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

Physically Uncloneable Functions in the Stand-Alone and Universally Composable Framework

In this thesis, we investigate the possibility of basing cryptographic primitives on Physically Uncloneable Functions (PUF). A PUF is a piece of hardware that can be seen as a source of randomness. When a PUF is evaluated on a physical stimulus, it answers with a noisy output. PUFs are unpredictable such that even if a chosen stimulus is given, it should be infeasible to predict the corresponding output without physically evaluating the PUF. Furthermore, PUFs are uncloneable, which means that even if all components of the system are known, it is computational infeasible to model their behavior. In the course of this dissertation, we discuss PUFs in the context of their implementation, their mathematical description, as well as their usage as a cryptographic primitive and in cryptographic protocols. We first give an overview of the most prominent PUF constructions in order to derive subsequently an appropriate mathematical PUF model. It turns out that this is a non- trivial task, because it is not certain which common security properties are generally necessary and achievable due to the numerous PUF implementations. Next, we consider PUFs in security applications. Due to the properties of PUFs, these hardware tokens are good to build authentication protocols that rely on challenge/response pairs. If the number of potential PUF-based challenge/response pairs is large enough, an adversary cannot measure all PUF responses. Therefore, the at- tacker will most likely not be able to answer the challenge of the issuing party even if he had physical access to the PUF for a short time. However, we show that some of the previously suggested protocols are not fully secure in the attacker model where the adversary has physical control of the PUF and the corresponding reader during a short time. Finally, we analyze PUFs in the universally composable (UC) framework for the first time. Although hardware tokens have been considered before in the UC framework, designing PUF-based protocols is fundamentally different from other hardware token approaches. One reason is that the manufacturer of the PUF creates a physical object that outputs pseudorandom values, but where no specific code is running. In fact, the functional behavior of the PUF is unpredictable even for the PUF creator. Thus, only the party in possession of the PUF has full access to the secrets. After formalizing PUFs in the UC framework, we derive efficient UC-secure protocols for basic tasks like oblivious transfer, commitments, and key exchange

Uncloneable Quantum Advice

The famous no-cloning principle has been shown recently to enable a number of uncloneable functionalities. Here we address for the first time unkeyed quantum uncloneablity, via the study of a complexity-theoretic tool that enables a computation, but that is natively unkeyed: quantum advice. Remarkably, this is an application of the no-cloning principle in a context where the quantum states of interest are not chosen by a random process. We show the unconditional existence of promise problems admitting uncloneable quantum advice, and the existence of languages with uncloneable advice, assuming the feasibility of quantum copy-protecting certain functions. Along the way, we note that state complexity classes, introduced by Rosenthal and Yuen (ITCS 2022) - which concern the computational difficulty of synthesizing sequences of quantum states - can be naturally generalized to obtain state cloning complexity classes. We make initial observations on these classes, notably obtaining a result analogous to the existence of undecidable problems. Our proof technique establishes the existence of ingenerable sequences of finite bit strings - essentially meaning that they cannot be generated by any uniform circuit family. We then prove a generic result showing that the difficulty of accomplishing a computational task on uniformly random inputs implies its difficulty on any fixed, ingenerable sequence. We use this result to derandomize quantum cryptographic games that relate to cloning, and then incorporate a result of Kundu and Tan (arXiv 2022) to obtain uncloneable advice. Applying this two-step process to a monogamy-of-entanglement game yields a promise problem with uncloneable advice, and applying it to the quantum copy-protection of pseudorandom functions with super-logarithmic output lengths yields a language with uncloneable advice.Comment: 58 pages, 6 figure

The entropy of keys derived from laser speckle

Laser speckle has been proposed in a number of papers as a high-entropy source of unpredictable bits for use in security applications. Bit strings derived from speckle can be used for a variety of security purposes such as identification, authentication, anti-counterfeiting, secure key storage, random number generation and tamper protection. The choice of laser speckle as a source of random keys is quite natural, given the chaotic properties of speckle. However, this same chaotic behaviour also causes reproducibility problems. Cryptographic protocols require either zero noise or very low noise in their inputs; hence the issue of error rates is critical to applications of laser speckle in cryptography. Most of the literature uses an error reduction method based on Gabor filtering. Though the method is successful, it has not been thoroughly analysed. In this paper we present a statistical analysis of Gabor-filtered speckle patterns. We introduce a model in which perturbations are described as random phase changes in the source plane. Using this model we compute the second and fourth order statistics of Gabor coefficients. We determine the mutual information between perturbed and unperturbed Gabor coefficients and the bit error rate in the derived bit string. The mutual information provides an absolute upper bound on the number of secure bits that can be reproducibly extracted from noisy measurements

Comparison of Quantum PUF models

Physical unclonable functions (PUFs) are hardware structures in a physical system (e.g. semiconductor, crystals etc.) that are used to enable unique identification of the semiconductor or to secure keys for cryptographic processes. A PUF thus generates a noisy secret reproducible at runtime. This secret can either be used to authenticate the chip, or it is available as a cryptographic key after removing the noise. Latest advancements in the field of quantum hardware, in some cases claiming to achieve quantum supremacy, highly target the fragility of current RSA type classical cryptosystems. As a solution, one would like to develop Quantum PUFs to mitigate such problem. There are several approaches for this technology. In our work we compare these different approaches and introduce the requirements for QTOKSim, a quantum token based authentication simulator testing its performance on a multi-factor authentication protocol