Transciphering (or Hybrid-Homomorphic Encryption, HHE) is an es- tablished technique for avoiding ciphertext expansion in HE applications, saving communication and storage resources. Recently, it has also been shown to be a fundamental component in the practical construction of HE-based multi-party computation (MPC) protocols, being used both for input data and intermediary results (Smart, IMACC 2023). In these protocols, however, ciphers are used with keys that are jointly generated by...

This paper introduces zkFFT, a novel zero-knowledge argument designed to efficiently generate proofs for FFT (Fast Fourier Transform) relations. Our approach enables the verification that one committed vector is the FFT of another, addressing an efficiency need in general-purpose non-interactive zero-knowledge proof systems where the proof relation utilizes vector commitments inputs. We present a concrete enhancement to the Halo2 proving system, demonstrating how zkFFT optimizes proofs in...

Artificial Intelligence (AI) has steadily improved across a wide range of tasks, and a significant breakthrough towards general intelligence was achieved with the rise of generative deep models, which have garnered worldwide attention. However, the development and deployment of AI are almost entirely controlled by a few powerful organizations and individuals who are racing to create Artificial General Intelligence (AGI). These centralized entities make decisions with little public oversight,...

The traditional definition of fully homomorphic encryption (FHE) is not composable, i.e., it does not guarantee that evaluating two (or more) homomorphic computations in a sequence produces correct results. We formally define and investigate a stronger notion of homomorphic encryption which we call "fully composable homomorphic encryption", or "composable FHE". The definition is both simple and powerful: it does not directly involve the evaluation of multiple functions, and yet it...

In this paper, we initiate the study of multi-designated detector watermarking (MDDW) for large language models (LLMs). This technique allows model providers to generate watermarked outputs from LLMs with two key properties: (i) only specific, possibly multiple, designated detectors can identify the watermarks, and (ii) there is no perceptible degradation in the output quality for ordinary users. We formalize the security definitions for MDDW and present a framework for constructing MDDW...

Local Differential Privacy (LDP) provides a formal guarantee of privacy that enables the collection and analysis of sensitive data without revealing any individual's data. While LDP methods have been extensively studied, there is a lack of a systematic and empirical comparison of LDP methods for descriptive statistics. In this paper, we first provide a systematization of LDP methods for descriptive statistics, comparing their properties and requirements. We demonstrate that several mean...

With the increasing spread of fake videos for misinformation, proving the provenance of an edited video (without revealing the original one) becomes critical. To this end, we introduce Eva, the first cryptographic protocol for authenticating lossy-encoded videos. Compared to previous cryptographic methods for image authentication, Eva supports significantly larger amounts of data that undergo complex transformations during encoding. We achieve this by decomposing repetitive and manageable...

This paper presents a survey of the state-of-the-art pre-silicon security verification techniques for System-on-Chip (SoC) designs, focusing on ensuring that designs, implemented in hardware description languages (HDLs) and synthesized circuits, meet security requirements before fabrication in semiconductor foundries. Due to several factors, pre-silicon security verification has become an essential yet challenging aspect of the SoC hardware lifecycle. The modern SoC design process often...

Masking has become a widely applied and heavily researched method to protect cryptographic implementations against SCA attacks. The success of masking is primarily attributed to its strong theoretical foundation enabling it to formally prove security by modeling physical properties through so-called probing models. Specifically, the robust $d$-probing model enables us to prove the security for arbitrarily masked hardware circuits, manually or with the assistance of automated tools, even when...

Homomorphic Encryption (HE) is a cutting-edge cryptographic technique that enables computations on encrypted data to be mirrored on the original data. This has quickly attracted substantial interest from the research community due to its extensive practical applications, such as in cloud computing and privacy-preserving machine learning. In addition to confidentiality, the importance of authenticity has emerged to ensure data integrity during transmission and evaluation. To address...

Compilers often weaken or even discard software-based countermeasures commonly used to protect programs against side-channel attacks; worse, they may also introduce vulnerabilities that attackers can exploit. The solution to this problem is to develop compilers that preserve these countermeasures. Prior work establishes that (a mildly modified version of) the CompCert and Jasmin formally verified compilers preserve constant-time, an information flow policy that ensures that programs are...

In recent years, formal verification has emerged as a crucial method for assessing security against Side-Channel attacks of masked implementations, owing to its remarkable versatility and high degree of automation. However, formal verification still faces technical bottlenecks in balancing accuracy and efficiency, thereby limiting its scalability. Former tools like maskVerif and CocoAlma are very efficient but they face accuracy issues when verifying schemes that utilize properties of...

Side-channel-analysis (SCA) resistance with cost optimization in AES hardware implementations remains a significant challenge. While traditional masking-based schemes offer provable security, they often incur substantial resource overheads (latency, area, randomness, performance, power consumption). Alternatively, the RAMBAM scheme introduced a redundancy-based approach to control the signal-to-noise ratio, and achieves exponential leakage reduction as redundancy increases. This method...

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,...

This paper presents KyberSlash1 and KyberSlash2 – two timing vulnerabilities in several implementations (including the official reference code) of the Kyber Post-Quantum Key Encapsulation Mechanism, currently undergoing standardization as ML-KEM. We demonstrate the exploitability of both KyberSlash1 and KyberSlash2 on two popular platforms: the Raspberry Pi 2 (Arm Cortex-A7) and the Arm Cortex-M4 microprocessor. Kyber secret keys are reliably recovered within minutes for KyberSlash2 and a...

Internet of Medical Things (IoMT) has gained significant research focus in both academic and medical institutions. Nevertheless, the sensitive data involved in IoMT raises concerns regarding user validation and data privacy. To address these concerns, certificateless signcryption (CLSC) has emerged as a promising solution, offering authenticity, confidentiality, and unforgeability. Unfortunately, most existing CLSC schemes are impractical for IoMT due to their heavy computational and storage...

We present a novel approach to small area and low-latency first-order masking in hardware. The core idea is to separate the processing of shares in time in order to achieve non-completeness. Resulting circuits are proven first-order glitch-extended PINI secure. This means the method can be straightforwardly applied to mask arbitrary functions without constraints which the designer must take care of. Furthermore we show that an implementation can benefit from optimization through EDA tools...

Cryptographic hash functions are said to be the work-horses of modern cryptography. One of the strongest approaches to assess a cryptographic hash function's security is indifferentiability. Informally, indifferentiability measures to what degree the function resembles a random oracle when instantiated with an ideal underlying primitive. However, proving the indifferentiability security of hash functions has been challenging due to complex simulator designs and proof arguments. The Sponge...

Ring signatures allow members of a group (called "ring") to sign a message anonymously within the group, which is chosen ad hoc at the time of signing (the members do not need to have interacted before). In this paper, we propose a physical version of ring signatures. Our signature is based on one-out-of-many signatures, a method used in many real cryptographic ring signatures. It consists of boxes containing coins locked with padlocks that can only be opened by a particular group member. To...

Verifying the verifier in the context of zero-knowledge proof is an essential part of ensuring the long-term integrity of the zero-knowledge ecosystem. This is vital for both zero-knowledge rollups and also other industrial applications of ZK. In addition to further minimizing the required trust and reducing the trusted computing base (TCB), having a verified verifier opens the door to decentralized proof generation by potentially untrusted parties. We outline a research program and justify...

In our highly digitalized world, an adversary is not constrained to purely digital attacks but can monitor or influence the physical execution environment of a target computing device. Such side-channel or fault-injection analysis poses a significant threat to otherwise secure cryptographic implementations. Hence, it is important to consider additional adversarial capabilities when analyzing the security of cryptographic implementations besides the default black-box model. For side-channel...

Provable privacy typically requires involved analysis and is often associated with unacceptable accuracy loss. While many empirical verification or approximation methods, such as Membership Inference Attacks (MIA) and Differential Privacy Auditing (DPA), have been proposed, these do not offer rigorous privacy guarantees. In this paper, we apply recently-proposed Probably Approximately Correct (PAC) Privacy to give formal, mechanized, simulation-based proofs for a range of practical,...

FALCON is candidate for standardization of the new Post Quantum Cryptography (PQC) primitives by the National Institute of Standards and Technology (NIST). However, it remains a challenge to define efficient countermeasures against side-channel attacks (SCA) for this algorithm. FALCON is a lattice-based signature that relies on rational numbers which is unusual in the cryptography field. While recent work proposed a solution to mask the addition and the multiplication, some roadblocks...

We study selfish mining attacks in longest-chain blockchains like Bitcoin, but where the proof of work is replaced with efficient proof systems -- like proofs of stake or proofs of space -- and consider the problem of computing an optimal selfish mining attack which maximizes expected relative revenue of the adversary, thus minimizing the chain quality. To this end, we propose a novel selfish mining attack that aims to maximize this objective and formally model the attack as a Markov...

We present a framework for privacy-preserving streaming algorithms which combine the memory-efficiency of streaming algorithms with strong privacy guarantees. These algorithms enable some number of servers to compute aggregate statistics efficiently on large quantities of user data without learning the user's inputs. While there exists limited prior work that fits within our model, our work is the first to formally define a general framework, interpret existing methods within this general...

Cryptographic primitives are often validated through rigorous security proofs, but insecure implementations or software-level attacks can compromise control flows, potentially undermining these guarantees. To address this issue, we introduce a new security notion, IND-CFA, which formalizes decryption security in the presence of adversarially controlled execution flows. Using this notion, we investigate the control flows under which a cryptographic scheme remains secure, providing insights...

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...

Motivated by secure database search, we present secure computation protocols for a function $f$ in the client-servers setting, where a client can obtain $f(x)$ on a private input $x$ by communicating with multiple servers each holding $f$. Specifically, we propose generic compilers from passively secure protocols, which only keep security against servers following the protocols, to actively secure protocols, which guarantee privacy and correctness even against malicious servers. Our...

Decoy accounts are often used as an indicator of the compromise of sensitive data, such as password files. An attacker targeting only specific known-to-be-real accounts might, however, remain undetected. A more effective method proposed by Juels and Rivest at CCS'13 is to maintain additional fake passwords associated with each account. An attacker who gains access to the password file is unable to tell apart real passwords from fake passwords, and the attempted usage of a false password...

Eligibility checks are often abstracted away or omitted in voting protocols, leading to situations where the voting server can easily stuff the ballot box. One reason for this is the difficulty of bootstraping the authentication material for voters without relying on trusting the voting server. In this paper, we propose a new protocol that solves this problem by building on OpenID, a widely deployed authentication protocol. Instead of using it as a standard authentication means, we turn it...

We propose a modification to the fork choice rule of proof-of-work blockchains. Instead of choosing the heaviest chain, we choose the chain with the most intrinsic work. The intrinsic work of a block is roughly the number of zeroes at the front of its hash. This modification allows us to safely decrease the confirmations required, yielding a $28.5\%$ improvement in confirmation delay or, dually, safely increase the block production rate, yielding a $16.3\%$ improvement in throughput, as...

Several distributed protocols, including distributed key generation (DKG) and interactive consistency (IC), depend on $\mathcal{O}(n)$ instances of Byzantine Broadcast or Byzantine Agreement among $n$ nodes, resulting in ${\Theta}(n^3)$ communication overhead. In this paper, we provide a new methodology of realizing such broadcasts we call DRAGON: Decentralization at the cost of Representation after Arbitrary GrOupiNg. At the core of it, we arbitrarily group nodes into small ``shards''...

A major unresolved question in quantum cryptography is whether it is possible to obfuscate arbitrary quantum computation. Indeed, there is much yet to understand about the feasibility of quantum obfuscation even in the classical oracle model, where one is given for free the ability to obfuscate any classical circuit. In this work, we develop a new array of techniques that we use to construct a quantum state obfuscator, a powerful notion formalized recently by Coladangelo and Gunn...

A universal circuit (UC) can be thought of as a programmable circuit that can simulate any circuit up to a certain size by specifying its secret configuration bits. UCs have been incorporated into various applications, such as private function evaluation (PFE). Recently, studies have attempted to formalize the concept of semiconductor intellectual property (IP) protection in the context of UCs. This is despite the observations made in theory and practice that, in reality, the adversary may...

The popularity of cloud computing has made outsourced databases prevalent in real-world applications. To protect data security, numerous encrypted outsourced databases have been proposed for this paradigm. However, the maintenance of encrypted databases has scarcely been addressed. In this paper, we focus on a typical maintenance task --- functional dependency (FD) discovery. We develop novel FD protocols in encrypted databases while guaranteeing minimal leakages: nothing is revealed besides...

Key Encapsulation Mechanisms (KEMs) are a critical building block for hybrid encryption and modern security protocols, notably in the post-quantum setting. Given the asymmetric public key of a recipient, the primitive establishes a shared secret key between sender and recipient. In recent years, a large number of abstract designs and concrete implementations of KEMs have been proposed, e.g., in the context of the NIST process for post-quantum primitives. In this work, we (i)...

Given a fixed-size block, cryptographic block functions gen- erate outputs by a sequence of bitwise operations. Block functions are widely used in the design of hash functions and stream ciphers. Their correct implementations hence are crucial to computer security. We pro- pose a method that leverages logic equivalence checking to verify assem- bly implementations of cryptographic block functions. Logic equivalence checking is a well-established technique from hardware verification....

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...

Common block ciphers like AES specified by the NIST or KASUMI (A5/3) of GSM are extensively utilized by billions of individuals globally to protect their privacy and maintain confidentiality in daily communications. However, these ciphers lack comprehensive security proofs against the vast majority of known attacks. Currently, security proofs are limited to differential and linear attacks for both AES and KASUMI. For instance, the consensus on the security of AES is not based on formal...

Privacy enhancing technologies must not only protect sensitive data in-transit, but also locally at-rest. For example, anonymity networks hide the sender and/or recipient of a message from network adversaries. However, if a participating device is physically captured, its owner can be pressured to give access to the stored conversations. Therefore, client software should allow the user to plausibly deny the existence of meaningful data. Since biometrics can be collected without consent and...

Verification of program safety is often reducible to proving the unsatisfiability (i.e., validity) of a formula in Satisfiability Modulo Theories (SMT): Boolean logic combined with theories that formalize arbitrary first-order fragments. Zero-knowledge (ZK) proofs allow SMT formulas to be validated without revealing the underlying formulas or their proofs to other parties, which is a crucial building block for proving the safety of proprietary programs. Recently, Luo et al. (CCS 2022)...

The Internet Key Exchange version 2 (IKEv2) (RFC 7296) is a component of IPsec used to authenticate two parties (the initiator and responder) to each other and to establish a set of security parameters for the communications. The security parameters include secret keys to encrypt and authenticate data as well as the negotiation of a set of cryptographic algorithms. The core documentation uses exclusively Diffie-Hellman exchanges to agree the security information. However, this is not a...

Envelope encryption is a method to encrypt data with two distinct keys in its basic form. Data is first encrypted with a data-encryption key, and then the data-encryption key is encrypted with a key-encryption key. Despite its deployment in major cloud services, as far as we know, envelope encryption has not received any formal treatment. To address this issue, we first formalize the syntax and security requirements of envelope encryption in the symmetric-key setting. Then, we show that...

Fault attacks impose a serious threat against the practical implementations of cryptographic algorithms. Statistical Ineffective Fault Attacks (SIFA), exploiting the dependency between the secret data and the fault propagation overcame many of the known countermeasures. Later, several countermeasures have been proposed to tackle this attack using error detection methods. However, the efficiency of the countermeasures, in part governed by the number of error checks, still remains a...

Zero-Knowledge Virtual Machines (ZKVMs) have gained traction in recent years due to their potential applications in a variety of areas, particularly blockchain ecosystems. Despite tremendous progress on ZKVMs in the industry, no formal definitions or security proofs have been established in the literature. Due to this lack of formalization, existing protocols exhibit significant discrepancies in terms of problem definitions and performance metrics, making it difficult to analyze and compare...

The Boomerang attack was one of the first attempts to visualize a cipher ($E$) as a composition of two sub-ciphers ($E_0\circ E_1$) to devise and exploit two high-probability (say $p,q$) shorter trails instead of relying on a single low probability (say $s$) longer trail for differential cryptanalysis. The attack generally works whenever $p^2 \cdot q^2 > s$. However, it was later succeeded by the so-called ``sandwich attack'' which essentially splits the cipher in three parts $E'_0\circ E_m...

We give a semantic characterization of leakage-freeness through timing side-channels for Jasmin programs. Our characterization also covers probabilistic Jasmin programs that are not constant-time. In addition, we provide a characterization in terms of probabilistic relational Hoare logic and prove equivalence of both definitions. We also prove that our new characterizations are compositional. Finally, we relate new definitions to the existing ones from prior work which only apply to...

Many modern security protocols such as TLS, WPA2, WireGuard, and Signal use a cryptographic primitive called Authenticated Encryption (optionally with Authenticated Data), also known as an AEAD scheme. AEAD is a variant of symmetric encryption that additionally provides authentication. While authentication may seem to be a straightforward additional requirement, it has in fact turned out to be complex: many different security notions for AEADs are still being proposed, and several recent...

The recent technological advances in Post-Quantum Cryptography (PQC) raise the questions of robust implementations of new asymmetric cryptography primitives in today's technology. This is the case for the lattice-based Module Lattice-Key Encapsulation Mechanism (ML-KEM) algorithm which is proposed by the National Institute of Standards and Technology (NIST) as the first standard for Key Encapsulation Mechanism (KEM), taking inspiration from CRYSTALS-Kyber. We must ensure that the ML-KEM...

This paper presents a provably secure, higher-order, and leakage-resilient (LR) rekeying scheme named LR Rekeying with Random oracle Repetition (LR4), along with a quantitative security evaluation methodology. Many existing LR primitives are based on a concept of leveled implementation, which still essentially require a leak-free sanctuary (i.e., differential power analysis (DPA)-resistant component(s)) for some parts. In addition, although several LR pseudorandom functions (PRFs) based on...

The stream cipher ChaCha is one of the most widely used ciphers in the real world, such as in TLS, SSH and so on. In this paper, we study the security of ChaCha via differential cryptanalysis based on probabilistic neutrality bits (PNBs). We introduce the \textit{syncopation} technique for the PNB-based approximation in the backward direction, which significantly amplifies its correlation by utilizing the property of ARX structure. In virtue of this technique, we present a new and efficient...

Zero-knowledge proof systems are becoming increasingly prevalent and being widely used to secure decentralized financial systems and protect the privacy of users. Given the sensitivity of these applications, zero-knowledge proof systems are a natural target for formal verification methods. We describe methods for checking one such proof system: Halo2. We use abstract interpretation and an SMT solver to check various properties of Halo2 circuits. Using abstract interpretation, we can detect...

This paper introduces arithmetic sketching, an abstraction of a primitive that several previous works use to achieve lightweight, low-communication zero-knowledge verification of secret-shared vectors. An arithmetic sketching scheme for a language $\mathcal{L} \in \mathbb{F}^n$ consists of (1) a randomized linear function compressing a long input x to a short “sketch,” and (2) a small arithmetic circuit that accepts the sketch if and only if $x \in \mathcal{L}$, up to some small error. If...

Quantitative analyses of the costs of cryptographic attack algorithms play a central role in comparing cryptosystems, guiding the search for improved attacks, and deciding which cryptosystems to standardize. Unfortunately, these analyses often turn out to be wrong. Sometimes errors are not caught until years later. This paper introduces CryptAttackTester (CAT), a software framework for high-assurance quantification of attack effectiveness. CAT enforces complete definitions of attack...

We implement the Schnorr protocol in assembler via the Jasmin toolchain, and prove the security (proof-of-knowledge and zero-knowledge properties) and the absence of leakage through timing side-channels of that implementation in EasyCrypt. In order to do so, we provide a semantic characterization of leakage-freeness for probabilistic Jasmin programs (that are not constant-time). We design a library for multiple-precision integer arithmetic in Jasmin -- the "libjbn'' library. Among others,...

In this paper, we take inspiration from an invited talk presented at CBCrypto'23 to design identification protocols and signature schemes from group actions using the MPC-in-the-head paradigm. We prove the security of the given identification schemes and rely on the Fiat-Shamir transformation to turn them into signatures. We also establish a parallel with the technique used for the MPC-in-the-head approach and the seed tree method that has been recently used in some signature and ring...

A zero-knowledge proof (ZKP) is a powerful cryptographic primitive used in many decentralized or privacy-focused applications. However, the high overhead of ZKPs can restrict their practical applicability. We design a programming language, Ou, aimed at easing the programmer's burden when writing efficient ZKPs, and a compiler framework, Lian, that automates the analysis and distribution of statements to a computing cluster. Lian uses programming language semantics, formal methods, and...

Succinct Non-interactive Arguments of Knowledge (SNARKs) have seen interest and development from the cryptographic community over recent years, and there are now constructions with very small proof size designed to work well in practice. A SNARK protocol can only be widely accepted as secure, however, if a rigorous proof of its security properties has been vetted by the community. Even then, it is sometimes the case that these security proofs are flawed, and it is then necessary for further...

Cryptographic hash functions are used inside many applications that critically rely on their resistance against cryptanalysis attacks and the correctness of their implementations. Nevertheless, vulnerabilities in cryptographic hash function implementations can remain unnoticed for more than a decade, as shown by the recent discovery of a buffer overflow in the implementation of SHA-3 in the eXtended Keccak Code Package (XKCP), impacting Python, PHP, and several other software projects. This...

In this work, we present a novel generic construction for a Distributed Key Generation (DKG) scheme. Our generic construction relies on three modular cryptographic building blocks. The first is an aggregatable Verifiable Secret Sharing (AgVSS) scheme, the second is a Non-Interactive Key Exchange (NIKE) scheme, and the third is a secure hash function. We give formal definitions for the AgVSS and NIKE schemes, as well as concrete constructions. The utility of this generic construction is...

This paper presents a correct-by-construction method of designing an FHE model based on the automated program verifier Dafny. We model FHE operations from the ground up, including fundamentals like GCD, coprimality, Montgomery multiplications, and polynomial operations, etc., and higher level optimizations such as Residue Number System (RNS) and Number Theoretic Transform (NTT). The fully formally verified FHE model serves as a reference design for both software stack development and...

In the recent work of (Cheon & Lee, Eurocrypt'22), the concept of a degree-$D$ packing method was formally introduced, which captures the idea of embedding multiple elements of a smaller ring into a larger ring, so that element-wise multiplication in the former is somewhat "compatible" with the product in the latter. Then, several optimal bounds and results are presented, and furthermore, the concept is generalized from one multiplication to degrees larger than two. These packing...

Critical and widely used cryptographic protocols have repeatedly been found to contain flaws in their design and their implementation. A prominent class of such vulnerabilities is logical attacks, e.g. attacks that exploit flawed protocol logic. Automated formal verification methods, based on the Dolev-Yao (DY) attacker, formally define and excel at finding such flaws, but operate only on abstract specification models. Fully automated verification of existing protocol implementations is...

Developers of computer-aided cryptographic tools are optimistic that formal methods will become a vital part of developing new cryptographic systems. We study the use of such tools to specify and verify the implementation of Classic McEliece, one of the code-based cryptography candidates in the fourth round of the NIST Post-Quantum standardisation Process. From our case study we draw conclusions about the practical applicability of these methods to the development of novel cryptography.

This work introduces DSKE, digital signatures with key extraction. In a DSKE scheme, the private key can be extracted if more than a threshold of signatures on different messages are ever created while, within the threshold, each signature continues to authenticate the signed message. We give a formal definition of DSKE, as well as two provably secure constructions, one from hash-based digital signatures and one from polynomial commitments. We demonstrate that DSKE is useful for...

Modern attestation based on Trusted Execution Environments (TEEs) can significantly reduce the risk of secret compromise, allowing users to securely perform sensitive computations such as running cryptographic protocols for authentication across security critical services. However, this has also made TEEs a high-value attack target, driving an arms race between novel compromise attacks and continuous TEEs updates. Ideally, we want to achieve Post-Compromise Security (PCS): even after a...

There has been intense interest over the last decade in implementations of _probabilistic proofs_ (IPs, SNARKs, PCPs, and so on): protocols in which an untrusted party proves to a verifier that a given computation was executed properly, possibly in zero knowledge. Nevertheless, implementations still do not scale beyond small computations. A central source of overhead is the _front-end_: translating from the abstract computation to a set of equivalent arithmetic constraints. This paper...

The privacy set intersection (PSI) protocol with the oblivious pseudorandom function (OPRF) as the core component is a crucial member of PSI family, and the most efficient PSI protocol at present also belongs to this category. Based on DDH assumption, Hash Diffie-Hellman (HashDH) PSI is one of the most classical PSI protocols. Benefiting by its low communication overhead, it still has tremendous research value today. The OPRF subprotocol at the bottom of classical DH-PSI protocol falls into...

A cryptographic accumulator is a space- and time-efficient data structure with associated algorithms used for secure membership testing. In the growing space of digital credentials, accumulators found in managing a set of valid credentials, giving efficient and anonymous methods for credential holders to prove their validity. Unlike traditional credentials like digital signatures, one can easily revoke credentials with an accumulator; however, each revocation forces existing credential...

Success Rate (SR) is one of the most popular security metrics measuring the efficiency of side-channel attacks. Theoretical expression reveals the functional dependency on critical parameters such as number of measurements and Signal-to-Noise Ratio (SNR), helping evaluators understand the threat of an attack as well as how one can mitigate it with proper countermeasures. However so far, existing works have exposed fundamental problems such as: (i) the evaluations are restricted to a very...

The current gold standard of cryptographic software is to write efficient libraries with systematic protections against timing attacks. In order to meet this goal, cryptographic engineers increasingly use high-assurance cryptography tools. These tools guide programmers and provide rigorous guarantees that can be verified independently by library users. However, high-assurance tools reason about overly simple execution models that elide micro-architectural leakage. Thus, implementations...

Secure messaging schemes such as the Signal protocol rely on out-of-band channels to verify the authenticity of long-running communication. Such out-of-band checks however are only rarely actually performed by users in practice. In this paper, we propose a new method for performing continuous authentication during a secure messaging session, without the need for an out-of-band channel. Leveraging the users' long-term secrets, our Authentication Steps extension guarantees authenticity as...

At Eurocrypt 2022, May et al. proposed a partial key exposure (PKE) attack on CRT-RSA that efficiently factors $N$ knowing only a $\frac{1}{3}$-fraction of either most significant bits (MSBs) or least significant bits (LSBs) of private exponents $d_p$ and $d_q$ for public exponent $e \approx N^{\frac{1}{12}}$. In practice, PKE attacks typically rely on the side-channel leakage of these exponents, while a side-channel resistant implementation of CRT-RSA often uses additively blinded exponents...

Elections are an important corner stone of democratic processes. In addition to publishing the final result (e.g., the overall winner), elections typically publish the full tally consisting of all (aggregated) individual votes. This causes several issues, including loss of privacy for both voters and election candidates as well as so-called Italian attacks that allow for easily coercing voters. Several e-voting systems have been proposed to address these issues by hiding (parts of) the...

During the last decades, many advances in the field of automated security protocol analysis have seen the field mature and grow from being applicable to toy examples, to modeling intricate protocol standards and finding real-world vulnerabilities that extensive manual analysis had missed. However, modern security protocols often contain elements for which such tools were not originally designed, such as protocols that construct, by design, terms of unbounded size, such as counters, trees,...

KEMTLS is a proposal for changing the TLS handshake to authenticate the handshake using long-term key encapsulation mechanism keys instead of signatures, motivated by trade-offs in the characteristics of post-quantum algorithms. Prior proofs of security of KEMTLS and its variant KEMTLS-PDK have been hand-written proofs in the reductionist model under computational assumptions. In this paper, we present computer-verified symbolic analyses of KEMTLS and KEMTLS-PDK using two distinct Tamarin...

Consider a computer user who needs to update a piece of software installed on their computing device. To do so securely, a commonly accepted ad-hoc method stipulates that the old software version first retrieves the update information from the vendor's public repository, then checks that a cryptographic signature embedded into it verifies with the vendor's public key, and finally replaces itself with the new version. This updating method seems to be robust and lightweight, and to reliably...

We formalize security properties of zero-knowledge protocols and their proofs in EasyCrypt. Specifically, we focus on sigma-protocols (three-round protocols). Most importantly, we also cover properties whose security proofs require the use of rewinding; prior work has focused on properties that do not need this more advanced technique. On our way we give generic definitions of the main properties associated with sigma protocols, both in the computational and ...

Recently, Chaum et al. (ACNS'21) introduced $\mathcal{S}_{leeve}$, which describes an extra security layer for signature schemes, i.e., ECDSA. This distinctive feature is a new key generation mechanism, allowing users to generate a ''back up key'' securely nested inside the secret key of a signature scheme. Using this novel construction, the ''back up key'', which is secret, can be used to generate a ''proof of ownership'', i.e., only the rightful owner of this secret key can generate such...

Symbolic security protocol verifiers have reached a high degree of automation and maturity. Today, experts can model real-world protocols, but this often requires model-specific encodings and deep insight into the strengths and weaknesses of each of those tools. With Sapic , we introduce a protocol verification platform that lifts this burden and permits choosing the right tool for the job, at any development stage. We build on the existing compiler from Sapic to Tamarin, and extend it with...

In theory, Fully Homomorphic Encryption schemes allow users to compute any operation over encrypted data. However in practice, one of the major difficulties lies into determining secure cryptographic parameters that minimize the computational cost of evaluating a circuit. In this paper, we propose a solution to solve this open problem. Even though it mainly focuses on TFHE, the method is generic enough to be adapted to all the current FHE schemes. TFHE is particularly suited, for small...

We propose a novel obfuscation technique that can be used to outsource hard satisfiability (SAT) formulas to the cloud. Servers with large computational power are typically used to solve SAT instances that model real-life problems in task scheduling, AI planning, circuit verification and more. However, outsourcing data to the cloud may lead to privacy and information breaches since satisfying assignments may reveal considerable information about the underlying problem modeled by SAT. In...

Cryptographic constant-time (CT) is a popular programming disci- pline used by cryptographic libraries to protect themselves against timing attacks. The CT discipline aims to enforce that program ex- ecution does not leak secrets, where leakage is defined by a formal leakage model. In practice, different leakage models coexist, some- times even within a single library, both to reflect different architec- tures and to accommodate different security-efficiency trade-offs. Constant-timeness is...

The celebrated Nakamoto consensus protocol ushered in several new consensus applications including cryptocurrencies. A few recent works have analyzed important properties of blockchains, including most significantly, consistency, which is a guarantee that all honest parties output the same sequence of blocks throughout the execution of the protocol. To establish consistency, the prior analysis of Pass, Seeman and shelat required a careful counting of certain combinatorial events that was...

The protection of cryptographic software implementations against power-analysis attacks is critical for applications in embedded systems. A commonly used algorithmic countermeasure against these attacks is masking, a secret-sharing scheme that splits a sensitive computation into computations on multiple random shares. In practice, the security of masking schemes relies on several assumptions that are often violated by microarchitectural side-effects of CPUs. Many past works address this...

We consider the problem of sampling random supersingular elliptic curves over finite fields of cryptographic size (SRS problem). The currently best-known method combines the reduction of a suitable complex multiplication (CM) $j$-invariant and a random walk over some supersingular isogeny graph. Unfortunately, this method is not suitable for numerous cryptographic applications because it gives information about the endomorphism ring of the generated curve. This motivates a stricter version...

Research on the design of masked cryptographic hardware circuits in the past has mostly focused on reducing area and randomness requirements. However, many embedded devices like smart cards and IoT nodes also need to meet certain performance criteria, which is why the latency of masked hardware circuits also represents an important metric for many practical applications. The root cause of latency in masked hardware circuits is the need for additional register stages that synchronize the...

An $\ell$-server Private Information Retrieval (PIR) scheme allows a client to retrieve the $\tau$-th element $a_\tau$ from a database $\bm{a}=(a_1,\ldots,a_n)$ which is replicated among $\ell$ servers. It is called $t$-private if any coalition of $t$ servers learns no information on $\tau$, and $b$-error correcting if a client can correctly compute $a_\tau$ from $\ell$ answers containing $b$ errors. This paper concerns the following problems: Is there a $t$-private $\ell$-server PIR scheme...

In 2002, Chow et al. initiated the formal study of white-box cryptography and introduced the CEJO framework. Since then, various white-box designs based on their framework have been proposed, all of them broken. Ranea and Preneel proposed a different method in 2020, called self-equivalence encodings and analyzed its security for AES. In this paper, we apply this method to generate the first academic white-box Speck implementations using self-equivalence encodings. Although we focus on Speck...

We provide the first mechanized post-quantum sound security protocol proofs. We achieve this by developing PQ-BC, a computational first-order logic that is sound with respect to quantum attackers, and corresponding mechanization support in the form of the PQ-Squirrel prover. Our work builds on the classical BC logic [Bana,Comon,CCS14] and its mechanization in the Squirrel prover [BDJKM,S&P21]. Our development of PQ-BC requires making the BC logic sound for a single interactive quantum...

A leading approach to enhancing the performance and scalability of permissionless blockchains is to use the payment channel, which allows two users to perform off-chain payments with almost unlimited frequency. By linking payment channels together to form a payment channel network, users connected by a path of channels can perform off-chain payments rapidly. However, payment channels risk encountering fund depletion, which threatens the availability of both the payment channel and network....

We introduce verifiable partially-decryptable commitments (VPDC), as a building block for constructing efficient privacy-preserving protocols supporting auditability by a trusted party. A VPDC is an extension of a commitment along with an accompanying proof, convincing a verifier that (i) the given commitment is well-formed and (ii) a certain part of the committed message can be decrypted using a (secret) trapdoor known to a trusted party. We first formalize VPDCs and then introduce a...

Related-key attacks (RKA) are powerful cryptanalytic attacks, where the adversary can tamper with the secret key of a cryptographic scheme. Since their invention, RKA security has been an important design goal in cryptography, and various works aim at designing cryptographic primitives that offer protection against related-key attacks. At EUROCRYPT'03, Bellare and Kohno introduced the first formal treatment of related-key attacks focusing on pseudorandom functions and permutations. This was...

In this work, we perform a formal analysis of definitions of non-malleability for commitment schemes in the EasyCrypt theorem prover. There are two distinct formulations of non-malleability found in the literature: the comparison-based definition and the simulation- based definition. In this paper, we do a formal analysis of both. We start by formally proving that the comparison-based definition which was originally introduced by Laur et al. is unsatisfiable. Also, we propose a novel...

We describe a technique to backdoor a prime factor of a composite odd integer $N$, so that an attacker knowing a possibly secret factor base $\mathcal{B}$, can efficiently retrieve it from $N$. Such method builds upon Complex Multiplication theory for elliptic curves, by generating primes $p$ associated to $\mathcal{B}$-smooth order elliptic curves over $\mathbb{F}_p$. When such primes $p$ divide an integer $N$, the latter can be efficiently factored using a generalization of Lenstra's...

We introduce Precio, a new secure aggregation method for computing layered histograms and sums over secret shared data in a client-server setting. Precio is motivated by ad conversion measurement scenarios, where online advertisers and ad networks want to measure the performance of ad campaigns without requiring privacy-invasive techniques, such as third-party cookies. Precio has linear (time and communication) complexity in the number of data points and guarantees differentially private...

Side-channel attacks are a powerful class of attacks targeting cryptographic devices. Masking is a popular protection technique to thwart such attacks as it can be theoretically proven secure. However, correctly implementing masking schemes is a non-trivial task and error-prone. If several techniques have been proposed to formally verify masked implementations, they all come with limitations regarding expressiveness, scalability or accuracy. In this work, we propose a symbolic approach,...

Homomorphic Encryption (HE) is a useful primitive for secure computation, but it is not generally applicable when multiple parties are involved, as the authority is solely concentrated in a single party, the secret key owner. To solve this issue, several variants of HE have emerged in the context of multiparty setting, resulting in two major lines of work -- Multi-Party HE (MPHE) and Multi-Key HE (MKHE). In short, MPHEs tend to be more efficient, but all parties should be specified at the...

In many cryptocurrencies, the problem of key management has become one of the most fundamental security challenges. Typically, keys are kept in designated schemes called 'Wallets', whose main purpose is to store these keys securely. One such system is the BIP32 wallet (Bitcoin Improvement Proposal 32), which since its introduction in 2012 has been adopted by countless Bitcoin users and is one of the most frequently used wallet system today. Surprisingly, very little is known about the...

In this paper we derive a suite of lemmas which allows users to internally reflect EasyCrypt programs into distributions which correspond to their denotational semantics (probabilistic reflection). Based on this we develop techniques for reasoning about rewinding of adversaries in EasyCrypt. (A widely used technique in cryptology.) We use our reflection and rewindability results to prove the security of a coin-toss protocol.