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arXiv:2602.04859

Digital signatures with classical shadows on near-term quantum computers

Pradeep Niroula, Minzhao Liu, Sivaprasad Omanakuttan, David Amaro, Shouvanik Chakrabarti, Soumik Ghosh, Zichang He, Yuwei Jin, Fatih Kaleoglu, Steven Kordonowy, Rohan Kumar, Michael A. Perlin, Akshay Seshadri, Matthew Steinberg, Joseph Sullivan, Jacob Watkins, Henry Yuen, Ruslan Shaydulin

quant-phcs.CRSubmitted Feb 4, 2026• Announced Feb 5, 2026• 0 citations
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Abstract

Quantum mechanics provides cryptographic primitives whose security is grounded in hardness assumptions independent of those underlying classical cryptography. However, existing proposals require low-noise quantum communication and long-lived quantum memory, capabilities which remain challenging to realize in practice. In this work, we introduce a quantum digital signature scheme that operates with only classical communication, using the classical shadows of states produced by random circuits as public keys. We provide theoretical and numerical evidence supporting the conjectured hardness of learning the private key (the circuit) from the public key (the shadow). A key technical ingredient enabling our scheme is an improved state-certification primitive that achieves higher noise tolerance and lower sample complexity than prior methods. We realize this certification by designing a high-rate error-detecting code tailored to our random-circuit ensemble and experimentally generating shadows for 32-qubit states using circuits with $\geq 80$ logical ($\geq 582$ physical) two-qubit gates, attaining 0.90 $\pm$ 0.01 fidelity. With increased number of measurement samples, our hardware-demonstrated primitives realize a proof-of-principle quantum digital signature, demonstrating the near-term feasibility of our scheme.

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IACR Communications in Cryptology•Apr 8, 2025•1 citation
Quantum mechanics provides cryptographic primitives whose security is grounded in hardness assumptions independent of those underlying classical cryptography. However, existing proposals require low-noise quantum communication and long-lived quantum memory, capabilities which remain challenging to realize in practice. In this work, we introduce a quantum digital signature scheme that operates with only classical communication, using the classical shadows of states produced by random circuits as public keys. We provide theoretical and numerical evidence supporting the conjectured hardness of learning the private key (the circuit) from the public key (the shadow). A key technical ingredient enabling our scheme is an improved state-certification primitive that achieves higher noise tolerance and lower sample complexity than prior methods. We realize this certification by designing a high-rate error-detecting code tailored to our random-circuit ensemble and experimentally generating shadows for 32-qubit states using circuits with ≥80\geq 80≥80 logical (≥582\geq 582≥582 physical) two-qubit gates, attaining 0.90 ±\pm± 0.01 fidelity. With increased number of measurement samples, our hardware-demonstrated primitives realize a proof-of-principle quantum digital signature, demonstrating the near-term feasibility of our scheme.
We show that concrete hardness assumptions about learning or cloning the output state of a random quantum circuit can be used as the foundation for secure quantum cryptography. In particular, under these assumptions we construct secure one-way state generators (OWSGs), digital signature schemes, quantum bit commitments, and private key encryption schemes. We also discuss evidence for these hardness assumptions by analyzing the best-known quantum learning algorithms, as well as proving black-box lower bounds for cloning and learning given state preparation oracles. Our random circuit-based constructions provide concrete instantiations of quantum cryptographic primitives whose security do not depend on the existence of one-way functions. The use of random circuits in our constructions also opens the door to NISQ-friendly quantum cryptography. We discuss noise tolerant versions of our OWSG and digital signature constructions which can potentially be implementable on noisy quantum computers connected by a quantum network. On the other hand, they are still secure against noiseless quantum adversaries, raising the intriguing possibility of a useful implementation of an end-to-end cryptographic protocol on near-term quantum computers. Finally, our explorations suggest that the rich interconnections between learning theory and cryptography in classical theoretical computer science also extend to the quantum setting.
ABSTRACT We propose a novel quantum digital signature protocol that eliminates the need for a trusted third‐party, a common limitation in existing quantum signature schemes. While the concept of third‐party‐free quantum signatures has been discussed in earlier works, such as Gottesman and Chuang (2001), no concrete protocol with provable information‐theoretic security has been presented to date. Recent studies have explored computationally secure quantum signature schemes without trusted parties, but their security relies on assumptions about quantum computational hardness. In contrast, our protocol achieves information‐theoretic unforgeability based solely on the non‐cloning property of quantum states. It uses classical private keys and quantum public keys, and requires only single‐qubit operations. The scheme also satisfies key security properties, including asymmetry, undeniability, and expandability, making it suitable for implementation in near‐term quantum technologies.
Founding Quantum Cryptography on Quantum Advantage, or, Towards Cryptography from #P\mathsf{\#P}#P-Hardness
Recent oracle separations [Kretschmer, TQC'21, Kretschmer et. al., STOC'23] have raised the tantalizing possibility of building quantum cryptography from sources of hardness that persist even if the polynomial hierarchy collapses. We realize this possibility by building quantum bit commitments and secure computation from unrelativized, well-studied mathematical problems that are conjectured to be hard for P#P\mathsf{P^{\#P}}P#P -- such as approximating the permanents of complex Gaussian matrices, or approximating the output probabilities of random quantum circuits. Indeed, we show that as long as any one of the conjectures underlying sampling-based quantum advantage (e.g., BosonSampling, Random Circuit Sampling, IQP, etc.) is true, quantum cryptography can be based on the extremely mild assumption that P#P⊈(io)BQP/qpoly\mathsf{P^{\#P}} \not\subseteq \mathsf{(io)BQP/qpoly}P#P⊆(io)BQP/qpoly. We prove that the following hardness assumptions are equivalent. (1) The hardness of approximating the probability assigned to a randomly chosen string in the support of certain efficiently sampleable distributions (upto inverse polynomial multiplicative error).(2) The existence of one-way puzzles, where a quantum sampler outputs a pair of classical strings -- a puzzle and its key -- and where the hardness lies in finding the key corresponding to a random puzzle. These are known to imply quantum bit commitments [Khurana and Tomer, STOC'24]. (3) The existence of state puzzles, or one-way state synthesis, where it is hard to synthesize a secret quantum state given a public classical identifier. These capture the hardness of search problems with quantum inputs (secrets) and classical outputs (challenges). These are the first constructions of quantum cryptographic primitives (one-way puzzles, quantum bit commitments, state puzzles) from concrete, well-founded mathematical assumptions that do not imply the existence of classical cryptography.
We provide a pre-obfuscation circuit-level implementation of an efficient one shot signature scheme, which has known applications to delegated signatures, secure token transfer, and publicly verifiable randomness. The algorithm consists of two stages: a key generation stage where a classical public key/quantum secret key pair is produced, and a signing stage where the quantum secret key is processed with a message string to produce a classical signature. There is no algorithmic error in the construction and the signed message can be efficiently checked by a classical verifier. Our scheme works by preparing a superposition over elements of a random affine coset determined by the output of a puncturable pseudorandom function, together with a circuit that tests coset membership. The logical qubit number scales like Θ(κlog⁡(r)+n+l)Θ( κ\log(r) + n + l)Θ(κlog(r)+n+l) and the gate complexity scales like Θ(n3+nl)Θ(n^3 + nl)Θ(n3+nl), where rrr is the public key size, n+ln+ln+l is the signature size, lll is the message size, and κ=Ω(n)κ= Ω(n)κ=Ω(n) is the cryptographic security parameter. We provide explicit qubit and gate counts before obfuscation. A secure instantiation additionally requires modified key generation and a security proof for the resulting obfuscated construction.
Extracting information efficiently from quantum systems is crucial for quantum information processing. Classical shadows enable predicting many properties of arbitrary quantum states using few measurements. While random single-qubit measurements are experimentally friendly and suitable for learning low-weight Pauli observables, they perform poorly for nonlocal observables. Introducing a shallow random quantum circuit before measurements improves sample efficiency for high-weight Pauli observables and low-rank properties. However, in practice, these circuits can be noisy and bias the measurement results. Here, we propose the robust shallow shadows, which employs Bayesian inference to learn and mitigate noise in postprocessing. We analyze noise effects on sample complexity and the optimal circuit depth. We provide theoretical guarantees for the success of error mitigation under a wide class of noise processes. Experimental validation on a superconducting quantum processor confirms the advantage of our method, even in the presence of realistic noise, over single-qubit measurements for predicting diverse state properties, such as fidelity and entanglement entropy. Our protocol thus offers a scalable, robust, and sample-efficient method for quantum state characterization on near-term quantum devices. Fast and reliable characterisation of quantum systems is a key part of quantum technologies development. Here, the authors propose and demonstrate a way to embed noise characterisation in classical shadow estimation of nonlocal properties, enabling an efficient way to extract information from noisy quantum systems.
We construct the following cryptographic primitives with unconditional security in a bounded-key model: * One-time public-key encryption, where the public keys are pure quantum states * One-time signatures, where the verification keys are pure quantum states. In our model, the adversary is given a bounded number of copies of the public key. We present efficient constructions and nearly-tight lower bounds for the size of the secret keys. Our security proofs are based on the quantum coupon collector problem, which was originally studied in the context of learning theory. The quantum coupon collector seeks to learn a set of strings (coupons) when given several copies of a superposition over the coupons. We make novel connections between this problem and cryptography. Our main technical ingredient is a family of coupon states, with randomized phases, that come with strong hardness properties. Our analysis improves on prior work by (i) showing that the number of quantum states needed to learn the entire set of coupons is identical to the number of random coupons needed in the classical coupon collector problem. (ii) Furthermore we prove that this result holds for a randomly chosen set of coupons, whereas prior work only lower-bounded the number of coupon states required to learn the worst-case set of coupons.