International Association for Cryptologic Research

International Association
for Cryptologic Research


Luowen Qian


Cryptography from Pseudorandom Quantum States 📺
Pseudorandom states, introduced by Ji, Liu and Song (Crypto'18), are efficiently-computable quantum states that are computationally indistinguishable from Haar-random states. One-way functions imply the existence of pseudorandom states, but Kretschmer (TQC'20) recently constructed an oracle relative to which there are no one-way functions but pseudorandom states still exist. Motivated by this, we study the intriguing possibility of basing interesting cryptographic tasks on pseudorandom states. We construct, assuming the existence of pseudorandom state generators that map a $\lambda$-bit seed to a $\omega(\log\lambda)$-qubit state, (a) statistically binding and computationally hiding commitments and (b) pseudo one-time encryption schemes. A consequence of (a) is that pseudorandom states are sufficient to construct maliciously secure multiparty computation protocols in the dishonest majority setting. Our constructions are derived via a new notion called pseudorandom function-like states (PRFS), a generalization of pseudorandom states that parallels the classical notion of pseudorandom functions. Beyond the above two applications, we believe our notion can effectively replace pseudorandom functions in many other cryptographic applications.
Collusion-Resistant Functional Encryption for RAMs
In recent years, functional encryption (FE) has established itself as one of the fundamental primitives in cryptography. The choice of model of computation to represent the functions associated with the functional keys plays a critical role in the complexity of the algorithms of an FE scheme. Historically, the functions are represented as circuits. However, this results in the decryption time of the FE scheme growing proportional to not only the worst case running time of the function but also the size of the input, which in many applications can be quite large. In this work, we present the first construction of a public-key collusion resistant FE scheme, where the functions, associated with the keys, are represented as random access machines (RAMs). We base the security of our construction on the existence of: (i) public-key collusion-resistant FE for circuits and, (ii) public-key doubly-efficient private-information retrieval [Boyle et al., Canetti et al., TCC 2017]. Our scheme enjoys many nice efficiency properties, including input-specific decryption time. We also show how to achieve FE for RAMs in the bounded-key setting with weaker efficiency guarantees from laconic oblivious transfer, which can be based on standard cryptographic assumptions. En route to achieving our result, we present conceptually simpler constructions of succinct garbling for RAMs [Canetti et al., Chen et al., ITCS 2016] from weaker assumptions.
Pseudorandom (Function-Like) Quantum State Generators: New Definitions and Applications
Pseudorandom quantum states (PRS) are efficiently constructible states that are computationally indistinguishable from being Haar-random, and have recently found cryptographic applications. We explore new definitions and applications of pseudorandom states, and present the following contributions: - We study variants of pseudorandom \emph{function-like} state (PRFS) generators, introduced by Ananth, Qian, and Yuen (CRYPTO'22), where the pseudorandomness property holds even when the generator can be queried adaptively or in superposition. We show feasibility of these variants assuming the existence of post-quantum one-way functions. - We show that PRS generators with logarithmic output length imply commitment and encryption schemes with \emph{classical communication}. Previous constructions of such schemes from PRS generators required quantum communication. - We give a simpler proof of the Brakerski--Shmueli (TCC'19) result that polynomially-many copies of uniform superposition states with random binary phases are indistinguishable from Haar-random states. - We also show that logarithmic output length is a sharp threshold where PRS generators start requiring computational assumptions.
Collusion-Resistant Copy-Protection for Watermarkable Functionalities
Copy-protection is the task of encoding a program into a quantum state to prevent illegal duplications. A line of recent works studied copy-protection schemes under "1 -> 2 attacks": the adversary receiving one program copy can not produce two valid copies. However, under most circumstances, vendors need to sell more than one copy of a program and still ensure that no duplicates can be generated. In this work, we initiate the study of collusion-resistant copy-protection in the plain model. Our results are twofold: * For the first time, we show that all major watermarkable functionalities can be copy-protected (including unclonable decryption, digital signatures, and PRFs). Among these, copy-protection of digital signature schemes is not known before. The feasibility of copy-protecting all watermarkable functionalities is an open question raised by Aaronson et al. (CRYPTO' 21) * We make all the above schemes k bounded collusion-resistant for any polynomial k, giving the first bounded collusion-resistant copy-protection for various functionalities in the plain model.
Adaptively Secure Garbling Schemes for Parallel Computations
Kai-Min Chung Luowen Qian
We construct the first adaptively secure garbling scheme based on standard public-key assumptions for garbling a circuit $$C: \{0, 1\}^n \mapsto \{0, 1\}^m$$ that simultaneously achieves a near-optimal online complexity $$n + m + \textsf {poly} (\lambda , \log |C|)$$ (where $$\lambda $$ is the security parameter) and preserves the parallel efficiency for evaluating the garbled circuit; namely, if the depth of C is d, then the garbled circuit can be evaluated in parallel time $$d \cdot \textsf {poly} (\log |C|, \lambda )$$ . In particular, our construction improves over the recent seminal work of [GS18], which constructs the first adaptively secure garbling scheme with a near-optimal online complexity under the same assumptions, but the garbled circuit can only be evaluated gate by gate in a sequential manner. Our construction combines their novel idea of linearization with several new ideas to achieve parallel efficiency without compromising online complexity.We take one step further to construct the first adaptively secure garbling scheme for parallel RAM (PRAM) programs under standard assumptions that preserves the parallel efficiency. Previous such constructions we are aware of is from strong assumptions like indistinguishability obfuscation. Our construction is based on the work of [GOS18] for adaptively secure garbled RAM, but again introduces several new ideas to handle parallel RAM computation, which may be of independent interests. As an application, this yields the first constant round secure computation protocol for persistent PRAM programs in the malicious settings from standard assumptions.