## CryptoDB

### Omri Shmueli

#### Publications

Year
Venue
Title
2021
CRYPTO
Abstract. We present a designated-verifier non-interactive zero-knowledge argument system for QMA with multi-theorem security under the Learning with Errors Assumption. All previous such protocols for QMA are only single-theorem secure. We also relax the setup assumption required in previous works. We prove security in the malicious designated-verifier (MDV-NIZK) model (Quach, Rothblum, and Wichs, EUROCRYPT 2019), where the setup consists of a mutually trusted random string and an untrusted verifier public key. Our main technical contribution is a general compiler that given a NIZK for NP and a quantum sigma protocol for QMA generates an MDV-NIZK protocol for QMA.
2021
TCC
We study post-quantum zero-knowledge (classical) protocols that are sound against quantum resetting attacks. Our model is inspired by the classical model of resetting provers (Barak-Goldreich-GoldwasserLindell, FOCS ‘01), providing a malicious efficient prover with oracle access to the verifier’s next-message-function, fixed to some initial random tape; thereby allowing it to effectively reset (or equivalently, rewind) the verifier. In our model, the prover has quantum access to the verifier’s function, and in particular can query it in superposition. The motivation behind quantum resettable soundness is twofold: First, ensuring a strong security guarantee in scenarios where quantum resetting may be possible (e.g., smart cards, or virtual machines). Second, drawing intuition from the classical setting, we hope to improve our understanding of basic questions regarding post-quantum zero knowledge. We prove the following results: – Black-Box Barriers. Quantum resetting exactly captures the power of black-box zero knowledge quantum simulators. Accordingly, resettable soundness cannot be achieved in conjunction with black-box zero knowledge, except for languages in BQP. Leveraging this, we prove that constant-round public-coin, or three message, protocols cannot be black-box post-quantum zero-knowledge. For this, we show how to transform such protocols into quantumly resettably sound ones. The transformations are similar to classical ones, but their analysis is very different due to the essential difference between classical and quantum resetting. – A Resettably-Sound Non-Black-Box Zero-Knowledge Protocol. Under the (quantum) Learning with Errors assumption and quantum fully-homomorphic encryption, we construct a post-quantum resettably sound zero knowledge protocol for NP. We rely on non-black-box simulation techniques, thus overcoming the black-box barrier for such protocols. – From Resettable Soundness to The Impossibility of Quantum Obfuscation. Assuming one-way functions, we prove that any quantumly resettably-sound zero-knowledge protocol for NP implies the impossibility of quantum obfuscation. Combined with the above result, this gives an alternative proof to several recent results on quantum unobfuscatability.
2020
CRYPTO
Efficiently sampling a quantum state that is hard to distinguish from a truly random quantum state is an elementary task in quantum information theory that has both computational and physical uses. This is often referred to as pseudorandom (quantum) state generator, or PRS generator for short. In existing constructions of PRS generators, security scales with the number of qubits in the states, i.e.\ the (statistical) security parameter for an $n$-qubit PRS is roughly $n$. Perhaps counter-intuitively, $n$-qubit PRS are not known to imply $k$-qubit PRS even for $k<n$. Therefore the question of \emph{scalability} for PRS was thus far open: is it possible to construct $n$-qubit PRS generators with security parameter $\secp$ for all $n, \secp$. Indeed, we believe that PRS with tiny (even constant) $n$ and large $\secp$ can be quite useful. We resolve the problem in this work, showing that any quantum-secure one-way function implies scalable PRS. We follow the paradigm of first showing a \emph{statistically} secure construction when given oracle access to a random function, and then replacing the random function with a quantum-secure (classical) pseudorandom function to achieve computational security. However, our methods deviate significantly from prior works since scalable pseudorandom states require randomizing the amplitudes of the quantum state, and not just the phase as in all prior works. We show how to achieve this using Gaussian sampling.
2019
TCC
We prove a quantum information-theoretic conjecture due to Ji, Liu and Song (CRYPTO 2018) which suggested that a uniform superposition with random binary phase is statistically indistinguishable from a Haar random state. That is, any polynomial number of copies of the aforementioned state is within exponentially small trace distance from the same number of copies of a Haar random state.As a consequence, we get a provable elementary construction of pseudorandom quantum states from post-quantum pseudorandom functions. Generating pseudorandom quantum states is desirable for physical applications as well as for computational tasks such as quantum money. We observe that replacing the pseudorandom function with a (2t)-wise independent function (either in our construction or in previous work), results in an explicit construction for quantum state t-designs for all t. In fact, we show that the circuit complexity (in terms of both circuit size and depth) of constructing t-designs is bounded by that of (2t)-wise independent functions. Explicitly, while in prior literature t-designs required linear depth (for $t > 2$), this observation shows that polylogarithmic depth suffices for all t.We note that our constructions yield pseudorandom states and state designs with only real-valued amplitudes, which was not previously known. Furthermore, generating these states require quantum circuit of restricted form: applying one layer of Hadamard gates, followed by a sequence of Toffoli gates. This structure may be useful for efficiency and simplicity of implementation.

#### Coauthors

Nir Bitansky (1)
Zvika Brakerski (2)
Michael Kellner (1)