Andrea Coladangelo

Device-independent self-testing refers to the certification of quantum states based entirely on the correlations exhibited by measurements on separate subsystems. The fact that such a certification is possible at all is remarkable in its own right, and is intimately connected to the violation Bell’s inequalities by entangled quantum systems. In the bipartite case, self-testing of states has been completely characterized, up to local isometries, as there exist protocols that self-test arbitrary pure states of any local dimension. Despite the growing interest in device-independent certification protocols, an analogous result in the general multipartite case has remained elusive. In this work, we give a complete characterization of the qubit case, showing that any multipartite entangled state of qubits can be self-tested.

We give a meta-complexity characterization of EFI pairs, which are considered the “minimal” primitive in quantum cryptography (due to their equivalence to quantum commitments and for being implied by almost all other known quantum cryptographic primitives). More precisely, we show that the existence of EFI pairs is equivalent to the following: there exists a non-uniformly samplable distribution over pure states such that the problem of estimating a certain Kolmogorov-like complexity measure is hard given a single copy. The complexity measure that we consider is a smoothed version of the algorithmic entropy notion introduced by Gács [Gác01]. A key technical step in our proof, which may be of independent interest, is to show that the existence of EFI pairs is equivalent to the existence of non-uniform single-copy secure pseudorandom state generators (nu 1-PRS). As a corollary, we get an alternative, arguably simpler, construction of a universal EFI pair.

In this work, we focus on the following question: what are the cryptographic implications of having access to an oracle that provides a single Haar random quantum state? We show, perhaps surprisingly, that such an oracle is sufficient to construct quantum pseudorandomness. Pseudorandom states (PRS) are a family of states for which it is hard to distinguish between polynomially many copies of either a state sampled uniformly from the family or a Haar random state. A weaker notion, called single-copy pseudorandom states (1PRS), satisfies this property with respect to a single copy. Our main result is that 1PRS (as well as bit-commitments) exist relative to an oracle that provides a single Haar random state. We build on this result to show the existence of an oracle relative to which 1PRS exist, but PRS do not. This provides one of the first black-box separations between different forms of quantum pseudorandomness.

We give a comprehensive characterization of the computational power of shallow quantum circuits combined with classical computation. Specifically, we show that the following statements hold relative to a random oracle: (a) latex mathsfBPP^QNC^BPP neq mathsfBQP. This refutes Jozsa's conjecture [Joz05] in the random oracle model. As a result, this gives the first instantiatable separation between the classes by replacing the oracle with a cryptographic hash function, yielding a resolution to one of Aaronson's ten semi-grand challenges in quantum computing. (b) latex mathsfQNC^BPP notsubseteq mathsfBPP^QNC and latex mathsfBPP^QNC notsubseteq mathsfQNC^BPP This shows that there is a subtle interplay between classical computation and shallow quantum computation. In fact, for the second separation, we establish that, for some problems, the ability to perform adaptive measurements in a single shallow quantum circuit, is more useful than the ability to perform polynomially many shallow quantum circuits without adaptive measurements. (c) There exists a 2-message proof of quantum depth protocol. Such a protocol allows a classical verifier to efficiently certify that a prover must be performing a computation of some minimum quantum depth. Our proof of quantum depth can be instantiated using the recent proof of quantumness construction by Yamakawa and Zhandry [YZ22]. [Joz05] - Richard Jozsa. ""An introduction to measurement based quantum computation."" In: Quantum Information Processing 199 (Sept. 2005). [YZ22] - Takashi Yamakawa and Mark Zhandry. ""Verifiable quantum advantage without structure."" In: 63rd Annual Symposium on Foundations of Computer Science (FOCS). IEEE, 2022.

We construct the first constant-round protocols for secure quantum computation in the two-party (2PQC) and multi-party (MPQC) settings with security against malicious adversaries. Our protocols are in the common random string (CRS) model. - Assuming two-message oblivious transfer (OT), we obtain (i) three-message 2PQC, and (ii) five-round MPQC with only three rounds of online (input-dependent) communication; such OT is known from quantum-hard Learning with Errors (QLWE). - Assuming sub-exponential hardness of QLWE, we obtain (i) three-round 2PQC with two online rounds and (ii) four-round MPQC with two online rounds. - When only one (out of two) parties receives output, we achieve minimal interaction (two messages) from two-message OT; classically, such protocols are known as non-interactive secure computation (NISC), and our result constitutes the first maliciously-secure quantum NISC. Additionally assuming reusable malicious designated-verifier NIZK arguments for NP (MDV-NIZKs), we give the first MDV-NIZKs for QMA that only require one copy of the witness. Finally, we perform a preliminary investigation into two-round secure quantum computation where each party must obtain output. On the negative side, we identify a broad class of simulation strategies that suffice for classical two-round secure computation that are unlikely to work in the quantum setting. Next, as a proof-of-concept, we show that two-round secure quantum computation exists with respect to a quantum oracle.

Device-independent protocols use untrusted quantum devices to achieve a cryptographic task. Such protocols are typically based on Bell inequalities and require the assumption that the quantum device is composed of separated non-communicating components. In this submission, we present protocols for self-testing and device-independent quantum key distribution (DIQKD) that are secure even if the components of the quantum device can exchange arbitrary quantum communication. Instead, we assume that the device cannot break a standard post-quantum cryptographic assumption. Importantly, the computational assumption only needs to hold during the protocol execution and only applies to the (adversarially prepared) device in possession of the (classical) user, while the adversary herself remains unbounded. The output of the protocol, e.g. secret keys in the case of DIQKD, is information-theoretically secure. For our self-testing protocol, we build on a recently introduced cryptographic tool (Brakerski et al., FOCS 2018; Mahadev, FOCS 2018) to show that a classical user can enforce a bipartite structure on the Hilbert space of a black-box quantum device, and certify that the device has prepared and measured a state that is entangled with respect to this bipartite structure. Using our self-testing protocol as a building block, we construct a protocol for DIQKD that leverages the computational assumption to produce information-theoretically secure keys. The security proof of our DIQKD protocol uses the self-testing theorem in a black-box way. Our self-testing theorem thus also serves as a first step towards a more general translation procedure for standard device-independent protocols to the setting of computationally bounded (but freely communicating) devices.

In 2012, Aaronson and Christiano introduced the idea of hidden subspace states to build public-key quantum money [STOC '12]. Since then, this idea has been applied to realize several other cryptographic primitives which enjoy some form of unclonability. In this work, we propose a generalization of hidden subspace states to hidden coset states. We study different unclonable properties of coset states and several applications: (*) We show that, assuming indistinguishability obfuscation (iO), hidden coset states possess a certain direct product hardness property, which immediately implies a tokenized signature scheme in the plain model. Previously, a tokenized signature scheme was known only relative to an oracle, from a work of Ben-David and Sattath [QCrypt '17]. (*) Combining a tokenized signature scheme with extractable witness encryption, we give a construction of an unclonable decryption scheme in the plain model. The latter primitive was recently proposed by Georgiou and Zhandry [ePrint '20], who gave a construction relative to a classical oracle. (*) We conjecture that coset states satisfy a certain natural monogamy-of-entanglement property. Assuming this conjecture is true, we remove the requirement for extractable witness encryption in our unclonable decryption construction. As potential evidence in support of the conjecture, we prove a weaker version of this monogamy property, which we believe will still be of independent interest. (*) Finally, we give the first construction of a copy-protection scheme for pseudorandom functions (PRFs) in the plain model. Our scheme is secure either assuming iO, onw-way functions (OWFs) and extractable witness encryption, or assuming iO, OWFs, compute-and-compare obfuscation and the conjectured monogamy property mentioned above. This is the first example of a copy-protection scheme with provable security in the plain model for a class of functions that is not evasive.

Abstract MiniQCrypt is a world where quantum-secure one-way functions exist, and quantum communication is possible. We construct an oblivious transfer (OT) protocol in MiniQCrypt that achieves simulation-security against malicious quantum polynomial-time adversaries, building on the foundational work of Bennett, Brassard, Crépeau and Skubiszewska (CRYPTO 1991). Combining the OT protocol with prior works, we obtain secure two-party and multi-party computation protocols also in MiniQCrypt. This is in contrast to the classical world, where it is widely believed that one-way functions alone do not give us OT.

Abstract A non-interactive zero-knowledge (NIZK) proof system for a language L in NP allows a prover (who is provided with an instance x and a witness w) to compute a classical certificate for the claim that x is in L, with the following properties: 1) the protocol can be verified efficiently, and 2) the protocol does not reveal any information about w, besides the fact that it exists (i.e., that x is in L). While NIZKs are known to be impossible in the plain model (i.e., with no additional trusted resource), they are well studied in alternative models and have seen widespread application in classical cryptography. Given the importance of NIZKs, and more generally zero-knowledge protocols, in classical cryptography, there has been a recent effort to achieve such protocols for QMA, a natural quantum analog of NP. However, all previous results only achieved interactive protocols, limiting their cryptographic use. Moreover, they all rely on quantum communication between the prover and the verifier, which may be difficult to achieve. In this submission, we present two NIZK protocols for QMA in the Common Reference String (CRS) model, with additional offline setup. Both protocols are achieved through the homomorphic computation of classical NIZKs for NP, and rely on the hardness of the Learning With Errors problem. However, each of them then combines this core idea with different (seemingly incomparable) techniques: 1) our first protocol makes use of quantum teleportation and quantum communication in an offline setup phase, with a classical online phase; our second protocol leverages techniques for classical verification of quantum computations, and is the only known NIZK for QMA to be completely classical, as well as reusable, meaning that a single setup allows to prove many theorems. Security of the latter is in the Quantum Random Oracle model.

Abstract Device-independent protocols use untrusted quantum devices to achieve a cryptographic task. Such protocols are typically based on Bell inequalities and require the assumption that the quantum device is composed of separated non-communicating components. In this submission, we present protocols for self-testing and device-independent quantum key distribution (DIQKD) that are secure even if the components of the quantum device can exchange arbitrary quantum communication. Instead, we assume that the device cannot break a standard post-quantum cryptographic assumption. Importantly, the computational assumption only needs to hold during the protocol execution and only applies to the (adversarially prepared) device in possession of the (classical) user, while the adversary herself remains unbounded. The output of the protocol, e.g. secret keys in the case of DIQKD, is information-theoretically secure. For our self-testing protocol, we build on a recently introduced cryptographic tool (Brakerski et al., FOCS 2018; Mahadev, FOCS 2018) to show that a classical user can enforce a bipartite structure on the Hilbert space of a black-box quantum device, and certify that the device has prepared and measured a state that is entangled with respect to this bipartite structure. Using our self-testing protocol as a building block, we construct a protocol for DIQKD that leverages the computational assumption to produce information-theoretically secure keys. The security proof of our DIQKD protocol uses the self-testing theorem in a black-box way. Our self-testing theorem thus also serves as a first step towards a more general translation procedure for standard device-independent protocols to the setting of computationally bounded (but freely communicating) devices.

Abstract We investigate the round complexity of maliciously-secure two-party quantum computation (2PQC) with setup, and obtain the following results: - A three-message protocol (two-message if only one party receives output) in the common random string (CRS) model assuming classical two-message oblivious transfer (OT) with post-quantum malicious security. This round complexity is optimal for the sequential communication setting. Under the additional assumption of reusable malicious designated-verifier non-interactive zero-knowledge (MDV-NIZK) arguments for NP, our techniques give an MDV-NIZK for QMA. Each of the assumptions mentioned above is known from the quantum hardness of learning with errors (QLWE). - A protocol with two simultaneous rounds of communication, in a quantum preprocessing model, assuming sub-exponential QLWE. In fact, we construct a three-round protocol in the CRS model with only two rounds of online communication, which implies the above result. Along the way, we develop a new delayed simulation technique that we call simulation via teleportation, which may be useful in other settings. In addition, we perform a preliminary investigation into barriers and possible approaches for two round 2PQC in the CRS model, including an impossibility result for a natural class of simulators, and a proof-of-concept construction from a strong form of quantum virtual black-box (VBB) obfuscation. Prior to our work, maliciously-secure 2PQC required round complexity linear in the size of the quantum circuit.

Abstract Copy-protection allows a software distributor to encode a program in such a way that it can be evaluated on any input, yet it cannot be ``pirated'' -- a notion that is impossible to achieve in a classical setting. Aaronson (CCC 2009) initiated the formal study of quantum copy-protection schemes, and speculated that quantum cryptography could offer a solution to the problem thanks to the quantum no-cloning theorem. In this work, we introduce a quantum copy-protection scheme for a large class of evasive functions known as ``compute-and-compare programs'' -- a more expressive generalization of point functions. A compute-and-compare program CC[f,y] is specified by a function f and a string y within its range: on input x, CC[f,y] outputs 1, if f(x) = y, and 0 otherwise. We prove that our scheme achieves non-trivial security against fully malicious adversaries in the quantum random oracle model (QROM), which makes it the first copy-protection scheme to enjoy any level of provable security in a standard cryptographic model. As a complementary result, we show that the same scheme fulfils a weaker notion of software protection, called ``secure software leasing'', introduced very recently by Ananth and La Placa (eprint 2020), with a standard security bound in the QROM, i.e. guaranteeing negligible adversarial advantage.

We put forward the idea that classical blockchains and smart contracts are potentially useful primitives not only for classical cryptography, but for quantum cryptography as well. Abstractly, a smart contract is a functionality that allows parties to deposit funds, and release them upon fulfillment of algorithmically checkable conditions, and can thus be employed as a formal tool to enforce monetary incentives. In this work, we give the first example of the use of smart contracts in a quantum setting. We describe a simple hybrid classical-quantum payment system whose main ingredients are a classical blockchain capable of handling stateful smart contracts, and quantum lightning, a strengthening of public-key quantum money introduced by Zhandry (Eurocrypt'19). Our hybrid payment system employs quantum states as banknotes and a classical blockchain to settle disputes and to keep track of the valid serial numbers. It has several desirable properties: it is decentralized, requiring no trust in any single entity; payments are as quick as quantum communication, regardless of the total number of users; when a quantum banknote is damaged or lost, the rightful owner can recover the lost value.

It is well-known that digital signatures can be constructed from one-way functions in a black-box way. While one-way functions are essentially the minimal assumption in classical cryptography, this is not the case in the quantum setting. A variety of qualitatively weaker and inherently quantum assumptions (e.g. EFI pairs, one-way state generators, and pseudorandom states) are known to be sufficient for non-trivial quantum cryptography. While it is known that commitments, zero-knowledge proofs, and even multiparty computation can be constructed from these assumptions, it has remained an open question whether the same is true for quantum digital signatures schemes (QDS). In this work, we show that there does not exist a black-box construction of a QDS scheme with classical signatures from pseudorandom states with linear, or greater, output length. Our result complements that of Morimae and Yamakawa (2022), who described a one-time secure QDS scheme with classical signatures, but left open the question of constructing a standard multi-time secure one.