Deniable authentication allows Alice to authenticate a message to Bob, while retaining deniability towards third parties. In particular, not even Bob can convince a third party that Alice authenticated that message. Clearly, in this setting Bob should not be considered trustworthy. Furthermore, deniable authentication is necessary for deniable key exchange, as explicitly desired by Signal and off-the-record (OTR) messaging. In this work we focus on (publicly verifiable) designated...

We study the (in)feasibility of quantum pseudorandom notions in a quantum analog of the random oracle model, where all the parties, including the adversary, have oracle access to the same Haar random unitary. In this model, we show the following: • (Unbounded-query secure) pseudorandom unitaries (PRU) exist. Moreover, the PRU construction makes two calls to the Haar oracle. • We consider constructions of PRUs making a single call to the Haar oracle. In this setting, we show that...

In this work, we revisit the Hosoyamada-Iwata (HI) proof for the quantum CPA security of the 4-round Luby-Rackoff construction and identify a gap that appears to undermine the security proof. We emphasize that this is not an attack, and the construction may still achieve the claimed security level. However, this gap raises concerns about the feasibility of establishing a formal security proof for the 4-round Luby-Rackoff construction. In fact, the issue persists even if the number of rounds...

In recent years quantum computing has developed rapidly. The security threat posed by quantum computing to cryptography makes it necessary to better evaluate the resource cost of attacking algorithms, some of which require quantum implementations of the attacked cryptographic building blocks. In this paper we manage to optimize quantum circuits of AES in several aspects. Firstly, based on de Brugière \textit{et al.}'s greedy algorithm, we propose an improved depth-oriented algorithm for...

Collision-resistant hashing, a fundamental primitive in modern cryptography, ensures that there is no efficient way to find distinct inputs that produce the same hash value. This property underpins the security of various cryptographic applications, making it crucial to understand its complexity. The complexity of this problem is well-understood in the classical setting and $\Theta(N^{1/2})$ queries are needed to find a collision. However, the advent of quantum computing has introduced new...

Threshold symmetric encryption (TSE), introduced by Agrawal et al. [DiSE, CCS 2018], provides scalable and decentralized solution for symmetric encryption by ensuring that the secret-key stays distributed at all times. They avoid having a single point of attack or failure, while achieving the necessary security requirements. TSE was further improved by Christodorescu et al. [ATSE, CCS 2021] to support an amortization feature which enables a “more privileged” client to encrypt records in bulk...

In Crypto 2019, Zhandry showed how to define compressed oracles, which record quantum superposition queries to the quantum random oracle. In this paper, we extend Zhandry's compressed oracle technique to non-uniformly distributed functions with independently sampled outputs. We define two quantum oracles $\mathsf{CStO}_D$ and $\mathsf{CPhsO}_D$, which are indistinguishable to the non-uniform quantum random oracle where quantum access is given to a random function $H$ whose images $H(x)$...

Many classical secure structures are broken by quantum attacks. Evaluating the quantum security of a structure and providing a tight security bound is a challenging research area. As a tweakable block cipher structure based on block ciphers, $\mathsf{TNT}$ was proven to have $O(2^{3n/4})$ CPA and $O(2^{n/2})$ CCA security in the classical setting. We prove that $\mathsf{TNT}$ is a quantum-secure tweakable block cipher with a bound of $O(2^{n/6})$. In addition, we show the tight quantum PRF...

Zero-knowledge elementary databases (ZK-EDBs) enable a prover to commit a database ${D}$ of key-value $(x,v)$ pairs and later provide a convincing answer to the query ``send me the value $D(x)$ associated with $x$'' without revealing any extra knowledge (including the size of ${D}$). After its introduction, several works extended it to allow more expressive queries, but the expressiveness achieved so far is still limited: only a relatively simple queries--range queries over the keys and...

The significant progress in the development of quantum computers has made the study of cryptanalysis based on quantum computing an active topic. To accurately estimate the resources required to carry out quantum attacks, the involved quantum algorithms have to be synthesized into quantum circuits with basic quantum gates. In this work, we present several generic synthesis and optimization techniques for circuits implementing the quantum oracles of iterative symmetric-key ciphers that are...

This paper introduces a new type of public-key encryption scheme, called Multi-Designated Receiver Signed Public Key Encryption (MDRS-PKE), which allows a sender to select a set of designated receivers and both encrypt and sign a message that only these receivers will be able to read and authenticate (confidentiality and authenticity). An MDRS-PKE scheme provides several additional security properties which allow for a fundamentally new type of communication not considered before. Namely, it...

HMAC and NMAC are the most basic and important constructions to convert Merkle-Damgård hash functions into message authentication codes (MACs) or pseudorandom functions (PRFs). In the quantum setting, at CRYPTO 2017, Song and Yun showed that HMAC and NMAC are quantum pseudorandom functions (qPRFs) under the standard assumption that the underlying compression function is a qPRF. Their proof guarantees security up to $O(2^{n/5})$ or $O(2^{n/8})$ quantum queries when the output length of HMAC...

In the quantum random oracle model, the adversary may make quantum superposition queries to the random oracle. Since even a single query can potentially probe exponentially many points, classical proof techniques are hard to be applied. For example, recording the oracle queries seemed difficult. In 2018, Mark Zhandry showed that, despite the apparent difficulties, it is in fact possible to ‘record’ the quantum queries. He has defined the compressed oracle, which is indistinguishable from...

This work studies signcryption of classical data in the quantum setting. Essentially, we investigate the quantum security of generic constructions of signcryption schemes based on three paradigms, viz., encrypt-then-sign (EtS), sign-then-encrypt (StE) and commit-then-encrypt-and-sign (CtE&S). For doing that we define the confidentiality and authenticity of signcryption for classical data both in insider and outsider models against quantum adversaries. In the insider model, we show that the...

We consider the theoretically-sound selection of cryptographic parameters, such as the size of algebraic groups or RSA keys, for TLS 1.3 in practice. While prior works gave security proofs for TLS 1.3, their security loss is quadratic in the total number of sessions across all users, which due to the pervasive use of TLS is huge. Therefore, in order to deploy TLS 1.3 in a theoretically-sound way, it would be necessary to compensate this loss with unreasonably large parameters that would be...

Recently, Beullens, Kleinjung, and Vercauteren (Asiacrypt'19) provided the first practical isogeny-based digital signature, obtained from the Fiat-Shamir (FS) paradigm. They worked with the CSIDH-512 parameters and passed through a new record class group computation. However, as with all standard FS signatures, the security proof is highly non-tight and the concrete parameters are set under the heuristic that the only way to attack the scheme is by finding collisions for a hash function. In...

We propose the General Sieve Kernel (G6K, pronounced /ʒe.si.ka/), an abstract stateful machine supporting a wide variety of lattice reduction strategies based on sieving algorithms. Using the basic instruction set of this abstract stateful machine, we first give concise formulations of previous sieving strategies from the literature and then propose new ones. We then also give a light variant of BKZ exploiting the features of our abstract stateful machine. This encapsulates several...

Deniable encryption (Canetti et al., Crypto 1996) enhances secret communication over public channels, providing the additional guarantee that the secrecy of communication is protected even if the parties are later coerced (or willingly bribed) to expose their entire internal states: plaintexts, keys and randomness. To date, constructions of deniable encryption --- and more generally, interactive deniable communication --- only address restricted cases where only one party is compromised...

We advance the study of secure stream-based channels (Fischlin et al., CRYPTO ’15) by considering the multiplexing of many data streams over a single channel, an essential feature of real world protocols such as TLS. Our treatment adopts the definitional perspective of Rogaway and Stegers (CSF ’09), which offers an elegant way to reason about what standardizing documents actually provide: a partial specification of a protocol that admits a collection of compliant, fully realized...

The quantum random oracle model (QROM) has become the standard model in which to prove the post-quantum security of random-oracle-based constructions. Unfortunately, none of the known proof techniques allow the reduction to record information about the adversary's queries, a crucial feature of many classical ROM proofs, including all proofs of indifferentiability for hash function domain extension. In this work, we give a new QROM proof technique that overcomes this ``recording barrier''. ...

Authentication and authenticated encryption with associated data (AEAD) are applied in cryptographic protocols to provide message integrity. The definitions in the literature and the constructions used in practice all protect against forgeries, but offer varying levels of protection against replays, reordering, and drops. As a result of the lack of a systematic hierarchy of authentication and AEAD security notions, gaps have arisen in the literature, specifically in the provable security...

TLS is one of the most widely deployed cryptographic protocols on the Internet; it is used to protect the confidentiality and integrity of transmitted data in various client-server protocols. Its non-standard use of cryptographic primitives, however, makes it hard to formally assess its security. It is in fact difficult to use traditional (well-understood) security notions for the key-exchange (here: handshake) and the encryption/authentication (here: record layer) parts of the protocol due...

We reformalize and recast dual-receiver encryption (DRE) proposed in CCS '04, a public-key encryption (PKE) scheme for encrypting to two independent recipients in one shot. We start by defining the crucial soundness property for DRE, which ensures that two recipients will get the same decryption result. While conceptually simple, DRE with soundness turns out to be a powerful primitive for various goals for PKE, such as complete non-malleability (CNM) and plaintext-awareness (PA). We then...

Ciphertext-Policy Attribute-Based Proxy Re-Encryption (CP-ABPRE) extends the traditional Proxy Re-Encryption (PRE) by allowing a semi-trusted proxy to transform a ciphertext under an access policy to the one with the same plaintext under another access policy (i.e.attribute-based re-encryption). The proxy, however, learns nothing about the underlying plaintext. CP-ABPRE has many real world applications, such as fine-grained access control in cloud storage systems and medical records sharing...

Computer-aided verification provides effective means of analyzing the security of cryptographic primitives. However, it has remained a challenge to achieve fully automated analyses yielding guarantees that hold against computational (rather than symbolic) attacks. This paper meets this challenge for public-key encryption schemes built from trapdoor permutations and hash functions. Using a novel combination of techniques from computational and symbolic cryptography, we present proof systems...

We present a protocol for anonymous access to a database where the different records have different access control permissions. These permissions could be attributes, roles, or rights that the user needs to have in order to access the record. Our protocol offers maximal security guarantees for both the database and the user, namely (1) only authorized users can access the record; (2) the database provider does not learn which record the user accesses; and (3) the database provider does not...