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1. On Quantum Chosen-Ciphertext Attacks and Learning with Errors

   Alagic, Gorjan; Jeffery, Stacey; Ozols, Maris; Poremba, Alexander (January 2019, Theory of Quantum Computing, Communication, and Cryptography 2019)

   Large-scale quantum computing is a significant threat to classical public-key cryptography. In strong "quantum access" security models, numerous symmetric-key cryptosystems are also vulnerable. We consider classical encryption in a model which grants the adversary quantum oracle access to encryption and decryption, but where the latter is restricted to non-adaptive (i.e., pre-challenge) queries only. We define this model formally using appropriate notions of ciphertext indistinguishability and semantic security (which are equivalent by standard arguments) and call it QCCA1 in analogy to the classical CCA1 security model. Using a bound on quantum random-access codes, we show that the standard PRF- and PRP-based encryption schemes are QCCA1-secure when instantiated with quantum-secure primitives. We then revisit standard IND-CPA-secure Learning with Errors (LWE) encryption and show that leaking just one quantum decryption query (and no other queries or leakage of any kind) allows the adversary to recover the full secret key with constant success probability. In the classical setting, by contrast, recovering the key uses a linear number of decryption queries, and this is optimal. The algorithm at the core of our attack is a (large-modulus version of) the well-known Bernstein-Vazirani algorithm. We emphasize that our results should *not* be interpreted as a weakness of these cryptosystems in their stated security setting (i.e., post-quantum chosen-plaintext secrecy). Rather, our results mean that, if these cryptosystems are exposed to chosen-ciphertext attacks (e.g., as a result of deployment in an inappropriate real-world setting) then quantum attacks are even more devastating than classical ones. 

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2. Minimizing Trust in Hardware Wallets with Two Factor Signatures

   Marcedone, Antonio; Pass, Rafael; shelat, abhi (January 2019, Financial Crypto)

   We introduce the notion of two-factor signatures (2FS), a generalization of a two-out-of-two threshold signature scheme in which one of the parties is a hardware token which can store a high-entropy secret, and the other party is a human who knows a low-entropy password. The security (unforgeability) property of 2FS requires that an external adversary corrupting either party (the token or the computer the human is using) cannot forge a signature. This primitive is useful in contexts like hardware cryptocurrency wallets in which a signature conveys the authorization of a transaction. By the above security property, a hardware wallet implementing a two-factor signature scheme is secure against attacks mounted by a malicious hardware vendor; in contrast, all currently used wallet systems break under such an attack (and as such are not secure under our definition). We construct efficient provably-secure 2FS schemes which produce either Schnorr signature (assuming the DLOG assumption), or EC-DSA signatures (assuming security of EC-DSA and the CDH assumption) in the Random Oracle Model, and evaluate the performance of implementations of them. Our EC-DSA based 2FS scheme can directly replace currently used hardware wallets for Bitcoin and other major cryptocurrencies to enable security against malicious hardware vendors. 

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3. Oblivious Transfer Is in MiniQCrypt.

   Grilo, Alex; Lin, Huijia; Song, Fang; Vaikuntanathan, Vinod (January 2021, EUROCRYPT)

   null (Ed.)

   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 in the plain model 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. 

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4. Efficient Simulation of Random States and Random Unitaries

   https://doi.org/10.1007/978-3-030-45727-3_26  

   Gorjan Alagic, Christian Majenz (May 2020, Annual International Conference on the Theory and Applications of Cryptographic Techniques (Eurocrypt))

   We consider the problem of efficiently simulating random quantum states and random unitary operators, in a manner which is convincing to unbounded adversaries with black-box oracle access. This problem has previously only been considered for restricted adversaries. Against adversaries with an a priori bound on the number of queries, it is well-known that t-designs suffice. Against polynomial-time adversaries, one can use pseudorandom states (PRS) and pseudorandom unitaries (PRU), as defined in a recent work of Ji, Liu, and Song; unfortunately, no provably secure construction is known for PRUs. In our setting, we are concerned with unbounded adversaries. Nonetheless, we are able to give stateful quantum algorithms which simulate the ideal object in both settings of interest. In the case of Haar-random states, our simulator is polynomial-time, has negligible error, and can also simulate verification and reflection through the simulated state. This yields an immediate application to quantum money: a money scheme which is information-theoretically unforgeable and untraceable. In the case of Haar-random unitaries, our simulator takes polynomial space, but simulates both forward and inverse access with zero error. These results can be seen as the first significant steps in developing a theory of lazy sampling for random quantum objects. 

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5. A Note on the Instantiability of the Quantum Random Oracle

   https://doi.org/10.1007/978-3-030-44223-1_27  

   Eaton, E; Song, F. (January 2020, Post-Quantum Cryptography. PQCrypto 2020. Lecture Notes in Computer Science)

   In a highly influential paper from fifteen years ago [10], Canetti, Goldreich, and Halevi showed a fundamental separation between the Random Oracle Model (ROM) and the standard model. They constructed a signature scheme which can be proven secure in the ROM, but is insecure when instantiated with any hash function (and thus insecure in the standard model). In 2011, Boneh et al. defined the notion of the Quantum Random Oracle Model (QROM), where queries to the random oracle may be made in quantum superposition. Because the QROM generalizes the ROM, a proof of security in the QROM is stronger than one in the ROM. This leaves open the possibility that security in the QROM could imply security in the standard model. In this work, we show that this is not the case, and that security in the QROM cannot imply standard-model security. We do this by showing that the original schemes that show a separation between the standard model and the ROM are also secure in the QROM. We consider two schemes that establish such a separation, one with length-restricted messages, and one without, and show both to be secure in the QROM. Our results give further understanding to the landscape of proofs in the ROM versus the QROM or standard model, and point towards the QROM and ROM being much closer to each other than either is to standard model security. 

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