Limitations on Uncloneable Encryption and Simultaneous One-Way-to-Hiding

We study uncloneable quantum encryption schemes for classical messages as recently proposed by Broadbent and Lord. We focus on the information-theoretic setting and give several limitations on the structure and security of these schemes: Concretely, 1) We give an explicit cloning-indistinguishable attack that succeeds with probability $\frac12 + \mu/16$ where $\mu$ is related to the largest eigenvalue of the resulting quantum ciphertexts. 2) For a uniform message distribution, we partially characterize the scheme with the minimal success probability for cloning attacks. 3) Under natural symmetry conditions, we prove that the rank of the ciphertext density operators has to grow at least logarithmically in the number of messages to ensure uncloneable security. 4) The \emph{simultaneous} one-way-to-hiding (O2H) lemma is an important technique in recent works on uncloneable encryption and quantum copy protection. We give an explicit example which shatters the hope of reducing the multiplicative "security loss" constant in this lemma to below 9/8.Comment: v2 and v3: several fixes, including a missing attribution to Broadbent and Lor

Uncloneable Quantum Encryption via Oracles

Quantum information is well-known to achieve cryptographic feats that are unattainable using classical information alone. Here, we add to this repertoire by introducing a new cryptographic functionality called uncloneable encryption. This functionality allows the encryption of a classical message such that two collaborating but isolated adversaries are prevented from simultaneously recovering the message, even when the encryption key is revealed. Clearly, such functionality is unattainable using classical information alone. We formally define uncloneable encryption, and show how to achieve it using Wiesner\u27s conjugate coding, combined with a quantum-secure pseudorandom function (qPRF). Modelling the qPRF as an oracle, we show security by adapting techniques from the quantum one-way-to-hiding lemma, as well as using bounds from quantum monogamy-of-entanglement games

Quantum Cryptography Beyond Quantum Key Distribution

Quantum cryptography is the art and science of exploiting quantum mechanical effects in order to perform cryptographic tasks. While the most well-known example of this discipline is quantum key distribution (QKD), there exist many other applications such as quantum money, randomness generation, secure two- and multi-party computation and delegated quantum computation. Quantum cryptography also studies the limitations and challenges resulting from quantum adversaries---including the impossibility of quantum bit commitment, the difficulty of quantum rewinding and the definition of quantum security models for classical primitives. In this review article, aimed primarily at cryptographers unfamiliar with the quantum world, we survey the area of theoretical quantum cryptography, with an emphasis on the constructions and limitations beyond the realm of QKD.Comment: 45 pages, over 245 reference

Device-independent uncloneable encryption

Uncloneable encryption, first introduced by Broadbent and Lord (TQC 2020) is a quantum encryption scheme in which a quantum ciphertext cannot be distributed between two non-communicating parties such that, given access to the decryption key, both parties cannot learn the underlying plaintext. In this work, we introduce a variant of uncloneable encryption in which several possible decryption keys can decrypt a particular encryption, and the security requirement is that two parties who receive independently generated decryption keys cannot both learn the underlying ciphertext. We show that this variant of uncloneable encryption can be achieved device-independently, i.e., without trusting the quantum states and measurements used in the scheme, and that this variant works just as well as the original definition in constructing quantum money. Moreover, we show that a simple modification of our scheme yields a single-decryptor encryption scheme, which was a related notion introduced by Georgiou and Zhandry. In particular, the resulting single-decryptor encryption scheme achieves device-independent security with respect to a standard definition of security against random plaintexts. Finally, we derive an "extractor" result for a two-adversary scenario, which in particular yields a single-decryptor encryption scheme for single bit-messages that achieves perfect anti-piracy security without needing the quantum random oracle model.Comment: Issue found in application of the extractor technique to uncloneable encryption; corresponding claims have been removed. Added generalization of our results to single-decryptor encryption, in which the extractor technique can indeed be applie

Tighter Post-quantum Secure Encryption Schemes Using Semi-classical Oracles

Krüpteerimisprotokollide analüüsimiseks kasutatakse tihti juhusliku oraakli mudelit (JOM), aga postkvant turvaliste protokollide analüüs tuleb läbi viiakvant juhusliku oraakli mudelis (KJOM). Kuna paljudel tõestamise tehnikatel ei ole kvant juhusliku oraakli mudelis analoogi, on KJOMis raske töötada. Seda probleemi aitab lahendada One-Way to Hiding (O2H) Teoreem, mille Unruh tõestas 2015. aastal.Ambainis, Hamburg ja Unruh esitasid teoreemi täiustatud versiooni 2018. aastal. See kasutab poolklassikalisi oraakleid, millel on suurem paindlikkus ja tihedamad piirid. Täiustatud versioon võimaldab tugevdada kõigi protokollide turvalisust, mis kasutasid vana versiooni. Me võtame ühe artikli, kus kasutati vana O2H Teoreemi versiooni, ja tõestame protokollide turvalisuse uuesti kasutades poolklassikalisi oraakleid.The random oracle model (ROM) has been widely used for analyzing cryptographic schemes. In the real world, a quantum adversary equipped with a quantum computer can execute hash functions on an arbitrary superposition of inputs. Therefore, one needs to analyze the post-quantum security in the quantum random oracle model (QROM). Unfortunately, working in the QROM is quite difficult because many proof techniques in the ROM have no analogue in the QROM. A technique that can help solve this problem is the One-Way to Hiding (O2H) Theorem, which was first proven in 2015 by Unruh. In 2018, Ambainis, Hamburg and Unruh presented an improved version of the O2H Theorem which uses so called semi-classical oracles and has higher flexibilityand tighter bounds. This improvement of the O2H Theorem should allow us to derive better security bounds for most schemes that used the old version. We take one paper that used the old version of the O2H Theorem to prove the security of different schemes in the QROM and give new proofs using semi-classical oracles

Building Unclonable Cryptography: A Tale of Two No-cloning Paradigms

Unclonable cryptography builds primitives that enjoy some form of unclonability, such as quantum money, software copy protection, and bounded execution programs. These are impossible in the classical model as classical data is inherently clonable. Quantum computing, with its no-cloning principle, offers a solution. However, it is not enough to realize bounded execution programs; these require one-time memory devices that self-destruct after a single data retrieval query. Very recently, a new no-cloning technology has been introduced [Eurocrypt\u2722], showing that unclonable polymers---proteins---can be used to build bounded-query memory devices and unclonable cryptographic applications. In this paper, we investigate the relation between these two technologies; whether one can replace the other, or complement each other such that combining them brings the best of both worlds. Towards this goal, we review the quantum and unclonable polymer models, and existing unclonable cryptographic primitives. Then, we discuss whether these primitives can be built using the other technology, and show alternative constructions and notions when possible. We also offer insights and remarks for the road ahead. We believe that this study will contribute in advancing the field of unclonable cryptography on two fronts: developing new primitives, and realizing existing ones using new constructions

Unforgeable Quantum Encryption

We study the problem of encrypting and authenticating quantum data in the presence of adversaries making adaptive chosen plaintext and chosen ciphertext queries. Classically, security games use string copying and comparison to detect adversarial cheating in such scenarios. Quantumly, this approach would violate no-cloning. We develop new techniques to overcome this problem: we use entanglement to detect cheating, and rely on recent results for characterizing quantum encryption schemes. We give definitions for (i.) ciphertext unforgeability , (ii.) indistinguishability under adaptive chosen-ciphertext attack, and (iii.) authenticated encryption. The restriction of each definition to the classical setting is at least as strong as the corresponding classical notion: (i) implies INT-CTXT, (ii) implies IND-CCA2, and (iii) implies AE. All of our new notions also imply QIND-CPA privacy. Combining one-time authentication and classical pseudorandomness, we construct schemes for each of these new quantum security notions, and provide several separation examples. Along the way, we also give a new definition of one-time quantum authentication which, unlike all previous approaches, authenticates ciphertexts rather than plaintexts.Comment: 22+2 pages, 1 figure. v3: error in the definition of QIND-CCA2 fixed, some proofs related to QIND-CCA2 clarifie