540 results sorted by ID
Shorter VOLE-in-the-Head-based Signatures from Vector Semi-Commitment
Seongkwang Kim, Byeonghak Lee, Mincheol Son
Public-key cryptography
The VOLE-in-the-Head (VOLEitH) paradigm transforms VOLE-based zero-knowledge proofs into post-quantum signature schemes by allowing public verification. We introduce reduced VOLE-in-the-Head (rVOLEitH), which incorporates the Vector Semi-Commitment (VSC) technique. VSC, originally developed for MPC-in-the-Head (MPCitH) schemes, reduces commitment size while maintaining security by relaxing the binding property. We adapt the ideal cipher version of VSC (IC-VSC) into the VOLEitH framework,...
Post-Quantum Security of Keyed Sponge-Based Constructions through a Modular Approach
Akinori Hosoyamada
Secret-key cryptography
Sponge-based constructions have successfully been receiving widespread adoption, as represented by the standardization of SHA-3 and Ascon by NIST. Yet, their provable security against quantum adversaries has not been investigated much. This paper studies the post-quantum security of some keyed sponge-based constructions in the quantum ideal permutation model, focusing on the Ascon AEAD mode and KMAC as concrete instances. For the Ascon AEAD mode, we prove the post-quantum security in the...
Incompressible Encryption with Everlasting Security
Eylon Yogev, Shany Ben-David
Foundations
Recently, the concept of incompressible encryption has emerged as a powerful enhancement to key-leakage resilience. In an incompressible encryption scheme, an adversary who intercepts ciphertexts is forced to dedicate a significant amount of memory to store them in full if they wish to extract any information about the plaintext later when the secret key becomes available. Given two messages, the security game involves two adversaries: the first adversary receives an encryption of one of the...
Quantum Security Analysis of the Key-Alternating Ciphers
Chen Bai, Mehdi Esmaili, Atul Mantri
Secret-key cryptography
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...
Blockcipher-Based Key Derivation without PRP/PRF Switching
Fabrice Benhamouda, Shai Halevi, Panos Kampanakis, Hugo Krawczyk
Secret-key cryptography
We examine the use of blockcipher-based key derivation beyond the birthday bound, arguing that the analysis step of PRP/PRF switching can be eliminated in many cases. To support this, we consider a modified ``ideal model'' for keying cryptographic applications in the multi-instance setting, where keys are chosen to be random \emph{but distinct}, rather than completely independent).
Our analysis shows that typical cryptographic applications remain secure in this model. One consequence is...
Blockcipher-Based Key Commitment for Nonce-Derived Schemes
Panos Kampanakis, Shai Halevi, Nevine Ebeid, Matt Campagna
Foundations
AES-GCM has been the status quo for efficient symmetric encryption for decades. As technology and cryptographic applications evolved over time, AES-GCM has posed some challenges to certain use-cases due to its default 96-bit nonce size, 128-bit block size, and lack of key commitment. Nonce-derived schemes are one way of addressing these challenges: Such schemes derive multiple keys from nonce values, then apply standard AES-GCM with the derived keys (and possibly another 96-bit nonce). The...
Quantum Lifting for Invertible Permutations and Ideal Ciphers
Alexandru Cojocaru, Minki Hhan, Qipeng Liu, Takashi Yamakawa, Aaram Yun
Foundations
In this work, we derive the first lifting theorems for establishing security in the quantum random permutation and ideal cipher models. These theorems relate the success probability of an arbitrary quantum adversary to that of a classical algorithm making only a small number of classical queries.
By applying these lifting theorems, we improve previous results and obtain new quantum query complexity bounds and post-quantum security results. Notably, we derive tight bounds for the quantum...
2025/728
Last updated: 2025-05-04
SNAIL: Verifiable Computation within 30% of Native Speed
Ole Hylland Spjeldnæs
Cryptographic protocols
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...
A Fiat-Shamir Transformation From Duplex Sponges
Alessandro Chiesa, Michele Orrù
Cryptographic protocols
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...
Tighter Concrete Security for the Simplest OT
Iftach Haitner, Gil Segev
Public-key cryptography
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...
Translating Between the Common Haar Random State Model and the Unitary Model
Eli Goldin, Mark Zhandry
Foundations
Black-box separations are a cornerstone of cryptography, indicating barriers to various goals. A recent line of work has explored black-box separations for quantum cryptographic primitives. Namely, a number of separations are known in the Common Haar Random State (CHRS) model, though this model is not considered a complete separation, but rather a starting point. A few very recent works have attempted to lift these separations to a unitary separation, which are considered complete...
On One-Shot Signatures, Quantum vs Classical Binding, and Obfuscating Permutations
Omri Shmueli, Mark Zhandry
Foundations
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...
CAKE requires programming - On the provable post-quantum security of (O)CAKE
Kathrin Hövelmanns, Andreas Hülsing, Mikhail Kudinov, Silvia Ritsch
In this work we revisit the post-quantum security of KEM-based password-authenticated key exchange (PAKE), specifically that of (O)CAKE. So far, these schemes evaded a security proof considering quantum adversaries. We give a detailed analysis of why this is the case, determining the missing proof techniques. To this end, we first provide a proof of security in the post-quantum setting, up to a single gap. This proof already turns out to be technically involved, requiring advanced techniques...
On the Security and Privacy of CKKS-based Homomorphic Evaluation Protocols
Intak Hwang, Seonhong Min, Jinyeong Seo, Yongsoo Song
Cryptographic protocols
CKKS is a homomorphic encryption (HE) scheme that supports arithmetic over complex numbers in an approximate manner.
Despite its utility in PPML protocols, formally defining the security of CKKS-based protocols is challenging due to its approximate nature.
To be precise, in a sender-receiver model, where the receiver holds input ciphertexts and the sender evaluates its private circuit, it is difficult to define sender's privacy in terms of indistinguishability, whereas receiver's privacy...
Random Number Generation from Pulsars
Hayder Tirmazi
Applications
Pulsars exhibit signals with precise inter-arrival times that are on the order of milliseconds to seconds, depending on the individual pulsar. There are subtle variations in the timing of pulsar signals. We show that these variations can serve as a natural entropy source for the creation of Random Number Generators (RNGs). We also explore the effects of using randomness extractors to increase the entropy of random bits extracted from Pulsar timing data. To evaluate the quality of the Pulsar...
Securely Instantiating 'Half Gates' Garbling in the Standard Model
Anasuya Acharya, Karen Azari, Mirza Ahad Baig, Dennis Hofheinz, Chethan Kamath
Foundations
Garbling is a fundamental cryptographic primitive, with numerous theoretical and practical applications. Since the first construction by Yao (FOCS’82, ’86), a line of work has concerned itself with reducing the communication and computational complexity of that construction. One of the most efficient garbling schemes presently is the ‘Half Gates’ scheme by Zahur, Rosulek, and Evans (Eurocrypt’15). Despite its widespread adoption, the provable security of this scheme has been based on...
UC-Security of Encrypted Key Exchange: A Tutorial
Jiayu Xu
Cryptographic protocols
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"...
NoIC: PAKE from KEM without Ideal Ciphers
Afonso Arriaga, Manuel Barbosa, Stanislaw Jarecki
Cryptographic protocols
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...
A Robust Variant of ChaCha20-Poly1305
Tim Beyne, Yu Long Chen, Michiel Verbauwhede
Secret-key cryptography
The ChaCha20-Poly1305 AEAD scheme is widely used as an alternative for AES-GCM on platforms without AES hardware instructions. Although recent analysis by Degabriele et al. shows that ChaCha20-Poly1305 provides adequate security in the conventional multiuser model, the construction is totally broken when a single nonce is repeated – a real-word scenario that can occur due to faulty implementations or the desire to use random nonces.
We present a new nonce-misuse resistant and...
Preprocessing Security in Multiple Idealized Models with Applications to Schnorr Signatures and PSEC-KEM
Jeremiah Blocki, Seunghoon Lee
Public-key cryptography
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...
Post-Quantum Threshold Ring Signature Applications from VOLE-in-the-Head
James Hsin-Yu Chiang, Ivan Damgård, William R. Duro, Sunniva Engan, Sebastian Kolby, Peter Scholl
Public-key cryptography
We propose efficient, post-quantum threshold ring signatures constructed from one-wayness of AES encryption and the VOLE-in-the-Head zero-knowledge proof system. Our scheme scales efficiently to large rings and extends the linkable ring signatures paradigm. We define and construct key-binding deterministic tags for signature linkability, that also enable succinct aggregation with approximate lower bound arguments of knowledge; this allows us to achieve succinct aggregation of our signatures...
BBB Secure Arbitrary Length Tweak TBC from n-bit Block Ciphers
Arghya Bhattacharjee, Ritam Bhaumik, Nilanjan Datta, Avijit Dutta, Shibam Ghosh, Sougata Mandal
Secret-key cryptography
At FSE'15, Mennink introduced the concept of designing beyond-the-birthday bound secure tweakable block cipher from an ideal block cipher. They proposed two tweakable block ciphers $\widetilde{F}[1]$ and $\widetilde{F}[2]$ that accepts $n$-bit tweak using a block cipher of $n$-bit key and $n$-bit data. Mennink proved that the constructions achieve security up to $2^{2n/3}$ and $2^n$ queries, respectively, assuming the underlying block cipher is ideal. Later, at ASIACRYPT'16, Wang et al....
NICE-PAKE: On the Security of KEM-Based PAKE Constructions without Ideal Ciphers
Nouri Alnahawi, Jacob Alperin-Sheriff, Daniel Apon, Gareth T. Davies, Alexander Wiesmaier
Cryptographic protocols
Password Authenticated Key Exchange (PAKE) is a fundamental
cryptographic component that allows two parties to establish a
shared key using only (potentially low-entropy) passwords. The interest
in realizing generic KEM-based PAKEs has increased significantly in the
last few years as part of the global migration effort to quantum-resistant
cryptography. One such PAKE is the CAKE protocol, proposed by Beguinet et al. (ACNS ’23). However, despite its simple design based on
the...
A Complete Characterization of One-More Assumptions In the Algebraic Group Model
Jake Januzelli, Jiayu Xu
Foundations
One-more problems like One-More Discrete Logarithm (OMDL) and One-More Diffie--Hellman (OMDH) have found wide use in cryptography, due to their ability to naturally model security definitions for interactive primitives like blind signatures and oblivious PRF. Furthermore, a generalization of OMDH called Threshold OMDH (TOMDH) has proven useful for building threshold versions of interactive protocols. However, due to their complexity it is often unclear how hard such problems actually are,...
Universally Composable Server-Supported Signatures for Smartphones
Nikita Snetkov, Jelizaveta Vakarjuk, Peeter Laud
Cryptographic 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...
SoK: On the Security Goals of Key Transparency Systems
Nicholas Brandt, Mia Filić, Sam A. Markelon
Applications
Key Transparency (KT) systems have emerged as a critical technology for adding verifiability to the distribution of public keys used in end-to-end encrypted messaging services. Despite substantial academic interest, increased industry adoption, and IETF standardization efforts, KT systems lack a holistic and formalized security model, limiting their resilience to practical threats and constraining future development.
In this paper, we survey the existing KT literature and present the...
Quantum One-Time Programs, Revisited
Aparna Gupte, Jiahui Liu, Justin Raizes, Bhaskar Roberts, Vinod Vaikuntanathan
Foundations
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...
Universally Composable and Reliable Password Hardening Services
Shaoqiang Wu, Ding Wang
Cryptographic protocols
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...
Towards Optimal Garbled Circuits in the Standard Model
Ruiyang Li, Chun Guo, Xiao Wang
Applications
State-of-the-art garbling schemes for boolean circuits roughly consist of two families, i.e., ideal model garbling that combines linear operations and ideal blockciphers (aiming at maximizing performance), and PRF-based garbling that insists on using theoretically sound assumptions. In the linear garbling framework introduced by Zahur, Rosulek, and Evans (Eurocrypt 2015), it was established that garbling an AND gate requires at least $2(\kappa +1)$ bits of ciphertext, with $\kappa$ as the...
Tweakable ForkCipher from Ideal Block Cipher
Sougata Mandal
Secret-key cryptography
In ASIACRYPT 2019, Andreeva et al. introduced a new symmetric key primitive called the $\textit{forkcipher}$, designed for lightweight applications handling short messages. A forkcipher is a keyed function with a public tweak, featuring fixed-length input and fixed-length (expanding) output. They also proposed a specific forkcipher, ForkSkinny, based on the tweakable block cipher SKINNY, and its security was evaluated through cryptanalysis. Since then, several efficient AEAD and MAC schemes...
Ideal Pseudorandom Codes
Omar Alrabiah, Prabhanjan Ananth, Miranda Christ, Yevgeniy Dodis, Sam Gunn
Foundations
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...
Cloning Games, Black Holes and Cryptography
Alexander Poremba, Seyoon Ragavan, Vinod Vaikuntanathan
Foundations
Quantum no-cloning is one of the most fundamental properties of quantum information. In this work, we introduce a new toolkit for analyzing cloning games; these games capture more quantitative versions of no-cloning and are central to unclonable cryptography. Previous works rely on the framework laid out by Tomamichel, Fehr, Kaniewski and Wehner to analyze both the $n$-qubit BB84 game and the subspace coset game. Their constructions and analysis face the following inherent limitations:
-...
Revisiting Leakage-Resilient MACs and Succinctly-Committing AEAD: More Applications of Pseudo-Random Injections
Mustafa Khairallah
Secret-key cryptography
Pseudo-Random Injections (PRIs) have been used in several applications in symmetric-key cryptography, such as in the idealization of Authenticated Encryption with Associated Data (AEAD) schemes, building robust AEAD, and, recently, in converting a committing AEAD scheme into a succinctly committing AEAD scheme. In Crypto 2024, Bellare and Hoang showed that if an AEAD scheme is already committing, it can be transformed into a succinctly committing scheme by encrypting part of the plaintext...
Pseudorandom Function-like States from Common Haar Unitary
Minki Hhan, Shogo Yamada
Foundations
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...
An Unstoppable Ideal Functionality for Signatures and a Modular Analysis of the Dolev-Strong Broadcast
Ran Cohen, Jack Doerner, Eysa Lee, Anna Lysyanskaya, Lawrence Roy
Cryptographic protocols
Many foundational results in the literature of consensus follow the Dolev-Yao model (FOCS '81), which treats digital signatures as ideal objects with perfect correctness and unforgeability. However, no work has yet formalized an ideal signature scheme that is both suitable for this methodology and possible to instantiate, or a composition theorem that ensures security when instantiating it cryptographically.
The Universal Composition (UC) framework would ensure composition if we could...
Fine-Grained Non-Interactive Key-Exchange without Idealized Assumptions, and Lower Bounds
Yuyu Wang, Chuanjie Su, Jiaxin Pan, Chunxiang Xu
Public-key cryptography
In this paper, we study multi-party non-interactive key exchange (NIKE) in the fine-grained setting. More precisely, we propose three multi-party NIKE schemes in three computation models, namely, the bounded parallel-time, bounded time, and bounded storage models. Their security is based on a very mild assumption (e.g., NC1 ⊊ ⊕L/poly) or even without any complexity assumption. This improves the recent work of Afshar, Couteau, Mahmoody, and Sadeghi (EUROCRYPT 2023) that requires idealized...
Offline-Online Indifferentiability of Cryptographic Systems
Ashrujit Ghoshal, Ilan Komargodski, Gil Segev
Foundations
The indifferentiability framework has become a standard methodology that enables us to study the security of cryptographic constructions in idealized models of computation. Unfortunately, while indifferentiability provides strong guarantees whenever the security of a construction is captured by a ``single-stage'' security game, it may generally provide no meaningful guarantees when the security is captured by a ``multi-stage'' one. In particular, the indifferentiability framework does not...
Pseudorandom Obfuscation and Applications
Pedro Branco, Nico Döttling, Abhishek Jain, Giulio Malavolta, Surya Mathialagan, Spencer Peters, Vinod Vaikuntanathan
Foundations
We introduce the notion of pseudorandom obfuscation, a way to obfuscate (keyed) pseudorandom functions $f_K$ in an average-case sense. We study several variants of pseudorandom obfuscation and show a number of applications.
1. Applications in the iO World: Our weakest variant of pseudorandom obfuscation, named obfuscation for identical pseudorandom functions (iPRO), is weaker than indistinguishability obfuscation (iO): rather than obfuscating arbitrary circuits as in iO, iPRO only...
The Mysteries of LRA: Roots and Progresses in Side-channel Applications
Jiangshan Long, Changhai Ou, Zhu Wang, Fan Zhang
Attacks and cryptanalysis
Evaluation of cryptographic implementations with respect to side-channels has been mandated at high security levels nowadays. Typically, the evaluation involves four stages: detection, modeling, certification and secret recovery. In pursuit of specific goal at each stage, inherently different techniques used to be considered necessary. However, since the recent works of Eurocrypt2022 and Eurocrypt2024, linear regression analysis (LRA) has uniquely become the technique that is well-applied...
A Tight Lower Bound on the TdScrypt Trapdoor Memory-Hard Function
Jeremiah Blocki, Seunghoon Lee
Public-key cryptography
A trapdoor Memory-Hard Function is a function that is memory-hard to evaluate for any party who does not have a trapdoor, but is substantially less expensive to evaluate with the trapdoor. Biryukov and Perin (ASIACRYPT 2017) introduced the first candidate trapdoor Memory-Hard Function called Diodon which modifies a Memory-Hard Function called Scrypt by replacing a hash chain with repeated squaring modulo a composite number $N=pq$. The trapdoor, which consists of the prime factors $p$ and...
Block Ciphers in Idealized Models: Automated Proofs and New Security Results
Miguel Ambrona, Pooya Farshim, Patrick Harasser
Implementation
We develop and implement AlgoROM, a tool to systematically analyze the security of a wide class of symmetric primitives in idealized models of computation. The schemes that we consider are those that can be expressed over an alphabet consisting of XOR and function symbols for hash functions, permutations, or block ciphers.
We implement our framework in OCaml and apply it to a number of prominent constructions, which include the Luby–Rackoff (LR), key-alternating Feistel (KAF), and...
Efficient Pairing-Free Adaptable k-out-of-N Oblivious Transfer Protocols
Keykhosro Khosravani, Taraneh Eghlidos, Mohammad reza Aref
Cryptographic protocols
Oblivious Transfer (OT) is one of the fundamental building blocks in cryptography that enables various privacy-preserving applications. Constructing efficient OT schemes has been an active research area. This paper presents three efficient two-round pairing-free k-out-of-N oblivious transfer protocols with standard security. Our constructions follow the minimal communication pattern: the receiver sends k messages to the sender, who responds with n+k messages, achieving the lowest data...
Tightly Secure Threshold Signatures over Pairing-Free Groups
Renas Bacho, Benedikt Wagner
Cryptographic protocols
Threshold signatures have been drawing lots of attention in recent years. Of particular interest are threshold signatures that are proven secure under adaptive corruptions (NIST Call 2023). Sadly, existing constructions with provable adaptive security suffer from at least one of the following drawbacks: (i) strong idealizations such as the algebraic group model (AGM), (ii) an unnatural restriction on the corruption threshold being $t/2$ where $t$ is the signing threshold, or (iii)...
Universally Composable SNARKs with Transparent Setup without Programmable Random Oracle
Christian Badertscher, Matteo Campanelli, Michele Ciampi, Luigi Russo, Luisa Siniscalchi
Cryptographic protocols
Non-interactive zero-knowledge (NIZK) proofs enable a prover to convince a verifier of an NP statement’s validity using a single message, without disclosing any additional information. These proofs are widely studied and deployed, especially in their succinct form, where proof length is sublinear in the size of the NP relation. However, efficient succinct NIZKs typically require an idealized setup, such as a a common reference string, which complicates real-world deployment. A key challenge...
Challenges in Timed Cryptography: A Position Paper
Karim Eldefrawy, Benjamin Terner, Moti Yung
Foundations
Time-lock puzzles are unique cryptographic primitives that use computational complexity to keep information secret for some period of time, after which security expires. This topic, while over 25 years old, is still in a state where foundations are not well understood: For example, current analysis techniques of time-lock primitives provide no sound mechanism to build composed multi-party cryptographic protocols which use expiring security as a building block. Further, there are analyses...
Schnorr Signatures are Tightly Secure in the ROM under a Non-interactive Assumption
Gavin Cho, Georg Fuchsbauer, Adam O'Neill, Marek Sefranek
Public-key cryptography
We show that the widely-used Schnorr signature scheme meets existential unforgeability under chosen-message attack (EUF-CMA) in the random oracle model (ROM) if the circular discrete-logarithm (CDL) assumption holds in the underlying group. CDL is a new, non-interactive and falsifiable variant of the discrete-logarithm (DL) assumption that we introduce. Our reduction is completely tight, meaning the constructed adversary against CDL has essentially the same running time and success...
On the Complexity of Cryptographic Groups and Generic Group Models
Cong Zhang, Keyu Ji, Taiyu Wang, Bingsheng Zhang, Hong-Sheng Zhou, Xin Wang, Kui Ren
Foundations
Ever since the seminal work of Diffie and Hellman, cryptographic (cyclic) groups have served as a fundamental building block for constructing cryptographic schemes and protocols. The security of these constructions can often be based on the hardness of (cyclic) group-based computational assumptions. Then, the generic group model (GGM) has been studied as an idealized model (Shoup, EuroCrypt 1997), which justifies the hardness of many (cyclic) group-based assumptions and shows the limits of...
On the Relationship between Public Key Primitives via Indifferentiability
Shuang Hu, Bingsheng Zhang, Cong Zhang, Kui Ren
Foundations
Recently, Masny and Rindal [MR19] formalized a notion called Endemic Oblivious Transfer (EOT), and they proposed a generic transformation from Non-Interactive Key Exchange (NIKE) to EOT with standalone security in the random oracle (RO) model. However, from the model level, the relationship between idealized NIKE and idealized EOT and the relationship between idealized elementary public key primitives have been rarely researched.
In this work, we investigate the relationship between ideal...
AGATE: Augmented Global Attested Trusted Execution in the Universal Composability framework
Lorenzo Martinico, Markulf Kohlweiss
Foundations
A Trusted Execution Environment (TEE) is a security technology,
implemented by CPU manufacturers, which guarantees integrity and confidentiality
on a restricted execution environment to any remote verifier through attestation. TEEs are deployed
on various consumer and commercial hardware platforms, and have been widely adopted as a component in the design of cryptographic protocols both theoretical and practical.
Within the provable security community, the use of TEEs as a setup...
Efficient online and Non-Interactive Threshold Signatures with Identifiable Aborts for Identity-Based Signatures in the IEEE P1363 Standard
Yan Jiang, Youwen Zhu, Jian Wang, Yudi Zhang
Cryptographic protocols
Identity-based threshold signature (IDTS) enables the generation of valid signatures without revealing cryptographic keys in the signing process. While current protocols have achieved much progress in their efficiency, many schemes easily suffer from denial-of-service attacks in which misbehaving parties could keep from generating signatures without being caught. The identifiable abort property is designed to withstand such an attack in some recent IDTS protocols. However, all these schemes...
Protecting Cryptographic Code Against Spectre-RSB
Santiago Arranz Olmos, Gilles Barthe, Chitchanok Chuengsatiansup, Benjamin Grégoire, Vincent Laporte, Tiago Oliveira, Peter Schwabe, Yuval Yarom, Zhiyuan Zhang
Implementation
It is fundamental that executing cryptographic software must not leak secrets through side-channels. For software-visible side-channels, it was long believed that "constant-time" programming would be sufficient as a systematic countermeasure. However, this belief was shattered in 2018 by attacks exploiting speculative execution—so called Spectre attacks.
Recent work shows that language support suffices to protect cryptographic code with minimal overhead against one class of such attacks,...
Stochastic Secret Sharing with $1$-Bit Shares and Applications to MPC
Benny Applebaum, Eliran Kachlon
Foundations
The problem of minimizing the share size of threshold secret-sharing schemes is a basic research question that has been extensively studied. Ideally, one strives for schemes in which the share size equals the secret size. While this is achievable for large secrets (Shamir, CACM '79), no similar solutions are known for the case of binary, single-bit secrets. Current approaches often rely on so-called ramp secret sharing that achieves a constant share size at the expense of a slight gap...
On Sequential Functions and Fine-Grained Cryptography
Jiaxin Guan, Hart Montgomery
Foundations
A sequential function is, informally speaking, a function $f$ for which a massively parallel adversary cannot compute "substantially" faster than an honest user with limited parallel computation power. Sequential functions form the backbone of many primitives that are extensively used in blockchains such as verifiable delay functions (VDFs) and time-lock puzzles. Despite this widespread practical use, there has been little work studying the complexity or theory of sequential...
Adaptively Secure 5 Round Threshold Signatures from MLWE/MSIS and DL with Rewinding
Shuichi Katsumata, Michael Reichle, Kaoru Takemure
Cryptographic protocols
T-out-of-N threshold signatures have recently seen a renewed interest, with various types now available, each offering different tradeoffs.
However, one property that has remained elusive is adaptive security. When we target thresholdizing existing efficient signatures schemes based on the Fiat-Shamir paradigm such as Schnorr, the elusive nature becomes clear. This class of signature schemes typically rely on the forking lemma to prove unforgeability. That is, an adversary is rewound and...
Improved Reductions from Noisy to Bounded and Probing Leakages via Hockey-Stick Divergences
Maciej Obremski, João Ribeiro, Lawrence Roy, François-Xavier Standaert, Daniele Venturi
Attacks and cryptanalysis
There exists a mismatch between the theory and practice of cryptography in the presence of leakage. On the theoretical front, the bounded leakage model, where the adversary learns bounded-length but noiseless information about secret components, and the random probing model, where the adversary learns some internal values of a leaking implementation with some probability, are convenient abstractions to analyze the security of numerous designs. On the practical front, side-channel attacks...
Relaxed Vector Commitment for Shorter Signatures
Seongkwang Kim, Byeonghak Lee, Mincheol Son
Public-key cryptography
MPC-in-the-Head (MPCitH) has recently gained traction as a foundation for post-quantum signature schemes, offering robust security without trapdoors. Despite its strong security profile, MPCitH-based schemes suffer from high computational overhead and large signature sizes, limiting their practical application.
This work addresses these inefficiencies by relaxing vector commitments within MPCitH-based schemes. We introduce the concept of vector semi-commitment, which relaxes the binding...
SoK: Programmable Privacy in Distributed Systems
Daniel Benarroch, Bryan Gillespie, Ying Tong Lai, Andrew Miller
Applications
This Systematization of Knowledge conducts a survey of contemporary distributed blockchain protocols, with the aim of identifying cryptographic and design techniques which practically enable both expressive programmability and user data confidentiality. To facilitate a framing which supports the comparison of concretely very different protocols, we define an epoch-based computational model in the form of a flexible UC-style ideal functionality which divides the operation of...
VRaaS: Verifiable Randomness as a Service on Blockchains
Jacob Gorman, Lucjan Hanzlik, Aniket Kate, Easwar Vivek Mangipudi, Pratyay Mukherjee, Pratik Sarkar, Sri AravindaKrishnan Thyagarajan
Foundations
Web3 applications, such as on-chain games, NFT minting, and leader elections necessitate access to unbiased, unpredictable, and publicly verifiable randomness. Despite its broad use cases and huge demand, there is a notable absence of comprehensive treatments of on-chain verifiable randomness services. To bridge this, we offer an extensive formal analysis of on-chain verifiable randomness services.
We present the $first$ formalization of on-chain verifiable randomness in the...
An Explicit High-Moment Forking Lemma and its Applications to the Concrete Security of Multi-Signatures
Gil Segev, Liat Shapira
Foundations
In this work we first present an explicit forking lemma that distills the information-theoretic essence of the high-moment technique introduced by Rotem and Segev (CRYPTO '21), who analyzed the security of identification protocols and Fiat-Shamir signature schemes. Whereas the technique of Rotem and Segev was particularly geared towards two specific cryptographic primitives, we present a stand-alone probabilistic lower bound, which does not involve any underlying primitive or idealized...
Fine-Grained Non-Interactive Key Exchange, Revisited
Balthazar Bauer, Geoffroy Couteau, Elahe Sadeghi
Public-key cryptography
We revisit the construction of multiparty non-interactive key-exchange protocols with fine-grained security, which was recently studied in (Afshar et al., Eurocrypt 2023). Their work introduced a 4-party non-interactive key exchange with quadratic hardness, and proved it secure in Shoup's generic group model. This positive result was complemented with a proof that $n$-party non-interactive key exchange with superquadratic security cannot exist in Maurer's generic group model, for any $n\geq...
Let Attackers Program Ideal Models: Modularity and Composability for Adaptive Compromise
Joseph Jaeger
Foundations
We show that the adaptive compromise security definitions of Jaeger and Tyagi (Crypto '20) cannot be applied in several natural use-cases. These include proving multi-user security from single-user security, the security of the cascade PRF, and the security of schemes sharing the same ideal primitive. We provide new variants of the definitions and show that they resolve these issues with composition. Extending these definitions to the asymmetric settings, we establish the security of the...
Multi User Security of LightMAC and LightMAC_Plus
Nilanjan Datta, Shreya Dey, Avijit Dutta, Devdutto Kanungo
Secret-key cryptography
In FSE'16, Luykx et al. have proposed $\textsf{LightMAC}$ that provably achieves a query length independent PRF security bound. To be precise, the construction achieves security roughly in the order of $O(q^2/2^n)$, when instantiated with two independently keyed $n$-bit block ciphers and $q$ is the total number of queries made by the adversary. Subsequently, in ASIACRYPT'17, Naito proposed a beyond-birthday-bound variant of the $\textsf{LightMAC}$ construction, dubbed as...
Linicrypt in the Ideal Cipher Model
Zahra Javar, Bruce M. Kapron
Foundations
We extend the Linicrypt framework for characterizing hash function security as proposed by McQuoid, Swope, and Rosulek (TCC 2018) to support constructions in the ideal cipher model.
In this setting, we give a characterization of collision- and second-preimage-resistance in terms of a linear-algebraic condition on Linicrypt programs, and present an efficient algorithm for determining whether a program satisfies the condition. As an application, we consider the case of the block cipherbased...
2024/693
Last updated: 2025-04-01
A Note on Gröbner Bases for Anemoi
Pierre Briaud
Attacks and cryptanalysis
This paper focuses on algebraic attacks on the $\mathsf{Anemoi}$ family of arithmetization-oriented permutations [Crypto '23]. We consider a slight variation of the naive modeling of the $\mathsf{CICO}$ problem associated to the primitive, for which we can very easily obtain a Gröbner basis and prove the degree of the associated ideal. For inputs in $\mathbb{F}_{q}^2$ when $q$ is an odd prime, we recover the same degree as conjectured for alternative polynomial systems used in other recent...
Fast Parallelizable Misuse-Resistant Authenticated Encryption: Low Latency (Decryption-Fast) SIV
Mustafa Khairallah
Secret-key cryptography
MRAE security is an important goal for many AEAD applications where the nonce uniqueness cannot be maintained and security risks are significant. However, MRAE schemes can be quite expensive. Two of the SoTA MRAE-secure schemes; Deoxys-II and AES-GCM-SIV rely on internal parallelism and special instructions to achieve competitive performance. However, they both suffer from the same bottleneck, they have at least one call to the underlying primitive that cannot be parallelized to any other...
CryptoVampire: Automated Reasoning for the Complete Symbolic Attacker Cryptographic Model
Simon Jeanteur, Laura Kovács, Matteo Maffei, Michael Rawson
Cryptographic protocols
Cryptographic protocols are hard to design and prove correct, as witnessed by the ever-growing list of attacks even on protocol standards. Symbolic models of cryptography enable automated formal security proofs of such protocols against an idealized cryptographic model, which abstracts away from the algebraic properties of cryptographic schemes and thus misses attacks. Computational models of cryptography yield rigorous guarantees but support at present only interactive proofs and/or...
CCA Secure Updatable Encryption from Non-Mappable Group Actions
Jonas Meers, Doreen Riepel
Cryptographic protocols
Ciphertext-independent updatable encryption (UE) allows to rotate encryption keys and update ciphertexts via a token without the need to first download the ciphertexts. Although, syntactically, UE is a symmetric-key primitive, ciphertext-independent UE with forward secrecy and post-compromise security is known to imply public-key encryption (Alamati, Montgomery and Patranabis, CRYPTO 2019).
Constructing post-quantum secure UE turns out to be a difficult task. While lattices offer the...
Two-Round Threshold Signature from Algebraic One-More Learning with Errors
Thomas Espitau, Shuichi Katsumata, Kaoru Takemure
Cryptographic protocols
Threshold signatures have recently seen a renewed interest due to applications in cryptocurrency while NIST has released a call for multi-party threshold schemes, with a deadline for submission expected for the first half of 2025. So far, all lattice-based threshold signatures requiring less than two-rounds are based on heavy tools such as (fully) homomorphic encryption (FHE) and homomorphic trapdoor commitments (HTDC). This is not unexpected considering that most efficient two-round...
HW-token-based Common Random String Setup
István Vajda
Applications
In the common random string model, the parties executing a protocol have access to a uniformly random bit string. It is known that under standard intractability assumptions, we can realize any ideal functionality with universally composable (UC) security if a trusted common random string (CrS) setup is available. It was always a question of where this CrS should come from since the parties provably could not compute it themselves. Trust assumptions are required, so minimizing the level of...
Threshold Structure-Preserving Signatures: Strong and Adaptive Security under Standard Assumptions
Aikaterini Mitrokotsa, Sayantan Mukherjee, Mahdi Sedaghat, Daniel Slamanig, Jenit Tomy
Public-key cryptography
Structure-preserving signatures (SPS) have emerged as an important cryptographic building block, as their compatibility with the Groth-Sahai (GS) NIZK framework allows to construct protocols under standard assumptions with reasonable efficiency.
Over the last years there has been a significant interest in the design of threshold signature schemes. However, only very recently Crites et al. (ASIACRYPT 2023) have introduced threshold SPS (TSPS) along with a fully non-interactive construction....
C'est très CHIC: A compact password-authenticated key exchange from lattice-based KEM
Afonso Arriaga, Manuel Barbosa, Stanislaw Jarecki, Marjan Skrobot
Cryptographic protocols
Driven by the NIST's post-quantum standardization efforts and the selection of Kyber as a lattice-based Key-Encapsulation Mechanism (KEM), several Password Authenticated Key Exchange (PAKE) protocols have been recently proposed that leverage a KEM to create an efficient, easy-to-implement and secure PAKE. In two recent works, Beguinet et al. (ACNS 2023) and Pan and Zeng (ASIACRYPT 2023) proposed generic compilers that transform KEM into PAKE, relying on an Ideal Cipher (IC) defined over a...
A New Approach to Generic Lower Bounds: Classical/Quantum MDL, Quantum Factoring, and More
Minki Hhan
Foundations
This paper studies the limitations of the generic approaches to solving cryptographic problems in classical and quantum settings in various models.
- In the classical generic group model (GGM), we find simple alternative proofs for the lower bounds of variants of the discrete logarithm (DL) problem: the multiple-instance DL and one-more DL problems (and their mixture). We also re-prove the unknown-order GGM lower bounds, such as the order finding, root extraction, and repeated squaring.
-...
Faster Signatures from MPC-in-the-Head
Dung Bui, Eliana Carozza, Geoffroy Couteau, Dahmun Goudarzi, Antoine Joux
Cryptographic protocols
We revisit the construction of signature schemes using the MPC-in-the-head paradigm. We obtain two main contributions:
– We observe that previous signatures in the MPC-in-the-head paradigm must rely on a salted version of the GGM puncturable pseudorandom function (PPRF) to avoid collision attacks. We design a new efficient PPRF construction that is provably secure in the multi-instance setting. The security analysis of our PPRF, in the ideal cipher model, is quite involved and forms a...
Robust Additive Randomized Encodings from IO and Pseudo-Non-linear Codes
Nir Bitansky, Sapir Freizeit
Cryptographic protocols
Additive randomized encodings (ARE), introduced by Halevi, Ishai, Kushilevitz, and Rabin (CRYPTO 2023), reduce the computation of a k-party function $f (x_1, . . . , x_k )$ to locally computing encodings $\hat{x}_i$ of each input xi and then adding them together over some Abelian group into an output encoding $\hat{y} = ∑ \hat{x}_i$, which reveals nothing but the result. In robust ARE (RARE) the sum of any subset of $\hat{x}_i$, reveals only the residual function obtained by restricting the...
Direct FSS Constructions for Branching Programs and More from PRGs with Encoded-Output Homomorphism
Elette Boyle, Lisa Kohl, Zhe Li, Peter Scholl
Cryptographic protocols
Function secret sharing (FSS) for a class $\cal{F}$ allows to split a secret function $f \in \cal{F}$ into (succinct) secret shares $f_0,f_1$, such that for all $x\in \{0,1\}^n$ it holds $f_0(x)-f_1(x)=f(x)$. FSS has numerous applications, including private database queries, nearest neighbour search, private heavy hitters and secure computation in the preprocessing model, where the supported class $\cal{F}$ translates to richness in the application. Unfortunately, concretely efficient FSS...
Constant-Size zk-SNARKs in ROM from Falsifiable Assumptions
Helger Lipmaa, Roberto Parisella, Janno Siim
Cryptographic protocols
We prove that the seminal KZG polynomial commitment scheme (PCS) is black-box extractable under a simple falsifiable assumption ARSDH. To create an interactive argument, we construct a compiler that combines a black-box extractable non-interactive PCS and a polynomial IOP (PIOP). The compiler incurs a minor cost per every committed polynomial. Applying the Fiat-Shamir transformation, we obtain slightly less efficient variants of well-known PIOP-based zk-SNARKs, such as Plonk, that are...
Zero-Knowledge Proofs of Training for Deep Neural Networks
Kasra Abbaszadeh, Christodoulos Pappas, Jonathan Katz, Dimitrios Papadopoulos
Cryptographic protocols
A zero-knowledge proof of training (zkPoT) enables a party to prove that they have correctly trained a committed model based on a committed dataset without revealing any additional information about the model or the dataset. An ideal zkPoT should offer provable security and privacy guarantees, succinct proof size and verifier runtime, and practical prover efficiency. In this work, we present \name, a zkPoT targeted for deep neural networks (DNNs) that achieves all these goals at once. Our...
Layout Graphs, Random Walks and the t-wise Independence of SPN Block Ciphers
Tianren Liu, Angelos Pelecanos, Stefano Tessaro, Vinod Vaikuntanathan
Secret-key cryptography
We continue the study of $t$-wise independence of substitution-permutation networks (SPNs) initiated by the recent work of Liu, Tessaro, and Vaikuntanathan (CRYPTO 2021).
Our key technical result shows that when the S-boxes are randomly and independently chosen and kept secret, an $r$-round SPN with input length $n = b \cdot k$ is $2^{-\Theta(n)}$-close to $t$-wise independent within $r = O(\min\{k, \log t\})$ rounds for any $t$ almost as large as $2^{b/2}$. Here, $b$ is the input length of...
On Efficient and Secure Compression Modes for Arithmetization-Oriented Hashing
Elena Andreeva, Rishiraj Bhattacharyya, Arnab Roy, Stefano Trevisani
Secret-key cryptography
ZK-SNARKs, a fundamental component of privacy-oriented payment systems, identity protocols, or anonymous voting systems, are advanced cryptographic protocols for verifiable computation: modern SNARKs allow to encode the invariants of a program, expressed as an arithmetic circuit, in an appropriate constraint language from which short, zero-knowledge proofs for correct computations can be constructed.
One of the most important computations that is run through SNARK systems is the...
Towards Compact Identity-based Encryption on Ideal Lattices
Huiwen Jia, Yupu Hu, Chunming Tang, Lin Wang
Public-key cryptography
Basic encryption and signature on lattices have comparable efficiency to their classical counterparts in terms of speed and key size. However, Identity-based Encryption (IBE) on lattices is much less efficient in terms of compactness, even when instantiated on ideal lattices and in the Random Oracle Model (ROM). This is because the underlying preimage sampling algorithm used to extract the users' secret keys requires huge public parameters. In this work, we specify a compact IBE...
Lattice-based Programmable Hash Functions and Applications
Jiang Zhang, Yu Chen, Zhenfeng Zhang
Public-key cryptography
Driven by the open problem raised by Hofheinz and Kiltz
(Journal of Cryptology, 2012), we study the formalization of lattice-based programmable hash function (PHF), and give three types of concrete constructions by using several techniques such as a novel combination of cover-free sets and lattice trapdoors. Under the Inhomogeneous Small Integer Solution (ISIS) assumption, we show that any (non-trivial) lattice-based PHF is a collision-resistant hash function, which gives a direct...
Unclonable Cryptography with Unbounded Collusions and Impossibility of Hyperefficient Shadow Tomography
Alper Çakan, Vipul Goyal
Foundations
Quantum no-cloning theorem gives rise to the intriguing possibility of quantum copy protection where we encode a program or functionality in a quantum state such that a user in possession of k copies cannot create k+1 copies, for any k. Introduced by Aaronson (CCC'09) over a decade ago, copy protection has proven to be notoriously hard to achieve. Previous work has been able to achieve copy-protection for various functionalities only in restricted models: (i) in the bounded collusion...
CASE: A New Frontier in Public-Key Authenticated Encryption
Shashank Agrawal, Shweta Agrawal, Manoj Prabhakaran, Rajeev Raghunath, Jayesh Singla
Public-key cryptography
We introduce a new cryptographic primitive, called Completely Anonymous Signed Encryption (CASE). CASE is a public-key authenticated encryption primitive, that offers anonymity for senders as well as receivers. A "case-packet" should appear, without a (decryption) key for opening it, to be a blackbox that reveals no information at all about its contents. To decase a case-packet fully - so that the message is retrieved and authenticated - a verifcation key is also required.
Defining security...
Adaptively Secure Consensus with Linear Complexity and Constant Round under Honest Majority in the Bare PKI Model, and Separation Bounds from the Idealized Message-Authentication Model
Matthieu Rambaud
Foundations
We consider the mainstream model in secure computation known as the bare PKI setup, also as the {bulletin-board PKI}. It allows players to broadcast once and non-interactively before they receive their inputs and start the execution. A bulletin-board PKI is essentially the minimum setup known so far to implement the model known as {messages-authentication}, i.e., when $P$ is forwarded a signed message, it considers it to be issued by $R$ if and only if $R$ signed it. It is known since...
On Overidealizing Ideal Worlds: Xor of Two Permutations and its Applications
Wonseok Choi, Minki Hhan, Yu Wei, Vassilis Zikas
Secret-key cryptography
Security proofs of symmetric-key primitives typically consider an idealized world with access to a (uniformly) random function. The starting point of our work is the observation that such an ideal world can lead to underestimating the actual security of certain primitives. As a demonstrating example, $\mathsf{XoP2}$, which relies on two independent random permutations, has been proven to exhibit superior concrete security compared to $\mathsf{XoP}$, which employs a single permutation with...
Security Bounds for Proof-Carrying Data from Straightline Extractors
Alessandro Chiesa, Ziyi Guan, Shahar Samocha, Eylon Yogev
Foundations
Proof-carrying data (PCD) is a powerful cryptographic primitive that allows mutually distrustful parties to perform distributed computation in an efficiently verifiable manner. Real-world deployments of PCD have sparked keen interest within the applied community and industry.
Known constructions of PCD are obtained by recursively-composing SNARKs or related primitives. Unfortunately, known security analyses incur expensive blowups, which practitioners have disregarded as the analyses...
Unmodified Half-Gates is Adaptively Secure - So is Unmodified Three-Halves
Xiaojie Guo, Kang Yang, Xiao Wang, Yu Yu, Zheli Liu
Cryptographic protocols
Adaptive security is a crucial property for garbling schemes in pushing the communication of garbled circuits to an offline phase when the input is unknown. In this paper, we show that the popular half-gates scheme by Zahur et al. (Eurocrypt'15), without any modification, is adaptively secure in the non-programmable random permutation model (npRPM). Since real implementations of selective-secure half-gates are already based on npRPM, our result shows that these implementations are already...
Making an Asymmetric PAKE Quantum-Annoying by Hiding Group Elements
Marcel Tiepelt, Edward Eaton, Douglas Stebila
Cryptographic protocols
The KHAPE-HMQV protocol is a state-of-the-art highly efficient asymmetric password-authenticated key exchange protocol that provides several desirable security properties, but has the drawback of being vulnerable to quantum adversaries due to its reliance on discrete logarithm-based building blocks: solving a single discrete logarithm allows the attacker to perform an offline dictionary attack and recover the password. We show how to modify KHAPE-HMQV to make the protocol quantum-annoying: a...
Efficient Agreement Over Byzantine Gossip
Ran Cohen, Julian Loss, Tal Moran
Cryptographic protocols
Byzantine agreement (BA) asks for a set of parties to reach agreement in an adversarial setting. A central question is how to construct efficient BA protocols that scale well with the number of parties. In particular, the communication complexity is a critical barrier for large-scale implementations.
State-of-the-art, scalable BA protocols typically work by sampling a small, unpredictable committee of parties that will send messages in each round. These messages must reach all honest...
Towards post-quantum secure PAKE - A tight security proof for OCAKE in the BPR model
Nouri Alnahawi, Kathrin Hövelmanns, Andreas Hülsing, Silvia Ritsch, Alexander Wiesmaier
Cryptographic protocols
We revisit OCAKE (ACNS 23), a generic recipe that constructs password-based authenticated key exchange (PAKE) from key encapsulation mechanisms (KEMs), to allow instantiations with post-quantums KEM like KYBER.
The ACNS23 paper left as an open problem to argue security against quantum attackers, with its security proof being in the universal composability (UC) framework. This is common for PAKE, however, at the time of this submission’s writing, it was not known how to prove (computational)...
On the Security of KZG Commitment for VSS
Atsuki Momose, Sourav Das, Ling Ren
Cryptographic protocols
The constant-sized polynomial commitment scheme by Kate, Zaverucha, and Goldberg (Asiscrypt 2010), also known as the KZG commitment, is an essential component in designing bandwidth-efficient verifiable secret-sharing (VSS) protocols. We point out, however, that the KZG commitment is missing two important properties that are crucial for VSS protocols.
First, the KZG commitment has not been proven to be degree binding in the standard adversary model without idealized group assumptions. In...
A Generic Construction of Tightly Secure Password-based Authenticated Key Exchange
Jiaxin Pan, Runzhi Zeng
Public-key cryptography
We propose a generic construction of password-based authenticated key exchange (PAKE) from key encapsulation mechanisms (KEM). Assuming that the KEM is oneway secure against plaintext-checkable attacks (OW-PCA), we prove that our PAKE protocol is \textit{tightly secure} in the Bellare-Pointcheval-Rogaway model (EUROCRYPT 2000). Our tight security proofs require ideal ciphers and random oracles. The OW-PCA security is relatively weak and can be implemented tightly with the Diffie-Hellman...
Verifiable random function from the Deuring correspondence and higher dimensional isogenies
Antonin Leroux
Cryptographic protocols
In this paper, we introduce $\mathsf{DeuringVUF}$, a new Verifiable Unpredictable Function (VUF) protocol based on isogenies between supersingular curves.
The most interesting application of this VUF is $\mathsf{DeuringVRF}$ a post-quantum Verifiable Random Function (VRF). The main advantage of this new scheme is its compactness, with combined public key and proof size of roughly 450 bytes, which is orders of magnitude smaller than other generic purpose post-quantum VRF...
A Methodology to Achieve Provable Side-Channel Security in Real-World Implementations
Sonia Belaïd, Gaëtan Cassiers, Camille Mutschler, Matthieu Rivain, Thomas Roche, François-Xavier Standaert, Abdul Rahman Taleb
A wide range of countermeasures have been proposed to defend against
side-channel attacks, with masking being one of the most effective and commonly
used techniques. While theoretical models provide formal security proofs, these often
rely on assumptions—sometimes implicit—that can be difficult to assess in practice.
As a result, the design of secure masked implementations frequently combines proven
theoretical arguments with heuristic and empirical validation. Despite the...
Streebog as a Random Oracle
Liliya Akhmetzyanova, Alexandra Babueva, Andrey Bozhko
Secret-key cryptography
The random oracle model is an instrument used for proving that protocol has no structural flaws when settling with standard hash properties is impossible or fairly difficult. In practice, however, random oracles have to be instantiated with some specific hash functions, which are not random oracles. Hence, in the real world, an adversary has broader capabilities than considered in the random oracle proof — it can exploit the peculiarities of a specific hash function to achieve its goal. In a...
How to Compile Polynomial IOP into Simulation-Extractable SNARKs: A Modular Approach
Markulf Kohlweiss, Mahak Pancholi, Akira Takahashi
Foundations
Most succinct arguments (SNARKs) are initially only proven knowledge sound (KS).
We show that the commonly employed compilation strategy from polynomial interactive oracle proofs (PIOP) via polynomial commitments to knowledge sound SNARKS actually also achieves other desirable properties: weak unique response (WUR) and trapdoorless zero-knowledge (TLZK); and that together they imply simulation extractability (SIM-EXT).
The factoring of SIM-EXT into KS + WUR + TLZK is becoming a...
Limits in the Provable Security of ECDSA Signatures
Dominik Hartmann, Eike Kiltz
Foundations
Digital Signatures are ubiquitous in modern computing. One of the most widely used digital signature schemes is ECDSA due to its use in TLS, various Blockchains such as Bitcoin and Etherum, and many other applications. Yet the formal analysis of ECDSA is comparatively sparse. In particular, all known security results for ECDSA rely on some idealized model such as the generic group model or the programmable (bijective) random oracle model.
In this work, we study the question whether these...
Practical Schnorr Threshold Signatures Without the Algebraic Group Model
Hien Chu, Paul Gerhart, Tim Ruffing, Dominique Schröder
Public-key cryptography
Threshold signatures are digital signature schemes in which a set of $n$ signers specify a threshold $t$ such that any subset of size $t$ is authorized to produce signatures on behalf of the group. There has recently been a renewed interest in this primitive, largely driven by the need to secure highly valuable signing keys, e.g., DNSSEC keys or keys protecting digital wallets in the cryptocurrency ecosystem. Of special interest is FROST, a practical Schnorr threshold signature scheme, which...
Additive Randomized Encodings and Their Applications
Shai Halevi, Yuval Ishai, Eyal Kushilevitz, Tal Rabin
Foundations
Addition of $n$ inputs is often the easiest nontrivial function to compute securely. Motivated by several open questions, we ask what can be computed securely given only an oracle that computes the sum. Namely, what functions can be computed in a model where parties can only encode their input locally, then sum up the encodings over some Abelian group $\G$, and decode the result to get the function output.
An *additive randomized encoding* (ARE) of a function $f(x_1,\ldots,x_n)$ maps...
On the (Im)possibility of Distributed Samplers: Lower Bounds and Party-Dynamic Constructions
Damiano Abram, Maciej Obremski, Peter Scholl
Cryptographic protocols
Distributed samplers, introduced by Abram, Scholl and Yakoubov (Eurocrypt ’22), are a one-round, multi-party protocol for securely sampling from any distribution. We give new lower and upper bounds for constructing distributed samplers in challenging scenarios. First, we consider the feasibility of distributed samplers with a malicious adversary in the standard model; the only previous construction in this setting relies on a random oracle. We show that for any UC-secure construction in the...
The VOLE-in-the-Head (VOLEitH) paradigm transforms VOLE-based zero-knowledge proofs into post-quantum signature schemes by allowing public verification. We introduce reduced VOLE-in-the-Head (rVOLEitH), which incorporates the Vector Semi-Commitment (VSC) technique. VSC, originally developed for MPC-in-the-Head (MPCitH) schemes, reduces commitment size while maintaining security by relaxing the binding property. We adapt the ideal cipher version of VSC (IC-VSC) into the VOLEitH framework,...
Sponge-based constructions have successfully been receiving widespread adoption, as represented by the standardization of SHA-3 and Ascon by NIST. Yet, their provable security against quantum adversaries has not been investigated much. This paper studies the post-quantum security of some keyed sponge-based constructions in the quantum ideal permutation model, focusing on the Ascon AEAD mode and KMAC as concrete instances. For the Ascon AEAD mode, we prove the post-quantum security in the...
Recently, the concept of incompressible encryption has emerged as a powerful enhancement to key-leakage resilience. In an incompressible encryption scheme, an adversary who intercepts ciphertexts is forced to dedicate a significant amount of memory to store them in full if they wish to extract any information about the plaintext later when the secret key becomes available. Given two messages, the security game involves two adversaries: the first adversary receives an encryption of one of the...
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...
We examine the use of blockcipher-based key derivation beyond the birthday bound, arguing that the analysis step of PRP/PRF switching can be eliminated in many cases. To support this, we consider a modified ``ideal model'' for keying cryptographic applications in the multi-instance setting, where keys are chosen to be random \emph{but distinct}, rather than completely independent). Our analysis shows that typical cryptographic applications remain secure in this model. One consequence is...
AES-GCM has been the status quo for efficient symmetric encryption for decades. As technology and cryptographic applications evolved over time, AES-GCM has posed some challenges to certain use-cases due to its default 96-bit nonce size, 128-bit block size, and lack of key commitment. Nonce-derived schemes are one way of addressing these challenges: Such schemes derive multiple keys from nonce values, then apply standard AES-GCM with the derived keys (and possibly another 96-bit nonce). The...
In this work, we derive the first lifting theorems for establishing security in the quantum random permutation and ideal cipher models. These theorems relate the success probability of an arbitrary quantum adversary to that of a classical algorithm making only a small number of classical queries. By applying these lifting theorems, we improve previous results and obtain new quantum query complexity bounds and post-quantum security results. Notably, we derive tight bounds for the quantum...
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...
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...
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...
Black-box separations are a cornerstone of cryptography, indicating barriers to various goals. A recent line of work has explored black-box separations for quantum cryptographic primitives. Namely, a number of separations are known in the Common Haar Random State (CHRS) model, though this model is not considered a complete separation, but rather a starting point. A few very recent works have attempted to lift these separations to a unitary separation, which are considered complete...
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...
In this work we revisit the post-quantum security of KEM-based password-authenticated key exchange (PAKE), specifically that of (O)CAKE. So far, these schemes evaded a security proof considering quantum adversaries. We give a detailed analysis of why this is the case, determining the missing proof techniques. To this end, we first provide a proof of security in the post-quantum setting, up to a single gap. This proof already turns out to be technically involved, requiring advanced techniques...
CKKS is a homomorphic encryption (HE) scheme that supports arithmetic over complex numbers in an approximate manner. Despite its utility in PPML protocols, formally defining the security of CKKS-based protocols is challenging due to its approximate nature. To be precise, in a sender-receiver model, where the receiver holds input ciphertexts and the sender evaluates its private circuit, it is difficult to define sender's privacy in terms of indistinguishability, whereas receiver's privacy...
Pulsars exhibit signals with precise inter-arrival times that are on the order of milliseconds to seconds, depending on the individual pulsar. There are subtle variations in the timing of pulsar signals. We show that these variations can serve as a natural entropy source for the creation of Random Number Generators (RNGs). We also explore the effects of using randomness extractors to increase the entropy of random bits extracted from Pulsar timing data. To evaluate the quality of the Pulsar...
Garbling is a fundamental cryptographic primitive, with numerous theoretical and practical applications. Since the first construction by Yao (FOCS’82, ’86), a line of work has concerned itself with reducing the communication and computational complexity of that construction. One of the most efficient garbling schemes presently is the ‘Half Gates’ scheme by Zahur, Rosulek, and Evans (Eurocrypt’15). Despite its widespread adoption, the provable security of this scheme has been based on...
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"...
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...
The ChaCha20-Poly1305 AEAD scheme is widely used as an alternative for AES-GCM on platforms without AES hardware instructions. Although recent analysis by Degabriele et al. shows that ChaCha20-Poly1305 provides adequate security in the conventional multiuser model, the construction is totally broken when a single nonce is repeated – a real-word scenario that can occur due to faulty implementations or the desire to use random nonces. We present a new nonce-misuse resistant and...
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...
We propose efficient, post-quantum threshold ring signatures constructed from one-wayness of AES encryption and the VOLE-in-the-Head zero-knowledge proof system. Our scheme scales efficiently to large rings and extends the linkable ring signatures paradigm. We define and construct key-binding deterministic tags for signature linkability, that also enable succinct aggregation with approximate lower bound arguments of knowledge; this allows us to achieve succinct aggregation of our signatures...
At FSE'15, Mennink introduced the concept of designing beyond-the-birthday bound secure tweakable block cipher from an ideal block cipher. They proposed two tweakable block ciphers $\widetilde{F}[1]$ and $\widetilde{F}[2]$ that accepts $n$-bit tweak using a block cipher of $n$-bit key and $n$-bit data. Mennink proved that the constructions achieve security up to $2^{2n/3}$ and $2^n$ queries, respectively, assuming the underlying block cipher is ideal. Later, at ASIACRYPT'16, Wang et al....
Password Authenticated Key Exchange (PAKE) is a fundamental cryptographic component that allows two parties to establish a shared key using only (potentially low-entropy) passwords. The interest in realizing generic KEM-based PAKEs has increased significantly in the last few years as part of the global migration effort to quantum-resistant cryptography. One such PAKE is the CAKE protocol, proposed by Beguinet et al. (ACNS ’23). However, despite its simple design based on the...
One-more problems like One-More Discrete Logarithm (OMDL) and One-More Diffie--Hellman (OMDH) have found wide use in cryptography, due to their ability to naturally model security definitions for interactive primitives like blind signatures and oblivious PRF. Furthermore, a generalization of OMDH called Threshold OMDH (TOMDH) has proven useful for building threshold versions of interactive protocols. However, due to their complexity it is often unclear how hard such problems actually are,...
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...
Key Transparency (KT) systems have emerged as a critical technology for adding verifiability to the distribution of public keys used in end-to-end encrypted messaging services. Despite substantial academic interest, increased industry adoption, and IETF standardization efforts, KT systems lack a holistic and formalized security model, limiting their resilience to practical threats and constraining future development. In this paper, we survey the existing KT literature and present the...
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...
State-of-the-art garbling schemes for boolean circuits roughly consist of two families, i.e., ideal model garbling that combines linear operations and ideal blockciphers (aiming at maximizing performance), and PRF-based garbling that insists on using theoretically sound assumptions. In the linear garbling framework introduced by Zahur, Rosulek, and Evans (Eurocrypt 2015), it was established that garbling an AND gate requires at least $2(\kappa +1)$ bits of ciphertext, with $\kappa$ as the...
In ASIACRYPT 2019, Andreeva et al. introduced a new symmetric key primitive called the $\textit{forkcipher}$, designed for lightweight applications handling short messages. A forkcipher is a keyed function with a public tweak, featuring fixed-length input and fixed-length (expanding) output. They also proposed a specific forkcipher, ForkSkinny, based on the tweakable block cipher SKINNY, and its security was evaluated through cryptanalysis. Since then, several efficient AEAD and MAC schemes...
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...
Quantum no-cloning is one of the most fundamental properties of quantum information. In this work, we introduce a new toolkit for analyzing cloning games; these games capture more quantitative versions of no-cloning and are central to unclonable cryptography. Previous works rely on the framework laid out by Tomamichel, Fehr, Kaniewski and Wehner to analyze both the $n$-qubit BB84 game and the subspace coset game. Their constructions and analysis face the following inherent limitations: -...
Pseudo-Random Injections (PRIs) have been used in several applications in symmetric-key cryptography, such as in the idealization of Authenticated Encryption with Associated Data (AEAD) schemes, building robust AEAD, and, recently, in converting a committing AEAD scheme into a succinctly committing AEAD scheme. In Crypto 2024, Bellare and Hoang showed that if an AEAD scheme is already committing, it can be transformed into a succinctly committing scheme by encrypting part of the plaintext...
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...
Many foundational results in the literature of consensus follow the Dolev-Yao model (FOCS '81), which treats digital signatures as ideal objects with perfect correctness and unforgeability. However, no work has yet formalized an ideal signature scheme that is both suitable for this methodology and possible to instantiate, or a composition theorem that ensures security when instantiating it cryptographically. The Universal Composition (UC) framework would ensure composition if we could...
In this paper, we study multi-party non-interactive key exchange (NIKE) in the fine-grained setting. More precisely, we propose three multi-party NIKE schemes in three computation models, namely, the bounded parallel-time, bounded time, and bounded storage models. Their security is based on a very mild assumption (e.g., NC1 ⊊ ⊕L/poly) or even without any complexity assumption. This improves the recent work of Afshar, Couteau, Mahmoody, and Sadeghi (EUROCRYPT 2023) that requires idealized...
The indifferentiability framework has become a standard methodology that enables us to study the security of cryptographic constructions in idealized models of computation. Unfortunately, while indifferentiability provides strong guarantees whenever the security of a construction is captured by a ``single-stage'' security game, it may generally provide no meaningful guarantees when the security is captured by a ``multi-stage'' one. In particular, the indifferentiability framework does not...
We introduce the notion of pseudorandom obfuscation, a way to obfuscate (keyed) pseudorandom functions $f_K$ in an average-case sense. We study several variants of pseudorandom obfuscation and show a number of applications. 1. Applications in the iO World: Our weakest variant of pseudorandom obfuscation, named obfuscation for identical pseudorandom functions (iPRO), is weaker than indistinguishability obfuscation (iO): rather than obfuscating arbitrary circuits as in iO, iPRO only...
Evaluation of cryptographic implementations with respect to side-channels has been mandated at high security levels nowadays. Typically, the evaluation involves four stages: detection, modeling, certification and secret recovery. In pursuit of specific goal at each stage, inherently different techniques used to be considered necessary. However, since the recent works of Eurocrypt2022 and Eurocrypt2024, linear regression analysis (LRA) has uniquely become the technique that is well-applied...
A trapdoor Memory-Hard Function is a function that is memory-hard to evaluate for any party who does not have a trapdoor, but is substantially less expensive to evaluate with the trapdoor. Biryukov and Perin (ASIACRYPT 2017) introduced the first candidate trapdoor Memory-Hard Function called Diodon which modifies a Memory-Hard Function called Scrypt by replacing a hash chain with repeated squaring modulo a composite number $N=pq$. The trapdoor, which consists of the prime factors $p$ and...
We develop and implement AlgoROM, a tool to systematically analyze the security of a wide class of symmetric primitives in idealized models of computation. The schemes that we consider are those that can be expressed over an alphabet consisting of XOR and function symbols for hash functions, permutations, or block ciphers. We implement our framework in OCaml and apply it to a number of prominent constructions, which include the Luby–Rackoff (LR), key-alternating Feistel (KAF), and...
Oblivious Transfer (OT) is one of the fundamental building blocks in cryptography that enables various privacy-preserving applications. Constructing efficient OT schemes has been an active research area. This paper presents three efficient two-round pairing-free k-out-of-N oblivious transfer protocols with standard security. Our constructions follow the minimal communication pattern: the receiver sends k messages to the sender, who responds with n+k messages, achieving the lowest data...
Threshold signatures have been drawing lots of attention in recent years. Of particular interest are threshold signatures that are proven secure under adaptive corruptions (NIST Call 2023). Sadly, existing constructions with provable adaptive security suffer from at least one of the following drawbacks: (i) strong idealizations such as the algebraic group model (AGM), (ii) an unnatural restriction on the corruption threshold being $t/2$ where $t$ is the signing threshold, or (iii)...
Non-interactive zero-knowledge (NIZK) proofs enable a prover to convince a verifier of an NP statement’s validity using a single message, without disclosing any additional information. These proofs are widely studied and deployed, especially in their succinct form, where proof length is sublinear in the size of the NP relation. However, efficient succinct NIZKs typically require an idealized setup, such as a a common reference string, which complicates real-world deployment. A key challenge...
Time-lock puzzles are unique cryptographic primitives that use computational complexity to keep information secret for some period of time, after which security expires. This topic, while over 25 years old, is still in a state where foundations are not well understood: For example, current analysis techniques of time-lock primitives provide no sound mechanism to build composed multi-party cryptographic protocols which use expiring security as a building block. Further, there are analyses...
We show that the widely-used Schnorr signature scheme meets existential unforgeability under chosen-message attack (EUF-CMA) in the random oracle model (ROM) if the circular discrete-logarithm (CDL) assumption holds in the underlying group. CDL is a new, non-interactive and falsifiable variant of the discrete-logarithm (DL) assumption that we introduce. Our reduction is completely tight, meaning the constructed adversary against CDL has essentially the same running time and success...
Ever since the seminal work of Diffie and Hellman, cryptographic (cyclic) groups have served as a fundamental building block for constructing cryptographic schemes and protocols. The security of these constructions can often be based on the hardness of (cyclic) group-based computational assumptions. Then, the generic group model (GGM) has been studied as an idealized model (Shoup, EuroCrypt 1997), which justifies the hardness of many (cyclic) group-based assumptions and shows the limits of...
Recently, Masny and Rindal [MR19] formalized a notion called Endemic Oblivious Transfer (EOT), and they proposed a generic transformation from Non-Interactive Key Exchange (NIKE) to EOT with standalone security in the random oracle (RO) model. However, from the model level, the relationship between idealized NIKE and idealized EOT and the relationship between idealized elementary public key primitives have been rarely researched. In this work, we investigate the relationship between ideal...
A Trusted Execution Environment (TEE) is a security technology, implemented by CPU manufacturers, which guarantees integrity and confidentiality on a restricted execution environment to any remote verifier through attestation. TEEs are deployed on various consumer and commercial hardware platforms, and have been widely adopted as a component in the design of cryptographic protocols both theoretical and practical. Within the provable security community, the use of TEEs as a setup...
Identity-based threshold signature (IDTS) enables the generation of valid signatures without revealing cryptographic keys in the signing process. While current protocols have achieved much progress in their efficiency, many schemes easily suffer from denial-of-service attacks in which misbehaving parties could keep from generating signatures without being caught. The identifiable abort property is designed to withstand such an attack in some recent IDTS protocols. However, all these schemes...
It is fundamental that executing cryptographic software must not leak secrets through side-channels. For software-visible side-channels, it was long believed that "constant-time" programming would be sufficient as a systematic countermeasure. However, this belief was shattered in 2018 by attacks exploiting speculative execution—so called Spectre attacks. Recent work shows that language support suffices to protect cryptographic code with minimal overhead against one class of such attacks,...
The problem of minimizing the share size of threshold secret-sharing schemes is a basic research question that has been extensively studied. Ideally, one strives for schemes in which the share size equals the secret size. While this is achievable for large secrets (Shamir, CACM '79), no similar solutions are known for the case of binary, single-bit secrets. Current approaches often rely on so-called ramp secret sharing that achieves a constant share size at the expense of a slight gap...
A sequential function is, informally speaking, a function $f$ for which a massively parallel adversary cannot compute "substantially" faster than an honest user with limited parallel computation power. Sequential functions form the backbone of many primitives that are extensively used in blockchains such as verifiable delay functions (VDFs) and time-lock puzzles. Despite this widespread practical use, there has been little work studying the complexity or theory of sequential...
T-out-of-N threshold signatures have recently seen a renewed interest, with various types now available, each offering different tradeoffs. However, one property that has remained elusive is adaptive security. When we target thresholdizing existing efficient signatures schemes based on the Fiat-Shamir paradigm such as Schnorr, the elusive nature becomes clear. This class of signature schemes typically rely on the forking lemma to prove unforgeability. That is, an adversary is rewound and...
There exists a mismatch between the theory and practice of cryptography in the presence of leakage. On the theoretical front, the bounded leakage model, where the adversary learns bounded-length but noiseless information about secret components, and the random probing model, where the adversary learns some internal values of a leaking implementation with some probability, are convenient abstractions to analyze the security of numerous designs. On the practical front, side-channel attacks...
MPC-in-the-Head (MPCitH) has recently gained traction as a foundation for post-quantum signature schemes, offering robust security without trapdoors. Despite its strong security profile, MPCitH-based schemes suffer from high computational overhead and large signature sizes, limiting their practical application. This work addresses these inefficiencies by relaxing vector commitments within MPCitH-based schemes. We introduce the concept of vector semi-commitment, which relaxes the binding...
This Systematization of Knowledge conducts a survey of contemporary distributed blockchain protocols, with the aim of identifying cryptographic and design techniques which practically enable both expressive programmability and user data confidentiality. To facilitate a framing which supports the comparison of concretely very different protocols, we define an epoch-based computational model in the form of a flexible UC-style ideal functionality which divides the operation of...
Web3 applications, such as on-chain games, NFT minting, and leader elections necessitate access to unbiased, unpredictable, and publicly verifiable randomness. Despite its broad use cases and huge demand, there is a notable absence of comprehensive treatments of on-chain verifiable randomness services. To bridge this, we offer an extensive formal analysis of on-chain verifiable randomness services. We present the $first$ formalization of on-chain verifiable randomness in the...
In this work we first present an explicit forking lemma that distills the information-theoretic essence of the high-moment technique introduced by Rotem and Segev (CRYPTO '21), who analyzed the security of identification protocols and Fiat-Shamir signature schemes. Whereas the technique of Rotem and Segev was particularly geared towards two specific cryptographic primitives, we present a stand-alone probabilistic lower bound, which does not involve any underlying primitive or idealized...
We revisit the construction of multiparty non-interactive key-exchange protocols with fine-grained security, which was recently studied in (Afshar et al., Eurocrypt 2023). Their work introduced a 4-party non-interactive key exchange with quadratic hardness, and proved it secure in Shoup's generic group model. This positive result was complemented with a proof that $n$-party non-interactive key exchange with superquadratic security cannot exist in Maurer's generic group model, for any $n\geq...
We show that the adaptive compromise security definitions of Jaeger and Tyagi (Crypto '20) cannot be applied in several natural use-cases. These include proving multi-user security from single-user security, the security of the cascade PRF, and the security of schemes sharing the same ideal primitive. We provide new variants of the definitions and show that they resolve these issues with composition. Extending these definitions to the asymmetric settings, we establish the security of the...
In FSE'16, Luykx et al. have proposed $\textsf{LightMAC}$ that provably achieves a query length independent PRF security bound. To be precise, the construction achieves security roughly in the order of $O(q^2/2^n)$, when instantiated with two independently keyed $n$-bit block ciphers and $q$ is the total number of queries made by the adversary. Subsequently, in ASIACRYPT'17, Naito proposed a beyond-birthday-bound variant of the $\textsf{LightMAC}$ construction, dubbed as...
We extend the Linicrypt framework for characterizing hash function security as proposed by McQuoid, Swope, and Rosulek (TCC 2018) to support constructions in the ideal cipher model. In this setting, we give a characterization of collision- and second-preimage-resistance in terms of a linear-algebraic condition on Linicrypt programs, and present an efficient algorithm for determining whether a program satisfies the condition. As an application, we consider the case of the block cipherbased...
This paper focuses on algebraic attacks on the $\mathsf{Anemoi}$ family of arithmetization-oriented permutations [Crypto '23]. We consider a slight variation of the naive modeling of the $\mathsf{CICO}$ problem associated to the primitive, for which we can very easily obtain a Gröbner basis and prove the degree of the associated ideal. For inputs in $\mathbb{F}_{q}^2$ when $q$ is an odd prime, we recover the same degree as conjectured for alternative polynomial systems used in other recent...
MRAE security is an important goal for many AEAD applications where the nonce uniqueness cannot be maintained and security risks are significant. However, MRAE schemes can be quite expensive. Two of the SoTA MRAE-secure schemes; Deoxys-II and AES-GCM-SIV rely on internal parallelism and special instructions to achieve competitive performance. However, they both suffer from the same bottleneck, they have at least one call to the underlying primitive that cannot be parallelized to any other...
Cryptographic protocols are hard to design and prove correct, as witnessed by the ever-growing list of attacks even on protocol standards. Symbolic models of cryptography enable automated formal security proofs of such protocols against an idealized cryptographic model, which abstracts away from the algebraic properties of cryptographic schemes and thus misses attacks. Computational models of cryptography yield rigorous guarantees but support at present only interactive proofs and/or...
Ciphertext-independent updatable encryption (UE) allows to rotate encryption keys and update ciphertexts via a token without the need to first download the ciphertexts. Although, syntactically, UE is a symmetric-key primitive, ciphertext-independent UE with forward secrecy and post-compromise security is known to imply public-key encryption (Alamati, Montgomery and Patranabis, CRYPTO 2019). Constructing post-quantum secure UE turns out to be a difficult task. While lattices offer the...
Threshold signatures have recently seen a renewed interest due to applications in cryptocurrency while NIST has released a call for multi-party threshold schemes, with a deadline for submission expected for the first half of 2025. So far, all lattice-based threshold signatures requiring less than two-rounds are based on heavy tools such as (fully) homomorphic encryption (FHE) and homomorphic trapdoor commitments (HTDC). This is not unexpected considering that most efficient two-round...
In the common random string model, the parties executing a protocol have access to a uniformly random bit string. It is known that under standard intractability assumptions, we can realize any ideal functionality with universally composable (UC) security if a trusted common random string (CrS) setup is available. It was always a question of where this CrS should come from since the parties provably could not compute it themselves. Trust assumptions are required, so minimizing the level of...
Structure-preserving signatures (SPS) have emerged as an important cryptographic building block, as their compatibility with the Groth-Sahai (GS) NIZK framework allows to construct protocols under standard assumptions with reasonable efficiency. Over the last years there has been a significant interest in the design of threshold signature schemes. However, only very recently Crites et al. (ASIACRYPT 2023) have introduced threshold SPS (TSPS) along with a fully non-interactive construction....
Driven by the NIST's post-quantum standardization efforts and the selection of Kyber as a lattice-based Key-Encapsulation Mechanism (KEM), several Password Authenticated Key Exchange (PAKE) protocols have been recently proposed that leverage a KEM to create an efficient, easy-to-implement and secure PAKE. In two recent works, Beguinet et al. (ACNS 2023) and Pan and Zeng (ASIACRYPT 2023) proposed generic compilers that transform KEM into PAKE, relying on an Ideal Cipher (IC) defined over a...
This paper studies the limitations of the generic approaches to solving cryptographic problems in classical and quantum settings in various models. - In the classical generic group model (GGM), we find simple alternative proofs for the lower bounds of variants of the discrete logarithm (DL) problem: the multiple-instance DL and one-more DL problems (and their mixture). We also re-prove the unknown-order GGM lower bounds, such as the order finding, root extraction, and repeated squaring. -...
We revisit the construction of signature schemes using the MPC-in-the-head paradigm. We obtain two main contributions: – We observe that previous signatures in the MPC-in-the-head paradigm must rely on a salted version of the GGM puncturable pseudorandom function (PPRF) to avoid collision attacks. We design a new efficient PPRF construction that is provably secure in the multi-instance setting. The security analysis of our PPRF, in the ideal cipher model, is quite involved and forms a...
Additive randomized encodings (ARE), introduced by Halevi, Ishai, Kushilevitz, and Rabin (CRYPTO 2023), reduce the computation of a k-party function $f (x_1, . . . , x_k )$ to locally computing encodings $\hat{x}_i$ of each input xi and then adding them together over some Abelian group into an output encoding $\hat{y} = ∑ \hat{x}_i$, which reveals nothing but the result. In robust ARE (RARE) the sum of any subset of $\hat{x}_i$, reveals only the residual function obtained by restricting the...
Function secret sharing (FSS) for a class $\cal{F}$ allows to split a secret function $f \in \cal{F}$ into (succinct) secret shares $f_0,f_1$, such that for all $x\in \{0,1\}^n$ it holds $f_0(x)-f_1(x)=f(x)$. FSS has numerous applications, including private database queries, nearest neighbour search, private heavy hitters and secure computation in the preprocessing model, where the supported class $\cal{F}$ translates to richness in the application. Unfortunately, concretely efficient FSS...
We prove that the seminal KZG polynomial commitment scheme (PCS) is black-box extractable under a simple falsifiable assumption ARSDH. To create an interactive argument, we construct a compiler that combines a black-box extractable non-interactive PCS and a polynomial IOP (PIOP). The compiler incurs a minor cost per every committed polynomial. Applying the Fiat-Shamir transformation, we obtain slightly less efficient variants of well-known PIOP-based zk-SNARKs, such as Plonk, that are...
A zero-knowledge proof of training (zkPoT) enables a party to prove that they have correctly trained a committed model based on a committed dataset without revealing any additional information about the model or the dataset. An ideal zkPoT should offer provable security and privacy guarantees, succinct proof size and verifier runtime, and practical prover efficiency. In this work, we present \name, a zkPoT targeted for deep neural networks (DNNs) that achieves all these goals at once. Our...
We continue the study of $t$-wise independence of substitution-permutation networks (SPNs) initiated by the recent work of Liu, Tessaro, and Vaikuntanathan (CRYPTO 2021). Our key technical result shows that when the S-boxes are randomly and independently chosen and kept secret, an $r$-round SPN with input length $n = b \cdot k$ is $2^{-\Theta(n)}$-close to $t$-wise independent within $r = O(\min\{k, \log t\})$ rounds for any $t$ almost as large as $2^{b/2}$. Here, $b$ is the input length of...
ZK-SNARKs, a fundamental component of privacy-oriented payment systems, identity protocols, or anonymous voting systems, are advanced cryptographic protocols for verifiable computation: modern SNARKs allow to encode the invariants of a program, expressed as an arithmetic circuit, in an appropriate constraint language from which short, zero-knowledge proofs for correct computations can be constructed. One of the most important computations that is run through SNARK systems is the...
Basic encryption and signature on lattices have comparable efficiency to their classical counterparts in terms of speed and key size. However, Identity-based Encryption (IBE) on lattices is much less efficient in terms of compactness, even when instantiated on ideal lattices and in the Random Oracle Model (ROM). This is because the underlying preimage sampling algorithm used to extract the users' secret keys requires huge public parameters. In this work, we specify a compact IBE...
Driven by the open problem raised by Hofheinz and Kiltz (Journal of Cryptology, 2012), we study the formalization of lattice-based programmable hash function (PHF), and give three types of concrete constructions by using several techniques such as a novel combination of cover-free sets and lattice trapdoors. Under the Inhomogeneous Small Integer Solution (ISIS) assumption, we show that any (non-trivial) lattice-based PHF is a collision-resistant hash function, which gives a direct...
Quantum no-cloning theorem gives rise to the intriguing possibility of quantum copy protection where we encode a program or functionality in a quantum state such that a user in possession of k copies cannot create k+1 copies, for any k. Introduced by Aaronson (CCC'09) over a decade ago, copy protection has proven to be notoriously hard to achieve. Previous work has been able to achieve copy-protection for various functionalities only in restricted models: (i) in the bounded collusion...
We introduce a new cryptographic primitive, called Completely Anonymous Signed Encryption (CASE). CASE is a public-key authenticated encryption primitive, that offers anonymity for senders as well as receivers. A "case-packet" should appear, without a (decryption) key for opening it, to be a blackbox that reveals no information at all about its contents. To decase a case-packet fully - so that the message is retrieved and authenticated - a verifcation key is also required. Defining security...
We consider the mainstream model in secure computation known as the bare PKI setup, also as the {bulletin-board PKI}. It allows players to broadcast once and non-interactively before they receive their inputs and start the execution. A bulletin-board PKI is essentially the minimum setup known so far to implement the model known as {messages-authentication}, i.e., when $P$ is forwarded a signed message, it considers it to be issued by $R$ if and only if $R$ signed it. It is known since...
Security proofs of symmetric-key primitives typically consider an idealized world with access to a (uniformly) random function. The starting point of our work is the observation that such an ideal world can lead to underestimating the actual security of certain primitives. As a demonstrating example, $\mathsf{XoP2}$, which relies on two independent random permutations, has been proven to exhibit superior concrete security compared to $\mathsf{XoP}$, which employs a single permutation with...
Proof-carrying data (PCD) is a powerful cryptographic primitive that allows mutually distrustful parties to perform distributed computation in an efficiently verifiable manner. Real-world deployments of PCD have sparked keen interest within the applied community and industry. Known constructions of PCD are obtained by recursively-composing SNARKs or related primitives. Unfortunately, known security analyses incur expensive blowups, which practitioners have disregarded as the analyses...
Adaptive security is a crucial property for garbling schemes in pushing the communication of garbled circuits to an offline phase when the input is unknown. In this paper, we show that the popular half-gates scheme by Zahur et al. (Eurocrypt'15), without any modification, is adaptively secure in the non-programmable random permutation model (npRPM). Since real implementations of selective-secure half-gates are already based on npRPM, our result shows that these implementations are already...
The KHAPE-HMQV protocol is a state-of-the-art highly efficient asymmetric password-authenticated key exchange protocol that provides several desirable security properties, but has the drawback of being vulnerable to quantum adversaries due to its reliance on discrete logarithm-based building blocks: solving a single discrete logarithm allows the attacker to perform an offline dictionary attack and recover the password. We show how to modify KHAPE-HMQV to make the protocol quantum-annoying: a...
Byzantine agreement (BA) asks for a set of parties to reach agreement in an adversarial setting. A central question is how to construct efficient BA protocols that scale well with the number of parties. In particular, the communication complexity is a critical barrier for large-scale implementations. State-of-the-art, scalable BA protocols typically work by sampling a small, unpredictable committee of parties that will send messages in each round. These messages must reach all honest...
We revisit OCAKE (ACNS 23), a generic recipe that constructs password-based authenticated key exchange (PAKE) from key encapsulation mechanisms (KEMs), to allow instantiations with post-quantums KEM like KYBER. The ACNS23 paper left as an open problem to argue security against quantum attackers, with its security proof being in the universal composability (UC) framework. This is common for PAKE, however, at the time of this submission’s writing, it was not known how to prove (computational)...
The constant-sized polynomial commitment scheme by Kate, Zaverucha, and Goldberg (Asiscrypt 2010), also known as the KZG commitment, is an essential component in designing bandwidth-efficient verifiable secret-sharing (VSS) protocols. We point out, however, that the KZG commitment is missing two important properties that are crucial for VSS protocols. First, the KZG commitment has not been proven to be degree binding in the standard adversary model without idealized group assumptions. In...
We propose a generic construction of password-based authenticated key exchange (PAKE) from key encapsulation mechanisms (KEM). Assuming that the KEM is oneway secure against plaintext-checkable attacks (OW-PCA), we prove that our PAKE protocol is \textit{tightly secure} in the Bellare-Pointcheval-Rogaway model (EUROCRYPT 2000). Our tight security proofs require ideal ciphers and random oracles. The OW-PCA security is relatively weak and can be implemented tightly with the Diffie-Hellman...
In this paper, we introduce $\mathsf{DeuringVUF}$, a new Verifiable Unpredictable Function (VUF) protocol based on isogenies between supersingular curves. The most interesting application of this VUF is $\mathsf{DeuringVRF}$ a post-quantum Verifiable Random Function (VRF). The main advantage of this new scheme is its compactness, with combined public key and proof size of roughly 450 bytes, which is orders of magnitude smaller than other generic purpose post-quantum VRF...
A wide range of countermeasures have been proposed to defend against side-channel attacks, with masking being one of the most effective and commonly used techniques. While theoretical models provide formal security proofs, these often rely on assumptions—sometimes implicit—that can be difficult to assess in practice. As a result, the design of secure masked implementations frequently combines proven theoretical arguments with heuristic and empirical validation. Despite the...
The random oracle model is an instrument used for proving that protocol has no structural flaws when settling with standard hash properties is impossible or fairly difficult. In practice, however, random oracles have to be instantiated with some specific hash functions, which are not random oracles. Hence, in the real world, an adversary has broader capabilities than considered in the random oracle proof — it can exploit the peculiarities of a specific hash function to achieve its goal. In a...
Most succinct arguments (SNARKs) are initially only proven knowledge sound (KS). We show that the commonly employed compilation strategy from polynomial interactive oracle proofs (PIOP) via polynomial commitments to knowledge sound SNARKS actually also achieves other desirable properties: weak unique response (WUR) and trapdoorless zero-knowledge (TLZK); and that together they imply simulation extractability (SIM-EXT). The factoring of SIM-EXT into KS + WUR + TLZK is becoming a...
Digital Signatures are ubiquitous in modern computing. One of the most widely used digital signature schemes is ECDSA due to its use in TLS, various Blockchains such as Bitcoin and Etherum, and many other applications. Yet the formal analysis of ECDSA is comparatively sparse. In particular, all known security results for ECDSA rely on some idealized model such as the generic group model or the programmable (bijective) random oracle model. In this work, we study the question whether these...
Threshold signatures are digital signature schemes in which a set of $n$ signers specify a threshold $t$ such that any subset of size $t$ is authorized to produce signatures on behalf of the group. There has recently been a renewed interest in this primitive, largely driven by the need to secure highly valuable signing keys, e.g., DNSSEC keys or keys protecting digital wallets in the cryptocurrency ecosystem. Of special interest is FROST, a practical Schnorr threshold signature scheme, which...
Addition of $n$ inputs is often the easiest nontrivial function to compute securely. Motivated by several open questions, we ask what can be computed securely given only an oracle that computes the sum. Namely, what functions can be computed in a model where parties can only encode their input locally, then sum up the encodings over some Abelian group $\G$, and decode the result to get the function output. An *additive randomized encoding* (ARE) of a function $f(x_1,\ldots,x_n)$ maps...
Distributed samplers, introduced by Abram, Scholl and Yakoubov (Eurocrypt ’22), are a one-round, multi-party protocol for securely sampling from any distribution. We give new lower and upper bounds for constructing distributed samplers in challenging scenarios. First, we consider the feasibility of distributed samplers with a malicious adversary in the standard model; the only previous construction in this setting relies on a random oracle. We show that for any UC-secure construction in the...
