This paper studies the security of key derivation functions (KDFs), a central class of cryptographic algorithms used to derive multiple independent-looking keys (each associated with a particular context) from a single secret. The main security requirement is that these keys are pseudorandom (i.e., the KDF is a pseudorandom function). This paper initiates the study of an additional security property, called key control (KC) security, first informally put forward in a recent update to NIST...

We develop a toolbox for modular construction and analysis of succinct, non-interactive commitments and vector commitments in the random oracle model, while guaranteeing universally composable security. To demonstrate its power, we use the toolbox to construct and analyze a modular variant of the Kilian-Micali ZK-SNARK. Along the way we also propose a new UC formulation of a global random oracle, that avoids a weakness in existing formulations and also enables expressing more nuanced,...

Private Set Intersection (PSI) enables two parties to compute the intersection of their private sets without revealing any additional information. While maliciously secure PSI protocols prevent many attacks, adversaries can still exploit them by using inconsistent inputs across multiple sessions. This limitation stems from the definition of malicious security in secure multiparty computation, but is particularly problematic in PSI because: (1) real-world applications---such as Apple’s PSI...

FROST and its variants are state-of-the-art protocols for threshold Schnorr signatures that are used in real-world applications. While static security of these protocols has been shown by several works, the security of these protocols under adaptive corruptions—where an adversary can choose which parties to corrupt at any time based on information it learns during protocol executions—has remained a notorious open problem that has received renewed attention due to recent standardization...

Anamorphic signatures allow covert communication through signatures in environments where encryption is restricted. They enable trusted recipients with a double key to extract hidden messages while the signature remains indistinguishable from a fresh and regular one. However, the traditional notion of anamorphic signatures suffers from vulnerabilities, particularly when a single recipient or sender is compromised, exposing all hidden messages and providing undeniable proof that citizens are...

We construct distributed broadcast encryption and registered attribute-based encryption (ABE) that support an arbitrary polynomial of users from the succinct LWE assumption. Specifically, if we take $\lambda$ to be the security parameter and $N$ to be the number of users, we obtain the following: * We obtain a distributed broadcast encryption scheme where the size of the public parameters, user public/secret keys, and ciphertexts are optimal (i.e., have size $\mathsf{poly}(\lambda, \log...

In this work, we consider secure multiparty computation (MPC) in the asynchronous network setting. MPC allows $n$ parties to compute a public function on their private inputs against an adversary corrupting at most $t$ of them. We consider both communication complexity and round complexity of asynchronous MPC (AMPC) with the optimal resilience $n=3t+1$. Without fully homomorphic encryptions, the best-known result in this setting is achieved by Coretti, Garay, Hirt, and Zikas...

A pseudorandom code is a keyed error-correction scheme with the property that any polynomial number of encodings appear random to any computationally bounded adversary. We show that the pseudorandomness of any code tolerating a constant rate of random errors cannot be based on black-box reductions to almost any generic cryptographic primitive: for instance, anything that can be built from random oracles, generic multilinear groups, and virtual black-box obfuscation. Our result is optimal, as...

Pseudorandom codes (PRCs) are error-correcting codes with the distinguishing feature that their codewords are computationally indistinguishable from random strings. Introduced by Christ and Gunn (CRYPTO 2024), PRCs have found applications in areas such as AI watermarking, where both robustness and pseudorandomness are essential. All known constructions of PRCs rely on coding-theoretic hardness assumptions. In this work, we study how inherent the use of coding-theoretic hardness is in the...

Threshold signatures are one of the most important cryptographic primitives in distributed systems. Of particular interest is the threshold Schnorr signature, a pairing-free signature with efficient verification, compatible with standardized EdDSA (non-threshold) signature. However, most threshold Schnorr signatures have only been proven secure against a static adversary, which has to declare its corruptions before the protocol execution. Many existing adaptively secure constructions require...

A Password-Authenticated Key Exchange (PAKE) protocol allows two parties to jointly establish a cryptographic key, where the only information shared in advance is a low-entropy password. The first efficient PAKE protocol whose security does not rely on the random oracle model is the one by Katz, Ostrovsky and Yung (KOY, EUROCRYPT 2001). Unfortunately, the KOY protocol has only been proven secure in the game-based setting, and it is unclear whether KOY is secure in the stronger Universal...

We revisit the quantum security (in the QROM) of digital signature schemes that follow the Fiat-Shamir-with-aborts (FSwA) or the probabilistic hash-and-sign with retry/abort (HSwA) design paradigm. Important examples of such signature schemes are Dilithium, SeaSign, Falcon+ and UOV. In particular, we are interested in the UF-CMA-to-UF-NMA reduction for such schemes. We observe that previous such reductions have a reduction loss that is larger than what one would hope for, or require a more...

We initiate the study of a new abstraction called incremental decentralized data archival (${\sf iDDA}$). Specifically, imagine that there is an ever-growing, massive database such as a blockchain, a comprehensive human knowledge base like Wikipedia, or the Internet archive. We want to build a decentralized archival of such datasets to ensure long-term robustness and sustainability. We identify several important properties that an ${\sf iDDA}$ scheme should satisfy. First,...

In this work, we study the quantum security of key-alternating ciphers (KAC), a natural multi-round generalization of the Even–Mansour (EM) cipher underlying many block cipher constructions, including AES. While the classical security of KAC and the quantum security of the $1$-round KAC (i.e. Even-Mansour) cipher are well understood, the quantum resistance of multi-round KAC remains largely unexplored. We focus on the $2$-round KAC construction, defined using public $n$-bit permutations...

Deterministic signatures are often used to mitigate the risks associated with poor-quality randomness, where the randomness in the signing process is generated by a pseudorandom function that takes a message as input. However, some studies have shown that such signatures are vulnerable to fault-injection attacks. To strike a balance, recent signature schemes often adopt "hedged" randomness generation, where the pseudorandom function takes both a message and a nonce as input. Aranha et al....

We study the security of a popular paradigm for constructing SNARGs, closing a key security gap left open by prior work. The paradigm consists of two steps: first, construct a public-coin succinct interactive argument by combining a functional interactive oracle proof (FIOP) and a functional commitment scheme (FC scheme); second, apply the Fiat–Shamir transformation in the random oracle model. Prior work did not consider this generalized setting nor prove the security of this second step...

Blind signature schemes are essential for privacy-preserving applications such as electronic voting, digital currencies or anonymous credentials. In this paper, we revisit Fischlin's framework for round-optimal blind signature schemes and its recent efficient lattice-based instantiations. Our proposed framework compiles any post-quantum hash-and-sign signature scheme into a blind signature scheme. The resulting scheme ensures blindness by design and achieves one-more unforgeability, relying...

We establish new lower bounds on the size of practical garbled circuits, which hold against any scheme satisfying the following simple properties: (1) Its security is based on symmetric-key cryptography only. More formally, security holds in Minicrypt, a model in which a random oracle is the only available source of cryptography. (2) The evaluation algorithm makes non-adaptive queries to the random oracle. (3) The evaluation algorithm "knows" which of its oracle queries are...

We initiate the study of multi-authority attribute-based functional encryption for noisy inner-product functionality, and propose two new primitives: (1) multi-authority attribute-based (noisy) inner-product functional encryption (MA-AB(N)IPFE), and (2) multi-authority attribute-based evasive inner-product functional encryption (MA-ABevIPFE). The MA-AB(N)IPFE primitive generalizes the existing multi-authority attribute-based inner-product functional encryption schemes by Agrawal et al....

We present conceptually simple and practically competitive constructions of verifiable random functions (VRF) that fulfill strong notions of unbiasability recently introduced by Giunta and Stewart. VRFs with such strong properties were previously only known in the random oracle model or from the decisional Diffie–Hellman assumption with preprocessing. In contrast, our constructions are based on generic assumptions and are thus the first to be plausibly post-quantum secure in the standard...

Proof-carrying data (PCD) is a powerful cryptographic primitive for computational integrity in a distributed setting. State-of-the-art constructions of PCD are based on accumulation schemes (and, closely related, folding schemes). We present WARP, the first accumulation scheme with linear prover time and logarithmic verifier time. Our scheme is hash-based (secure in the random oracle model), plausibly post-quantum secure, and supports unbounded accumulation depth. We achieve our result...

As quantum computing matures, its impact on traditional cryptographic protocols becomes increasingly critical, especially for data-at-rest scenarios where large data sets remain encrypted for extended periods of time. This paper addresses the pressing need to transition away from pre-quantum algorithms by presenting an agile cryptosystem that securely and efficiently supports post-quantum Key Encapsulation Mechanisms (KEMs). The proposed solution is based on combining a CCA-secure KEM with...

The sponge is a cryptographic construction that turns a public permutation into a hash function. When instantiated with the Keccak permutation, the sponge forms the NIST SHA-3 standard. SHA-3 is a core component of most post-quantum public-key cryptography schemes slated for worldwide adoption. While one can consider many security properties for the sponge, the ultimate one is \emph{indifferentiability from a random oracle}, or simply \emph{indifferentiability}. The sponge was proved...

SNAIL (Succinct, Non-interactive, Alon-compressed, Instant argument for Layered circuits) turns any depth-\(d\) arithmetic circuit into a non-interactive argument whose prover runs within \(1 + c(d,k,n)\) of plain circuit execution, where \(c(d,k,n) = \frac{3\,(k+n+1)}{k\,d + n + 1}\). For the representative choice \(k = n = 4\) and \(24 \le d \le 32\) this means only 21–28 % overhead. Core idea: A constant-round zerocheck based on a difference-driven Alon decomposition...

Threshold zero-knowledge protocols have not been widely adopted, presumably due to the relevant network overhead, complicated certification processes and thus limited interoperability chances. In this work, we propose $\mathsf{OSST}$, a Schnorr-based threshold identification scheme that is both non-interactive and non-reliant on the public shares. Given a $(n, t)$-shared secret $x$, the proposed protocol allows any $t^* \ge t$ (but no less) shareholders to collectively prove that...

We show that concrete hardness assumptions about learning or cloning the output state of a random quantum circuit can be used as the foundation for secure quantum cryptography. In particular, under these assumptions we construct secure one-way state generators (OWSGs), digital signature schemes, quantum bit commitments, and private key encryption schemes. We also discuss evidence for these hardness assumptions by analyzing the best-known quantum learning algorithms, as well as proving...

The major Fully Homomorphic Encryption (FHE) schemes guarantee the privacy of the encrypted message only in the honest-but-curious setting, when the server follows the protocol without deviating. However, various attacks in the literature show that an actively malicious server can recover sensitive information by executing incorrect functions, tampering with ciphertexts, or observing the client’s reaction during decryption. Existing integrity solutions for FHE schemes either fail to...

An interactive aggregate signature scheme allows $n$ signers, each with their own secret/public key pair $(sk_i, pk_i)$ and message $m_i$, to jointly produce a short signature that simultaneously witnesses that $m_i$ has been signed under $pk_i$ for every $i \in \{1, \dots, n\}$. Despite the large potential for savings in terms of space and verification time, which constitute the two main bottlenecks for large blockchain systems such as Bitcoin, aggregate signatures have received much less...

The Secure Shell (SSH) protocol is one of the first security protocols on the Internet to upgrade itself to resist attacks against future quantum computers, with the default adoption of the "quantum (otherwise, classically)" secure hybrid key exchange in OpenSSH from April 2022. However, there is a lack of a comprehensive security analysis of this quantum-resistant version of SSH in the literature: related works either focus on the hybrid key exchange in isolation and do not consider...

Multisignature schemes are crucial for secure operations in digital wallets and escrow services within smart contract platforms, particularly in the emerging post-quantum era. Existing post-quantum multisignature constructions either do not address the stringent requirements of the Quantum Random Oracle Model (QROM) or fail to achieve practical efficiency due to suboptimal parameter choices. In this paper, we present a novel Dilithium-based multisignature scheme designed to be secure in...

We prove that it is impossible to construct perfect-complete quantum public-key encryption (QPKE) with classical keys from quantumly secure one-way functions (OWFs) in a black-box manner, resolving a long-standing open question in quantum cryptography. Specifically, in the quantum random oracle model (QROM), no perfect-complete QPKE scheme with classical keys, and classical/quantum ciphertext can be secure. This improves the previous works which require either unproven conjectures or...

A threshold signature scheme allows distributing a signing key to $n$ users, such that any $t$ of them can jointly sign, but any $t-1$ cannot. It is desirable to prove adaptive security of threshold signature schemes, which considers adversaries that can adaptively corrupt honest users even after interacting with them. For a class of signatures that relies on security proofs with rewinding, such as Schnorr signatures, proving adaptive security entails significant challenges. This work...

In this paper, we present an improved framework for proving query bounds in the Quantum Random Oracle Model (QROM) for algorithms with both quantum and classical query interfaces, where the classical input is partially controlled by the adversary. By extending existing techniques, we develop a method to bound the progress an adversary can make with such partial-control classical queries. While this framework is applicable to different hash function properties, we decided to demonstrate the...

eIDAS 2.0 (electronic IDentification, Authentication and trust Services) is a very ambitious regulation aimed at equipping European citizens with a personal digital identity wallet (EU Digital Identity Wallet) on a mobile phone that not only needs to achieve a high level of security, but also needs to be available as soon as possible for a large number of citizens and respect their privacy (as per GDPR - General Data Protection Regulation). In this paper, we introduce the foundations of...

A Random Oracle Combiner (ROC), introduced by Dodis et al. (CRYPTO ’22), takes two hash functions $h_1, h_2$ from m bits to n bits and outputs a new hash function $C$ from $m$' to $n$' bits. This function C is guaranteed to be indifferentiable from a fresh random oracle as long as one of $h_1$ and $h_2$ (say, $h_1$) is a random oracle, while the other h2 can “arbitrarily depend” on $h_1$. The work of Dodis et al. also built the first length-preserving ROC, where $n$′ = $n$. Unfortunately,...

The Middle-Product Learning with Errors (MPLWE) assumption is a variant of the Learning with Errors (LWE) assumption. The MPLWE assumption reduces the key size of corresponding LWE-based schemes by setting keys as sets of polynomials. Moreover, MPLWE has more robust security than other LWE variants such as Ring-LWE and Module-LWE. Lombardi et al. proposed an identity-based encryption (IBE) scheme (LVV-IBE) based on the MPLWE assumption in the random oracle model (ROM) by following Gentry et...

Yao's garbled circuits have received huge attention in both theory and practice. While garbled circuits can be constructed using minimal assumption (i.e., the existence of pseudorandom functions or one-way functions), the state-of-the-art constructions (e.g., Rosulek-Roy, Crypto 2021) are based on stronger assumptions. In particular, the ``Free-XOR'' technique (Kolesnikov-Schneider, ICALP 2008) is essential in these state-of-the-art constructions, and their security can only be proven in the...

Conventional Publicly Verifiable Secret Sharing (PVSS) protocols allow a dealer to share a secret among $n$ parties without interaction, ensuring that any $t + 1$ parties (where $t+1 \le n$) can recover the secret, while anyone can publicly verify the validity of both the individual shares and the reconstructed secret. PVSS schemes are shown to be a key tool in a wide range of practical applications. In this paper, we introduce Pre-constructed PVSS (PPVSS), an extension of standard PVSS...

A (single server) private information retrieval (PIR) allows a client to read data from a public database held on a remote server, without revealing to the server which locations she is reading. In a doubly efficient PIR (DEPIR), the database is first preprocessed offline into a data structure, which then allows the server to answer any client query efficiently in sub-linear online time. Constructing DEPIR is a notoriously difficult problem, and this difficulty even extends to a weaker...

The Fiat-Shamir transformation underlies numerous non-interactive arguments, with variants that differ in important ways. This paper addresses a gap between variants analyzed by theoreticians and variants implemented (and deployed) by practitioners. Specifically, theoretical analyses typically assume parties have access to random oracles with sufficiently large input and output size, while cryptographic hash functions in practice have fixed input and output sizes (pushing practitioners...

In this work, we study the communication complexity of constant-round secure multiparty computation (MPC) against a fully malicious adversary and consider both the honest majority setting and the dishonest majority setting. In the (strong) honest majority setting (where $t=(1/2-\epsilon)n$ for a constant $\epsilon$), the best-known result without relying on FHE is given by Beck et al. (CCS 2023) based on the LPN assumption that achieves $O(|C|\kappa)$ communication, where $\kappa$ is the...

The Chou-Orlandi batch oblivious transfer (OT) protocol is a particularly attractive OT protocol that bridges the gap between practical efficiency and strong security guarantees and is especially notable due to its simplicity. The security analysis provided by Chou and Orlandi bases the security of their protocol on the hardness of the computational Diffie-Hellman ($\mathsf{CDH}$) problem in prime-order groups. Concretely, in groups in which no better-than-generic algorithms are known for...

One-shot signatures (OSS) were defined by Amos, Georgiou, Kiayias, and Zhandry (STOC'20). These allow for signing exactly one message, after which the signing key self-destructs, preventing a second message from ever being signed. While such an object is impossible classically, Amos et al observe that OSS may be possible using quantum signing keys by leveraging the no-cloning principle. OSS has since become an important conceptual tool with many applications in decentralized settings and for...

A sequential aggregate signature scheme (SAS) allows multiple potential signers to sequentially aggregate their respective signatures into a single compact signature. Typically, verification of a SAS signatures requires access to all messages and public key pairs utilized in the aggregate generation. However, efficiency is crucial for cryptographic protocols to facilitate their practical implementation. To this end, we propose a sequential aggregate signature scheme with lazy verification...

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...

Decentralization is a great enabler for adoption of modern cryptography in real-world systems. Widespread adoption of blockchains and secure multi-party computation protocols are perfect evidentiary examples for dramatic rise in deployment of decentralized cryptographic systems. Much of cryptographic research can be viewed as reducing (or eliminating) the dependence on trusted parties, while shielding from stronger adversarial threats. In this work, we study the problem of multi-authority...

Delegatable Attribute-Based Encryption (DABE) is a well-known generalization of ABE, proposed to mirror organizational hierarchies. We design a DABE scheme from witness encryption and other simple assumptions. Our construction does not rely on Random Oracles, and we provide a black-box reduction to polynomial hardness of underlying assumptions.

Abdalla et al. (ASIACRYPT 2020) introduced a notion of identity-based inner-product functional encryption (IBIPFE) that combines identity-based encryption and inner-product functional encryption (IPFE). Thus far, several pairing-based and lattice-based IBIPFE schemes have been proposed. However, there are two open problems. First, there are no known IBIPFE schemes that satisfy the adaptive simulation-based security. Second, known IBIPFE schemes that satisfy the adaptive...

We provide a generic construction of blind signatures from cryptographic group actions following the framework of the blind signature CSIOtter introduced by Katsumata et al. (CRYPTO'23) in the context of isogeny (commutative group action). We adapt and modify that framework to make it work even for non-commutative group actions. As a result, we obtain a blind signature from abstract group actions which are proven to be secure in the random oracle model. We also propose an instantiation based...

Updatable Public-Key Encryption (UPKE) augments the security of PKE with Forward Secrecy properties. While requiring more coordination between parties, UPKE enables much more efficient constructions than full-fledged Forward-Secret PKE. Alwen, Fuchsbauer and Mularczyk (AFM, Eurocrypt’24) presented the strongest security notion to date. It is the first to meet the needs of UPKE’s most important applications: Secure Group Messaging and Continuous Group Key Agreement. The authors provide a very...

Considering security against quantum adversaries, while it is important to consider the traditional existential unforgeability (EUF-CMA security), it is desirable to consider security against adversaries making quantum queries to the signing oracle: Plus-one security (PO security) and blind unforgeability (BU security) proposed by Boneh and Zhandry (Crypto 2013) and Alagic et al. (EUROCRYPT 2020), respectively. Hash-and-sign is one of the most common paradigms for constructing EUF-CMA-secure...

A memory checker is an algorithmic tool used to certify the integrity of a database maintained on a remote, unreliable, computationally bounded server. Concretely, it allows a user to issue instructions to the server and after every instruction, obtain either the correct value or a failure (but not an incorrect answer) with high probability. A recent result due to Boyle, Komargodski, and Vafa (BKV, STOC '24) showed a tradeoff between the size of the local storage and the number of queries...

Non-interactive zero-knowledge (NIZK) arguments allow a prover to convince a verifier about the truthfulness of an NP-statement by sending just one message, without disclosing any additional information. In several practical scenarios, the Fiat-Shamir transform is used to convert an efficient constant-round public-coin honest-verifier zero-knowledge proof system into an efficient NIZK argument system. This approach is provably secure in the random oracle model, crucially requires the...

A key encapsulation mechanism (KEM) allows two parties to establish a shared secret key using only public communication. For post-quantum KEMs, the most widespread approach is to design a passively secure public-key encryption (PKE) scheme and then apply the Fujisaki–Okamoto (FO) transform that turns any such PKE scheme into an IND-CCA secure KEM. While the base security requirement for KEMs is typically IND-CCA security, adversaries in practice can sometimes observe and attack many public...

Ratcheted key exchange (RKE) is at the heart of modern secure messaging, enabling protocol participants to continuously update their secret material to protect against full state exposure through forward security (protecting past secrets and messages) and post-compromise security (recovering from compromise). However, many practical attacks only provide the adversary with partial access to a party's secret state, an attack vector studied under the umbrella of leakage resilience. Existing...

The Fiat–Shamir transformation is a fundamental cryptographic technique widely used to convert public-coin interactive protocols into non-interactive ones. This transformation is crucial in both theoretical and practical applications, particularly in the construction of succinct non-interactive arguments (SNARKs). While its security is well-established in the random oracle model, practical implementations replace the random oracle with a concrete hash function, where security is merely...

Broadcast encryption allows a user to encrypt a message to $N$ recipients with a ciphertext whose size scales sublinearly with $N$. The natural security notion for broadcast encryption is adaptive security which allows an adversary to choose the set of recipients after seeing the public parameters. Achieving adaptive security in broadcast encryption is challenging, and in the plain model, the primary technique is the celebrated dual-systems approach, which can be implemented over groups with...

Recent applications and attacks have highlighted the need for authenticated encryption (AE) schemes to achieve the so-called committing security beyond privacy and authenticity. As a result, several generic solutions have been proposed to transform a non-committing AE scheme to a committing one, for both basic unique-nonce security and advanced misuse-resistant (MR) security. We observe that all existing practical generic transforms are subject to at least one of the following limitations:...

Succinct non-interactive arguments of knowledge (SNARKs) are variants of non-interactive zero-knowledge proofs (NIZKs) in which complex statements can be proven in a compact way. SNARKs have had tremendous impact in several areas of cryptography, including verifiable computing, blockchains, and anonymous communication. A recurring concept in many applications is the concept of recursive SNARKs, in which a proof references a previous proof to show an evolved statement. In this work, we...

Non-interactive key exchange (NIKE) is a simple and elegant cryptographic primitive that allows two or more users to agree on a secret shared key without any interaction. NIKE schemes have been formalized in different scenarios (such as the public-key, or the identity-based setting), and have found many applications in cryptography. In this work, we propose a NIKE variant that generalizes public-key and identity-based NIKE: a multi-authority identity-based NIKE (MA-ID-NIKE) is defined...

The concept of Anamorphic Encryption (Persiano, Phan and Yung, Eurocrypt '22), aims to enable private communication in settings where the usage of encryption is heavily controlled by a central authority (henceforth called the dictator) who can obtain users' secret keys. Since then, various works have improved our understanding of AE in several aspects, including its limitations. To this regard, two recent works constructed various Anamorphic-Resistant Encryption (ARE) schemes, i.e., schemes...

We present the first construction for adaptively secure HIBE, that does not rely on bilinear pairings or random oracle heuristics. Notably, we design an adaptively secure HIBE from any selectively secure IBE system in the standard model. Combining this with known results, this gives the first adaptively secure HIBE system from a wide variety of standard assumptions such as CDH/Factoring/LWE/LPN. We also extend our adaptively secure HIBE system to satisfy full anonymity, giving the first...

In this work, we consider the communication complexity of MPC protocols in honest majority setting achieving malicious security in both information-theoretic setting and computational setting. On the one hand, we study the possibility of basing honest majority MPC protocols on oblivious linear evaluation (OLE)-hybrid model efficiently with information-theoretic security. More precisely, we instantiate preprocessing phase of the recent work Sharing Transformation (Goyal, Polychroniadou, and...

We construct a novel code-based blind signature scheme, us- ing the Matrix Equivalence Digital Signature (MEDS) group action. The scheme is built using similar ideas to the Schnorr blind signature scheme and CSI-Otter, but uses additional public key and commitment informa- tion to overcome the difficulties that the MEDS group action faces: lack of module structure (present in Schnorr), lack of a quadratic twist (present in CSI-Otter), and non-commutativity of the acting group. We address...

Password-Authenticated Key Exchange (PAKE) is a type of key exchange protocols secure against man-in-the-middle adversaries, in the setting where the two parties only agree upon a low-entropy "password" in advance. The first and arguably most well-studied PAKE protocol is Encrypted Key Exchange (EKE) (Bellovin and Marritt, 1992), and the standard security notion for PAKE is in the Universal Composability (UC) framework (Canetti et al., 2005). While the UC-security of EKE has been "folklore"...

Anamorphic encryption (AE), introduced by Persiano, Phan and Yung at Eurocrypt `22, allows to establish secure communication in scenarios where users might be forced to hand over their decryption keys to some hostile authority. Over the last few years, several works have improved our understanding of the primitive by proposing novel realizations, new security notions and studying inherent limitations. This work makes progress, mainly, on this last line of research. We show concrete...

We show a generic compiler from KEM to (Universally Composable) PAKE in the Random Oracle Model (ROM) and without requiring an Ideal Cipher. The compiler is akin to Encrypted Key Exchange (EKE) by Bellovin-Merritt, but following the work of McQuoid et al. it uses only a 2-round Feistel to password-encrypt a KEM public key. The resulting PAKE incurs only insignificant cost overhead over the underlying KEM, and it is a secure UC PAKE if KEM is secure and key-anonymous under the...

We establish the following theorem: Let $\mathsf{O}_0, \mathsf{O}_1, \mathsf{R}$ be random functions from $\{0,1\}^n$ to $\{0,1\}^n$, $n \in \mathbb{N}$. For all polynomial-query-bounded distinguishers $\mathsf{D}$ making at most $q=\mathsf{poly}(n)$ queries to each oracle, there exists a poly-time oracle simulator $\mathsf{Sim}^{(\cdot)}$ and a constant $c>0$ such that the probability is negligible, that is ...

Quantum cryptography allows to achieve security goals which are unobtainable using classical cryptography alone: it offers the promise of everlasting privacy. Thatis, an adversary trying to attack a protocol must succeed during the run of the protocol. After the protocol has terminated, security holds unconditionally. In this work, we initiate the study of a new model which we call the quantum decoherence model (QDM). In a nutshell, this model captures adversaries that are computationally...

Rejection sampling is a crucial security mechanism in lattice-based signature schemes that follow the Fiat-Shamir with aborts paradigm, such as ML-DSA/CRYSTALS-Dilithium. This technique transforms secret-dependent signature samples into ones that are statistically close to a secret-independent distribution (in the random oracle model). While many side-channel attacks have directly targeted sensitive data such as nonces, secret keys, and decomposed commitments, fewer studies have explored the...

Distributed certification is a set of mechanisms that allows an all-knowing prover to convince the units of a communication network that the network's state has some desired property, such as being $3$-colorable or triangle-free. Classical mechanisms, such as proof labeling schemes (PLS), consist of a message from the prover to each unit, followed by on-e round of communication between each unit and its neighbors. Later works consider extensions, called distributed interactive proofs,...

It is often desirable to break cryptographic primitives into two components: an input-independent offline component, and a cheap online component used when inputs arrive. Security of such online/offline primitives must be proved in the input-adaptive setting: the adversary chooses its input adaptively, based on what it sees in the offline-phase. Proving security in the input-adaptive setting can be difficult, particularly when one wishes to achieve simulation security and avoid idealized...

We study error detection and correction in a computationally bounded world, where errors are introduced by an arbitrary $\textit{polynomial-time}$ adversarial channel. Our focus is on $\textit{seeded}$ codes, where the encoding and decoding procedures can share a public random seed, but are otherwise deterministic. We can ask for either $\textit{selective}$ or $\textit{adaptive}$ security, depending on whether the adversary can choose the message being encoded before or after seeing the...

Lund et al. (JACM 1992) invented the powerful Sumcheck protocol that has been extensively used in complexity theory and in designing concretely efficient (zero-knowledge) arguments. In this work, we systematically study Sumcheck in the context of secure multi-party computation (MPC). Our main result is a new generic framework for lifting semi-honest MPC protocols to maliciously secure ones, with a {\em constant} multiplicative overhead in {\em both} computation and communication, and in the...

We revisit Updatable Public-Key Encryption (UPKE), which was introduced as a practical mechanism for building forward-secure cryptographic protocols. We begin by observing that all UPKE notions to date are neither syntactically flexible nor secure enough for the most important multi-party protocols motivating UPKE. We provide an intuitive taxonomy of UPKE properties -- some partially or completely overlooked in the past -- along with an overview of known (explicit and implicit) UPKE...

Multi-Authority Functional Encryption ($\mathsf{MA}$-$\mathsf{FE}$) [Chase, TCC'07; Lewko-Waters, Eurocrypt'11; Brakerski et al., ITCS'17] is a popular generalization of functional encryption ($\mathsf{FE}$) with the central goal of decentralizing the trust assumption from a single central trusted key authority to a group of multiple, independent and non-interacting, key authorities. Over the last several decades, we have seen tremendous advances in new designs and constructions for...

In modern cryptography, relatively few instantiations of foundational cryptographic primitives are used across most cryptographic protocols. For example, elliptic curve groups are typically instantiated using P-256, P-384, Curve25519, or Curve448, while block ciphers are commonly instantiated with AES, and hash functions with SHA-2, SHA-3, or SHAKE. This limited diversity raises concerns that an adversary with nation-state-level resources could perform a preprocessing attack, generating a...

The Fiat-Shamir (FS) transform is a prolific and powerful technique for compiling public-coin interactive protocols into non-interactive ones. Roughly speaking, the idea is to replace the random coins of the verifier with the evaluations of a complex hash function. The FS transform is known to be sound in the random oracle model (i.e., when the hash function is modeled as a totally random function). However, when instantiating the random oracle using a concrete hash function, there...

We construct the \emph{first} tightly secure signature schemes in the multi-user setting with adaptive corruptions from static search assumptions, such as classical discrete logarithm, RSA, factoring, or post-quantum group action discrete logarithm assumptions. In contrast to our scheme, the previous tightly secure schemes are based on decisional assumptions (e.g., (group action) DDH) or interactive search assumptions (e.g., one-more CDH). The security of our schemes is independent of the...

Designing post-quantum digital signatures is a very active research area at present, with several protocols being developed, based on a variety of mathematical assumptions. Many of these signatures schemes can be used as a basis to define more advanced schemes, such as ring or threshold signatures, where multiple parties are involved in the signing process. Unfortunately, the majority of these protocols only considers a static adversary, that must declare which parties to corrupt at the...

We revisit a basic building block in the endeavor to migrate to post-quantum secure cryptography, Key Encapsulation Mechanisms (KEMs). KEMs enable the establishment of a shared secret key, using only public communication. When targeting chosen-ciphertext security against quantum attackers, the go-to method is to design a Public-Key Encryption (PKE) scheme and then apply a variant of the PKE-to-KEM conversion known as the Fujisaki-Okamoto (FO) transform, which we revisit in this work....

With the threat posed by quantum computers on the horizon, systems like Ethereum must transition to cryptographic primitives resistant to quantum attacks. One of the most critical of these primitives is the non-interactive multi-signature scheme used in Ethereum's proof-of-stake consensus, currently implemented with BLS signatures. This primitive enables validators to independently sign blocks, with their signatures then publicly aggregated into a compact aggregate signature. In this...

Registered attribute-based encryption (ABE) is a generalization of public-key encryption that enables fine-grained access control to encrypted data (like standard ABE), but without needing a central trusted authority. In a key-policy registered ABE scheme, users choose their own public and private keys and then register their public keys together with a decryption policy with an (untrusted) key curator. The key curator aggregates all of the individual public keys into a short master public...

In recent work [Crypto'24], Dodis, Jost, and Marcedone introduced Compact Key Storage (CKS) as a modern approach to backup for end-to-end (E2E) secure applications. As most E2E-secure applications rely on a sequence of secrets $(s_1,...,s_n)$ from which, together with the ciphertexts sent over the network, all content can be restored, Dodis et al. introduced CKS as a primitive for backing up $(s_1,...,s_n)$. The authors provided definitions as well as two practically efficient schemes (with...

Blind signatures are an important primitive for privacy-preserving technologies. To date, highly efficient pairing-free constructions rely on the random oracle model, and additionally, a strong assumption, such as interactive assumptions or the algebraic group model. In contrast, for signatures we know many efficient constructions that rely on the random oracle model and standard assumptions. In this work, we develop techniques to close this gap. Compared to the most efficient...

We construct the first blind signature scheme that achieves all of the following properties simultaneously: - it is tightly secure under a standard (i.e., non-interactive, non-\(q\)-type) computational assumption, - it does not require pairings, - it does not rely on generic, non-black-box techniques (like generic NIZK proofs). The third property enables a reasonably efficient solution, and in fact signatures in our scheme comprise 10 group elements and 29...

Garbled circuits are a foundational primitive in both theory and practice of cryptography. Given $(\hat{C}, K[x])$, where $\hat{C}$ is the garbling of a circuit C and $K[x] = \{K[i, x_i]\}$ are the input labels for an input $x$, anyone can recover $C(x)$, but nothing else about the input $x$. Most research efforts focus on minimizing the size of the garbled circuit $\hat{C}$. In contrast, the work by Applebaum, Ishai, Kushilevitz, and Waters (CRYPTO' 13) initiated the study of minimizing the...

The Gentry-Peikert-Vaikuntanathan (GPV) framework is utilized for constructing practical digital signatures, which is proven to be secure in both the classical/quantum random-oracle models. Falcon is such a signature scheme, recognized as a compact and efficient signature among NIST-standardized signature schemes. Recently, Guerreau et al. (CHES 2022) and Zhang et al. (Eurocrypt 2023) proposed the secret key recovery attack on Falcon utilizing signatures filtered by simple power analysis...

In this work we state and prove an abstract version of the multi-forking lemma of Pointcheval and Stern from EUROCRYPT'96. Earlier, Bellare and Neven had given an abstract version of forking lemma for two-collisions (CCS'06). While the original purpose of the forking lemma was to prove security of signature schemes in the random oracle methodology, the abstract forking lemma can be used to obtain security proofs for multi-signatures, group signatures, and compilation of interactive protocols...

Smart-ID is an application for signing and authentication provided as a service to residents of Belgium, Estonia, Latvia and Lithuania. Its security relies on multi-prime server-supported RSA, password-authenticated key shares and clone detection mechanism. Unfortunately, the security properties of the underlying protocol have been specified only in ``game-based'' manner. There is no corresponding ideal functionality that the actual protocol is shown to securely realize in the universal...

One-time programs (Goldwasser, Kalai and Rothblum, CRYPTO 2008) are functions that can be run on any single input of a user's choice, but not on a second input. Classically, they are unachievable without trusted hardware, but the destructive nature of quantum measurements seems to provide a quantum path to constructing them. Unfortunately, Broadbent, Gutoski and Stebila showed that even with quantum techniques, a strong notion of one-time programs, similar to ideal obfuscation, cannot be...

The password-hardening service (PH) is a crypto service that armors canonical password authentication with an external key against offline password guessing in case the password file is somehow compromised/leaked. The game-based formal treatment of PH was brought by Everspaugh et al. at USENIX Security'15. Their work is followed by efficiency-enhancing PO-COM (CCS'16), security-patching Phoenix (USENIX Security'17), and functionality-refining PW-Hero (SRDS'22). However, the issue of single...

The Messaging Layer Security (MLS) protocol, recently standardized in RFC 9420, aims to provide efficient asynchronous group key establishment with strong security guarantees. The main component of MLS, which is the source of its key efficiency and security properties, is a protocol called TreeKEM. Given that a major vision for the MLS protocol is for it to become the new standard for messaging applications like WhatsApp, Facebook Messenger, Signal, etc., it has the potential to be used by a...

We initiate the study of the black-box complexity of private-key functional encryption (FE). Of central importance in the private-key setting is the inner-product functionality, which is currently only known from assumptions that imply public-key encryption, such as Decisional Diffie-Hellman or Learning-with-Errors. As our main result, we rule out black-box constructions of private-key inner-product FE from random oracles. This implies a black-box separation between private-key...

In this work, we present a new paradigm for constructing Group Authenticated Key Exchange (GAKE). This result is the first tightly secure GAKE scheme in a strong security model that allows maximum exposure attacks (MEX) where the attacker is allowed to either reveal the secret session state or the long-term secret of all communication partners. Moreover, our protocol features the strong and realistic notion of (full) perfect forward secrecy (PFS), that allows the attacker to actively modify...

Pseudorandom codes are error-correcting codes with the property that no efficient adversary can distinguish encodings from uniformly random strings. They were recently introduced by Christ and Gunn [CRYPTO 2024] for the purpose of watermarking the outputs of randomized algorithms, such as generative AI models. Several constructions of pseudorandom codes have since been proposed, but none of them are robust to error channels that depend on previously seen codewords. This stronger kind of...

Is it possible to comprehensively destroy a piece of quantum information, so that nothing is left behind except the memory of that one had it at some point? For example, various works, most recently Morimae, Poremba, and Yamakawa (TQC '24), show how to construct a signature scheme with certified deletion where a user who deletes a signature on $m$ cannot later produce a signature for $m$. However, in all of the existing schemes, even after deletion the user is still able keep irrefutable...

We propose a Polynomial Commitment Scheme (PCS), called BrakingBase, which allows a prover to commit to multilinear (or univariate) polynomials with $n$ coefficients in $O(n)$ time. The evaluation protocol of BrakingBase operates with an $O(n)$ time-complexity for the prover, while the verifier time-complexity and proof-complexity are $O(\lambda \log^2 n)$, where $λ$ is the security parameter. Notably, BrakingBase is field-agnostic, meaning it can be instantiated over any field of...

We put forth Oblivious State Preparation (OSP) as a cryptographic primitive that unifies techniques developed in the context of a quantum server interacting with a classical client. OSP allows a classical polynomial-time sender to input a choice of one out of two public observables, and a quantum polynomial-time receiver to recover an eigenstate of the corresponding observable -- while keeping the sender's choice hidden from any malicious receiver. We obtain the following results: - The...

Recent active studies have demonstrated that cryptography without one-way functions (OWFs) could be possible in the quantum world. Many fundamental primitives that are natural quantum analogs of OWFs or pseudorandom generators (PRGs) have been introduced, and their mutual relations and applications have been studied. Among them, pseudorandom function-like state generators (PRFSGs) [Ananth, Qian, and Yuen, Crypto 2022] are one of the most important primitives. PRFSGs are a natural quantum...