub-exponential hardness of learning with errors.

   2. Key-revocable pseudorandom functions satisfying classical revocation from the polynomial hardness of learning with errors. Prior works relied upon unproven conjectures.

3. On the (Im)plausibility of Public-Key Quantum Money from Collision-Resistant Hash Functions

   Prabhanjan Ananth, Zihan Hu, and Henry Yuen

   Asiacrypt 2023

   Abs arXiv Slides

   Public-key quantum money is a cryptographic proposal for using highly entangled quantum states as currency that is publicly verifiable yet resistant to counterfeiting due to the laws of physics. Despite significant interest, constructing provably-secure public-key quantum money schemes based on standard cryptographic assumptions has remained an elusive goal. Even proposing plausibly-secure candidate schemes has been a challenge.

   These difficulties call for a deeper and systematic study of the structure of public-key quantum money schemes and the assumptions they can be based on. Motivated by this, we present the first black-box separation of quantum money and cryptographic primitives. Specifically, we show that collision-resistant hash functions cannot be used as a black-box to construct public-key quantum money schemes where the banknote verification makes classical queries to the hash function. Our result involves a novel combination of state synthesis techniques from quantum complexity theory and simulation techniques, including Zhandry’s compressed oracle technique.