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

Round-Preserving Parallel Composition of Probabilistic-Termination Protocols

An important benchmark for multi-party computation protocols (MPC) is their round complexity. For several important MPC tasks, (tight) lower bounds on the round complexity are known. However, for some of these tasks, such as broadcast, the lower bounds can be circumvented when the termination round of every party is not a priori known, and simultaneous termination is not guaranteed. Protocols with this property are called probabilistic-termination (PT) protocols. Running PT protocols in parallel affects the round complexity of the resulting protocol in somewhat unexpected ways. For instance, an execution of m protocols with constant expected round complexity might take O(log m) rounds to complete. In a seminal work, Ben-Or and El-Yaniv (Distributed Computing \u2703) developed a technique for parallel execution of arbitrarily many broadcast protocols, while preserving expected round complexity. More recently, Cohen et al. (CRYPTO \u2716) devised a framework for universal composition of PT protocols, and provided the first composable parallel-broadcast protocol with a simulation-based proof. These constructions crucially rely on the fact that broadcast is ``privacy free,\u27\u27 and do not generalize to arbitrary protocols in a straightforward way. This raises the question of whether it is possible to execute arbitrary PT protocols in parallel, without increasing the round complexity. In this paper we tackle this question and provide both feasibility and infeasibility results. We construct a round-preserving protocol compiler, secure against a dishonest minority of actively corrupted parties, that compiles arbitrary protocols into a protocol realizing their parallel composition, while having a black-box access to the underlying protocols. Furthermore, we prove that the same cannot be achieved, using known techniques, given only black-box access to the functionalities realized by the protocols, unless merely security against semi-honest corruptions is required, for which case we provide a protocol

Predictable arguments of knowledge

We initiate a formal investigation on the power of predictability for argument of knowledge systems for NP. Specifically, we consider private-coin argument systems where the answer of the prover can be predicted, given the private randomness of the verifier; we call such protocols Predictable Arguments of Knowledge (PAoK). Our study encompasses a full characterization of PAoK, showing that such arguments can be made extremely laconic, with the prover sending a single bit, and assumed to have only one round (i.e., two messages) of communication without loss of generality. We additionally explore PAoK satisfying additional properties (including zero-knowledge and the possibility of re-using the same challenge across multiple executions with the prover), present several constructions of PAoK relying on different cryptographic tools, and discuss applications to cryptography

A Survey of Leakage-Resilient Cryptography

In the past 15 years, cryptography has made considerable progress in expanding the adversarial attack model to cover side-channel attacks, and has built schemes to provably defend against some of them. This survey covers the main models and results in this so-called leakage-resilient cryptography

Fully Leakage-Resilient Codes

Leakage resilient codes (LRCs) are probabilistic encoding schemes that guarantee message hiding even under some bounded leakage on the codeword. We introduce the notion of \emph{fully} leakage resilient codes (FLRCs), where the adversary can leak some $\lambda_0$ bits from the encoding process, i.e., the message and the randomness involved during the encoding process. In addition the adversary can as usual leak from the codeword. We give a simulation-based definition requiring that the adversary\u27s leakage from the encoding process and the codework can be simulated given just $\lambda_0$ bits of leakage from the message. For $\lambda_0 = 0$ our new simulation-based notion is equivalent to the usual game-based definition. A FLRC would be interesting in its own right and would be useful in building other leakage-resilient primitives in a composable manner. We give a fairly general impossibility result for FLRCs in the popular split-state model, where the codeword is broken into independent parts and where the leakage occurs independently on the parts. We show that if the leakage is allowed to be any poly-time function of the secret and if collision-resistant hash functions exist, then there is no FLRC for the split-state model. The result holds only when the message length can be linear in the security parameter. However, we can extend the impossibility result to FLRCs for constant-length messages under assumptions related to differing-input obfuscation. These results show that it is highly unlikely that we can build FLRCs for the split-state model when the leakage can be any poly-time function of the secret state. We then give two feasibility results for weaker models. First, we show that for \NC^0-bounded leakage from the randomness and arbitrary poly-time leakage from the parts of the codeword the inner-product construction proposed by Daví \etal (SCN\u2710) and successively improved by Dziembowski and Faust (ASIACRYPT\u2711) is a FLRC for the split-state model. Second, we provide a compiler from any LRC to a FLRC in the common reference string model for any fixed leakage family of small cardinality. In particular, this compiler applies to the split-state model but also to many other models

Constant-round Leakage-resilient Zero-knowledge from Collision Resistance

In this paper, we present a constant-round leakage-resilient zero-knowledge argument system for NP under the assumption of the existence of collision-resistant hash function family. That is, using collision-resistant hash functions, we construct a constant-round zero-knowledge argument system that has the following zero-knowledge property: Even against any cheating verifier that obtains arbitrary amount of leakage on the prover\u27s internal secret state, a simulator can simulate the verifier\u27s view by obtaining the same amount of leakage on the witness. Previously, leakage-resilient zero-knowledge proofs/arguments for NP were constructed only under a relaxed security definition (Garg, Jain, and Sahai, CRYPTO\u2711) or under the DDH assumption (Pandey, TCC\u2714). Our leakage-resilient zero-knowledge argument system satisfies an additional property that it is simultaneously leakage-resilient zero-knowledge, meaning that both zero-knowledgeness and soundness hold in the presence of leakage

Cryptographic techniques for hardware security

Traditionally, cryptographic algorithms are designed under the so-called black-box model, which considers adversaries that receive black-box access to the hardware implementation. Although a "black-box" treatment covers a wide range of attacks, it fails to capture reality adequately, as real-world adversaries can exploit physical properties of the implementation, mounting attacks that enable unexpected, non-black-box access, to the components of the cryptographic system. This type of attacks is widely known as physical attacks, and has proven to be a significant threat to the real-world security of cryptographic systems. The present dissertation is (partially) dealing with the problem of protecting cryptographic memory against physical attacks, via the use of non-malleable codes, which is a notion introduced in a preceding work, aiming to provide privacy of the encoded data, in the presence of adversarial faults. In the present thesis we improve the current state-of-the-art on non-malleable codes and we provide practical solutions for protecting real-world cryptographic implementations against physical attacks. Our study is primarily focusing on the following adversarial models: (i) the extensively studied split-state model, which assumes that private memory splits into two parts, and the adversary tampers with each part, independently, and (ii) the model of partial functions, which is introduced by the current thesis, and models adversaries that access arbitrary subsets of codeword locations, with bounded cardinality. Our study is comprehensive, covering one-time and continuous, attacks, while for the case of partial functions, we manage to achieve a stronger notion of security, that we call non-malleability with manipulation detection, that in addition to privacy, it also guarantees integrity of the private data. It should be noted that, our techniques are also useful for the problem of establishing, private, keyless communication, over adversarial communication channels. Besides physical attacks, another important concern related to cryptographic hardware security, is that the hardware fabrication process is assumed to be trusted. In reality though, when aiming to minimize the production costs, or whenever access to leading-edge manufacturing facilities is required, the fabrication process requires the involvement of several, potentially malicious, facilities. Consequently, cryptographic hardware is susceptible to the so-called hardware Trojans, which are hardware components that are maliciously implanted to the original circuitry, having as a purpose to alter the device's functionality, while remaining undetected. Part of the present dissertation, deals with the problem of protecting cryptographic hardware against Trojan injection attacks, by (i) proposing a formal model for assessing the security of cryptographic hardware, whose production has been partially outsourced to a set of untrusted, and possibly malicious, manufacturers, and (ii) by proposing a compiler that transforms any cryptographic circuit, into another, that can be securely outsourced

Theory and application of computationally independent one-way functions: Interactive proof of ability - Revisited

We introduce the concept of computationally independent pair of one-way functions (CI-OWF). We also provide two rich classes of examples of such functions based on standard assumptions. We revisit two-party interactive protocols for proving possession of computational power and existing two-flow challenge-response protocols. We analyze existing protocols for proof of computation power and propose a new two-flow protocol using CI-OWF based on square Diffie-Hellman problem