A privately constrained pseudorandom function (pCPRF) is a PRF with the additional property that one can derive a constrained key that allows evaluating the PRF only on inputs satisfying a constraint predicate $C$, without revealing $C$ itself or leaking information about the PRF’s output on inputs that do not satisfy the constraint. Existing privately constrained PRFs face significant limitations: either (1) they rely on assumptions known to imply fully-homomorphic encryption or...

Recent server-side optimizations like speculative decoding significantly enhance the interactivity and resource efficiency of Large Language Model (LLM) services. However, we show that these optimizations inadvertently introduce new side-channel vulnerabilities through network packet timing and size variations that tend to be input-dependent. Network adversaries can leverage these side channels to learn sensitive information contained in \emph{encrypted} user prompts to and responses from...

This paper presents a novel approach to verifiable vote tallying using additive homomorphism, which can be appended to existing voting systems without modifying the underlying infrastructure. Existing End-to-End Verifiable (E2E-V) systems like Belenios and ElectionGuard rely on distributed trust models or are vulnerable to decryption compromises, making them less suitable for general elections. Our approach introduces a tamper-evident commitment to votes through cryptographic hashes...

In a single secret leader election (SSLE) protocol, all parties collectively and obliviously elect one leader. No one else should learn its identity unless it reveals itself as the leader. The problem is first formalized by Boneh \textit{et al.} (AFT'20), which proposes an efficient construction based on the Decision Diffie-Hellman (DDH) assumption. Considering the potential risk of quantum computers, several follow-ups focus on designing a post-quantum secure SSLE protocol based on pure...

Introduced by Halevi, Ishai, Kushilevitz, and Rabin (CRYPTO 2023), Additive randomized encodings (ARE) reduce the computation of a $k$-party function $f(x_1,\dots,x_k)$ to locally computing encodings $\hat x_i$ of each input $x_i$ and then adding them together over some Abelian group into an output encoding $\hat y = \sum \hat x_i$, which reveals nothing but the result. The appeal of ARE comes from the simplicity of the non-local computation, involving only addition. This gives rise for...

Threshold fully homomorphic encryption (ThFHE) enables multiple parties to compute functions over their sensitive data without leaking data privacy. Most of existing ThFHE schemes are restricted to full threshold and require the participation of all parties to output computing results. Compared with these full-threshold schemes, arbitrary threshold (ATh)-FHE schemes are robust to non-participants and can be a promising solution to many real-world applications. However, existing AThFHE...

Hybrid Homomorphic Encryption (HHE) is considered a promising solution for key challenges that emerge when adopting Homomorphic Encryption (HE). In cases such as communication and computation overhead for clients and storage overhead for servers, it combines symmetric cryptography with HE schemes. However, despite a decade of advancements, enhancing HHE usability, performance, and security for practical applications remains a significant stake. This work contributes to the field by...

Privacy and security have become critical priorities in many scenarios. Privacy-preserving computation (PPC) is a powerful solution that allows functions to be computed directly on encrypted data. Garbled circuit (GC) is a key PPC technology that enables secure, confidential computing. GC comes in two forms: Boolean GC supports all operations by expressing functions as logic circuits; arithmetic GC is a newer technique to efficiently compute a set of arithmetic operations like addition and...

We present an encrypted multi-map, a fundamental data structure underlying searchable encryption/structured encryption. Our protocol supports updates and is designed for applications demanding very strong data security. Not only it hides the information about queries and data, but also the query, access, and volume patterns. Our protocol utilizes a position-based ORAM and an encrypted dictionary. We provide two instantiations of the protocol, along with their operation-type-revealing...

Shadow is a family of lightweight block ciphers introduced by Guo, Li, and Liu in 2021, with Shadow-32 having a 32-bit block size and a 64-bit key, and Shadow-64 having a 64-bit block size and a 128-bit key. Both variants use a generalized Feistel network with four branches, incorporating the AND-Rotation-XOR operation similar to the Simon family for their bridging function. This paper reveals that the security claims of the Shadow family are not as strong as suggested. We present a key...

A garbling scheme transforms a program (e.g., circuit) $C$ into a garbled program $\hat{C}$, along with a pair of short keys $(k_{i,0},k_{i,1})$ for each input bit $x_i$, such that $(C,\hat{C}, \{k_{i,x_i}\})$ can be used to recover the output $z = C(x)$ while revealing nothing else about the input $x$. This can be naturally generalized to partial garbling, where part of the input is public, and a computation $z = C(x, y)$ is decomposed into a public part $C_{\text{pub}}(x)$, depending only...

This article presents a cryptanalysis of a 19th-century encrypted manuscript discovered in the archives of Conde de Siete Fuentes in Tenerife, Canary Islands, Spain. The manuscript, preserved by the heirs of the 6th Count of Valle de Salazar, utilizes a polyalphabetic substitution cipher. The cryptanalysis was performed by applying statistical frequency analysis and developing a Python script for decryption, resulting in the authors successfully deciphering the message. The decrypted letter...

Private set intersection (PSI) allows any two parties (say client and server) to jointly compute the intersection of their sets without revealing anything else. Fully homomorphic encryption (FHE)-based PSI is a cryptographic solution to implement PSI-based protocols. Most FHE-based PSI protocols implement hash function approach and oblivious transfer approach. The main limitations of their protocols are 1) high communication complexity, that is, $O(xlogy)$ (where $x$ is total number of...

The evasive LWE assumption, proposed by Wee [Eurocrypt'22 Wee] for constructing a lattice-based optimal broadcast encryption, has shown to be a powerful assumption, adopted by subsequent works to construct advanced primitives ranging from ABE variants to obfuscation for null circuits. However, a closer look reveals significant differences among the precise assumption statements involved in different works, leading to the fundamental question of how these assumptions compare to each other. In...

As dataset sizes grow, users increasingly rely on encrypted data and secure range queries on cloud servers, raising privacy concerns about potential data leakage. Order-revealing encryption (ORE) enables efficient operations on numerical datasets, and Delegatable ORE (DORE) extends this functionality to multi-client environments, but it faces risks of token forgery. Secure DORE (SEDORE) and Efficient DORE (EDORE) address some vulnerabilities, with EDORE improving speed and storage...

Private Set Intersection (PSI) is a cryptographic primitive that allows two parties to obtain the intersection of their private input sets while revealing nothing more than the intersection. PSI and its numerous variants, which compute on the intersection of items and their associated weights, have been widely studied. In this paper, we revisit the problem of finding the best item in the intersection according to weight sum introduced by Beauregard et al. (SCN '22), which is a special...

This paper introduces the Pad Thai message recovery attack on REDOG, a rank-metric code-based encryption scheme selected for the second round of evaluation in the Korean Post-Quantum Cryptography (KPQC) competition. The attack exploits the low rank weight of a portion of the ciphertext to construct multiple systems of linear equations, one of which is noise-free and can be solved to recover the secret message. The Pad Thai attack significantly undermines the security of...

Electronic voting (e-voting) systems have become more prevalent in recent years, but security concerns have also increased, especially regarding the privacy and verifiability of votes. As an essential ingredient for constructing secure e-voting systems, designers often employ zero-knowledge proofs (ZKPs), allowing voters to prove their votes are valid without revealing them. Invalid votes can then be discarded to protect verifiability without compromising the privacy of valid...

We extend the concept of access control for functional encryption, introduced by Abdalla et al. (ASIACRYPT 2020), to function-revealing encryption (Joy and Passelègue, SCN 2018). Here “access control” means that function evaluation is only possible when a specified access policy is met. Specifically, we introduce access-controlled inner product function-revealing encryption (AC-IPFRE) and give two applications. On the theoretical side, we use AC-IPFRE to show that function-hiding...

The no-cloning principle has played a foundational role in quantum information and cryptography. Following a long-standing tradition of studying quantum mechanical phenomena through the lens of interactive games, Broadbent and Lord (TQC 2020) formalized cloning games in order to quantitatively capture no-cloning in the context of unclonable encryption schemes. The conceptual contribution of this paper is the new, natural, notion of Haar cloning games together with two applications. In the...

Approximate nearest neighbor search (ANNS), also known as vector search, is an important building block for varies applications, such as databases, biometrics, and machine learning. In this work, we are interested in the private ANNS problem, where the client wants to learn (and can only learn) the ANNS results without revealing the query to the server. Previous private ANNS works either suffers from high communication cost (Chen et al., USENIX Security 2020) or works under a weaker...

Non-interactive zero-knowledge proofs (NIZK) are essential building blocks in threshold cryptosystems like multiparty signatures, distributed key generation, and verifiable secret sharing, allowing parties to prove correct behavior without revealing secrets. Furthermore, universally composable (UC) NIZKs enable seamless composition in the larger cryptosystems. A popular way to construct NIZKs is to compile interactive protocols using the Fiat-Shamir transform. Unfortunately, Fiat-Shamir...

Falcon is one of the three postquantum signature schemes already selected by NIST for standardization. It is the most compact among them, and offers excellent efficiency and security. However, it is based on a complex algorithm for lattice discrete Gaussian sampling which presents a number of implementation challenges. In particular, it relies on (possibly emulated) floating-point arithmetic, which is often regarded as a cause for concern, and has been leveraged in, e.g., side-channel...

We propose a dynamic, low-latency encrypted multi-map (EMM) that operates in two modes: low-leakage mode, which reveals minimal information such as data size, expected response length, and query arrival rate; and subliminal mode, which reveals only the data size while hiding metadata including query and update times, the number of operations executed, and even whether an operation was executed at all---albeit at the cost of full correctness. We achieve this by exploiting a tradeoff...

In aggregation queries, predicate parameters often reveal user intent. Protecting these parameters is critical for user privacy, regardless of whether the database is public or private. While most existing works focus on private data settings, we address a public data setting where the server has access to the database. Current solutions for this setting either require additional setups (e.g., noncolluding servers, hardware enclaves) or are inefficient for practical workloads. Furthermore,...

End-to-end encrypted cloud storage offers a way for individuals and organisations to delegate their storage needs to a third-party, while keeping control of their data using cryptographic techniques. We conduct a cryptographic analysis of various products in the ecosystem, showing that many providers fail to provide an adequate level of security. In particular, we provide an in-depth analysis of five end-to-end encrypted cloud storage systems, namely Sync, pCloud, Icedrive, Seafile, and...

We propose a new private Approximate Nearest Neighbor (ANN) search scheme named Pacmann that allows a client to perform ANN search in a vector database without revealing the query vector to the server. Unlike prior constructions that run encrypted search on the server side, Pacmann carefully offloads limited computation and storage to the client, no longer requiring computationally-intensive cryptographic techniques. Specifically, clients run a graph-based ANN search, where in each hop on...

In scenarios where a seller holds sensitive data $x$, like employee / patient records or ecological data, and a buyer seeks to obtain an evaluation of specific function $f$ on this data, solutions in trustless digital environments like blockchain-based Web3 systems typically fall into two categories: (1) Smart contract-powered solutions and (2) cryptographic solutions leveraging tools such as adaptor signatures. The former approach offers atomic transactions where the buyer learns the...

In August 2021, Liu et al. (IEEE TIFS'21) proposed a privacy-enhanced framework named PEFL to efficiently detect poisoning behaviours in Federated Learning (FL) using homomorphic encryption. In this article, we show that PEFL does not preserve privacy. In particular, we illustrate that PEFL reveals the entire gradient vector of all users in clear to one of the participating entities, thereby violating privacy. Furthermore, we clearly show that an immediate fix for this issue is still...

Forward-Secure Key-Encapsulation Mechanism (FS-KEM; Canetti et al. Eurocrypt 2003) allows Alice to encapsulate a key $k$ to Bob for some time $t$ such that Bob can decapsulate it at any time $t'\leq t$. Crucially, a corruption of Bob's secret key after time $t$ does not reveal $k$. In this work, we generalize and extend this idea by also taking Post-Compromise Security (PCS) into account and call it Interval Key-Encapsulation Mechanism (IKEM). Thus, we do not only protect confidentiality...

Private set union (PSU) is a cryptographic protocol that allows two parties to compute the union of their sets without revealing anything else. Despite some efficient PSU protocols that have been proposed, they mainly focus on the balanced setting, where the sets held by the parties are of similar size. Recently, Tu et al. (CCS 2023) proposed the first unbalanced PSU protocol which achieves sublinear communication complexity in the size of the larger set. In this paper, we are interested...

Always Encrypted (AE) is a Microsoft SQL Server feature that allows clients to encrypt sensitive data inside client applications and ensures that the sensitive data is hidden from untrusted servers and database administrators. AE offers two column-encryption options: deterministic encryption (DET) and randomized encryption (RND). In this paper, we explore the security implications of using AE with both DET and RND encryption modes by running Leakage Abuse Attacks (LAAs) against the system....

Making the most of TFHE programmable bootstrapping to evaluate functions or operators otherwise challenging to perform with only the native addition and multiplication of the scheme is a very active line of research. In this paper, we systematize this approach and apply it to build an 8-bit FHE processor abstraction, i.e., a software entity that works over FHE-encrypted 8-bit data and presents itself to the programmer by means of a conventional-looking assembly instruction set. In doing so,...

Censorship circumvention tools enable clients to access endpoints in a network despite the presence of a censor. Censors use a variety of techniques to identify content they wish to block, including filtering traffic patterns that are characteristic of proxy or circumvention protocols and actively probing potential proxy servers. Circumvention practitioners have developed fully encrypted protocols (FEPs), intended to have traffic that appears indistinguishable from random. A FEP is typically...

Garbled circuits (GC) are a secure multiparty computation protocol that enables two parties to jointly compute a function using their private data without revealing it to each other. While garbled circuits are proven secure at the protocol level, implementations can still be vulnerable to side-channel attacks. Recently, side-channel analysis of GC implementations has garnered significant interest from researchers. We investigate popular open-source GC frameworks and discover that the AES...

Privacy Set Intersection (PSI) has been an important research topic within privacy computation. Its main function is to allow two parties to compute the intersection of their private sets without revealing any other private information. Therefore, PSI can be applied to various real-world scenarios. Chase and Miao presented an impressive construction ``Private set intersection in the Internet setting from lightweight oblivious prf'' (CM20 for short) at Crypto 2020, highlighting its...

Message Authentication Codes (MACs) are ubiquitous primitives deployed in multiple flavors through standards such as HMAC, CMAC, GMAC, LightMAC, and many others. Its versatility makes it an essential building block in applications necessitating message authentication and integrity checks, in authentication protocols, authenticated encryption schemes, or as a pseudorandom or key derivation function. Its usage in this variety of settings makes it susceptible to a broad range of attack...

Private Set Union (PSU) protocol allows parties, each holding an input set, to jointly compute the union of the sets without revealing anything else. In the literature, scalable PSU protocols follow the “split-execute-assemble” paradigm (Kolesnikov et al., ASIACRYPT 2019); in addition, those fast protocols often use Oblivious Transfer as building blocks. Kolesnikov et al. (ASIACRYPT 2019) and Jia et al. (USENIX Security 2022), pointed out that certain security issues can be introduced in the...

Laconic function evaluation (LFE) allows us to compress a circuit $f$ into a short digest. Anybody can use this digest as a public-key to efficiently encrypt some input $x$. Decrypting the resulting ciphertext reveals the output $f(x)$, while hiding everything else about $x$. In this work we consider LFE for Random-Access Machines (RAM-LFE) where, instead of a circuit $f$, we have a RAM program $f_{\mathsf{DB}}$ that potentially contains some large hard-coded data $\mathsf{DB}$. The...

Boolean Searchable Symmetric Encryption (SSE) enables secure outsourcing of databases to an untrusted server in encrypted form and allows the client to execute secure Boolean queries involving multiple keywords. The leakage of keyword pair result pattern (KPRP) in a Boolean search poses a significant threat, which reveals the intersection of documents containing any two keywords involved in a search and can be exploited by attackers to recover plaintext information about searched keywords...

The proliferation of artificial intelligence and big data has resulted in a surge in data demand and increased data dimensionality. This escalation has consequently heightened the costs associated with storage and processing. Concurrently, the confidential nature of data collected by various institutions, which cannot be disclosed due to personal privacy concerns, has exacerbated the challenges associated with data analysis and machine learning model training. Therefore, designing a secure...

With additively homomorphic encryption (AHE), one can compute, from input ciphertexts $\mathsf{Enc}(x_1),\ldots,\mathsf{Enc}(x_n)$, and additional inputs $y_1,\ldots,y_k$, a ciphertext $c_\textit{f}=\mathsf{Enc}(f(x_1,\ldots,x_n,y_1,\ldots, y_k))$ for any polynomial $f$ in which each monomial has total degree at most $1$ in the $x$-variables (but can be arbitrary in the $y$-variables). For AHE that satisfies a set of natural requirements, we give a non-interactive zero-knowledge proof...

With the rising popularity of DeFi applications it is important to implement protections for regular users of these DeFi platforms against large parties with massive amounts of resources allowing them to engage in market manipulation strategies such as frontrunning/backrunning. Moreover, there are many situations (such as recovery of funds from vulnerable smart contracts) where a user may not want to reveal their transaction until it has been executed. As such, it is clear that preserving...

Privacy-preserving decision tree evaluation (PDTE) allows a client that holds feature vectors to perform inferences against a decision tree model on the server side without revealing feature vectors to the server. Our work focuses on the non-interactive batched setting where the client sends a batch of encrypted feature vectors and then obtains classifications, without any additional interaction. This is useful in privacy-preserving credit scoring, biometric authentication, and many more...

In recent years, quantum technology has been rapidly developed. As security analyses for symmetric ciphers continue to emerge, many require an evaluation of the resources needed for the quantum circuit implementation of the encryption algorithm. In this regard, we propose the quantum circuit decision problem, which requires us to determine whether there exists a quantum circuit for a given permutation f using M ancilla qubits and no more than K quantum gates within the circuit depth D....

Blind signatures enable a receiver to obtain signatures on messages of its choice without revealing any message to the signer. Round-optimal blind signatures are designed as a two-round interactive protocol between a signer and receiver. Incidentally, the choice of message is not important in many applications, and is routinely set as a random (unstructured) message by a receiver. With the goal of designing more efficient blind signatures for such applications, Hanzlik (Eurocrypt '23)...

Security proofs for cryptographic primitives typically assume operations are executed in the correct sequence; however, insecure implementations or software-level attacks can disrupt control flows, potentially invalidating 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...

Transformer neural networks have gained significant traction since their introduction, becoming pivotal across diverse domains. Particularly in large language models like Claude and ChatGPT, the transformer architecture has demonstrated remarkable efficacy. This paper provides a concise overview of transformer neural networks and delves into their security considerations, focusing on covert channel attacks and their implications for the safety of large language models. We present a covert...

Running machine learning algorithms on encrypted data is a way forward to marry functionality needs common in industry with the important concerns for privacy when working with potentially sensitive data. While there is already a growing field on this topic and a variety of protocols, mostly employing fully homomorphic encryption or performing secure multiparty computation (MPC), we are the first to propose a protocol that makes use of a specialized encryption scheme that allows to do secure...

Oblivious message retrieval (OMR) allows resource-limited recipients to outsource the message retrieval process without revealing which messages are pertinent to which recipient. Its realizations in recent works leave an open problem: can an OMR scheme be both practical and provably secure against spamming attacks from malicious senders (i.e., DoS-resistant) under standard assumptions? In this paper, we first prove that a prior construction $\mathsf{OMRp2}$ is DoS-resistant under a...

Policy-compliant signatures (PCS) are a recently introduced primitive by Badertscher et al. [TCC 2021] in which a central authority distributes secret and public keys associated with sets of attributes (e.g., nationality, affiliation with a specific department, or age) to its users. The authority also enforces a policy determining which senders can sign messages for which receivers based on a joint check of their attributes. For example, senders and receivers must have the same nationality,...

Secure multi-party computation aims to allow a set of players to compute a given function on their secret inputs without revealing any other information than the result of the computation. In this work, we focus on the design of secure multi-party protocols for shared polynomial operations. We consider the classical model where the adversary is honest-but-curious, and where the coefficients (or any secret values) are either encrypted using an additively homomorphic encryption scheme or...

In the rapidly evolving fields of encryption and blockchain technologies, the efficiency and security of cryptographic schemes significantly impact performance. This paper introduces a comprehensive framework for continuous benchmarking in one of the most popular cryptography Rust libraries, fastcrypto. What makes our analysis unique is the realization that automated benchmarking is not just a performance monitor and optimization tool, but it can be used for cryptanalysis and innovation...

Order-Revealing Encryption (ORE) provides a practical solution for conducting range queries over encrypted data. Achieving a desirable privacy-efficiency tradeoff in designing ORE schemes has posed a significant challenge. At Asiacrypt 2018, Cash et al. proposed Parameter-hiding ORE (pORE), which specifically targets scenarios where the data distribution shape is known, but the underlying parameters (such as mean and variance) need to be protected. However, existing pORE constructions rely...

Multiparty private set intersection (mPSI) protocol is capable of finding the intersection of multiple sets securely without revealing any other information. However, its limitation lies in processing only those elements present in every participant's set, which proves inadequate in scenarios where certain elements are common to several, but not all, sets. In this paper, we introduce an innovative variant of the mPSI protocol named unbalanced quorum PSI to fill in the gaps of the mPSI...

At CRYPTO 2019, Gohr demonstrated that differential-neural distinguishers (DNDs) for Speck32/64 can learn more features than classical cryptanalysis's differential distribution tables (DDT). Furthermore, a non-classical key recovery procedure is devised by combining the Upper Confidence Bound (UCB) strategy and the BayesianKeySearch algorithm. Consequently, the time complexity of 11-round key recovery attacks on Speck32/64 is significantly reduced compared with the state-of-the-art results...

Decentralized Anonymous Credential (DAC) systems are increasingly relevant, especially when enhancing revocation mechanisms in the face of complex traceability challenges. This paper introduces IDEA-DAC, a paradigm shift from the conventional revoke-and-reissue methods, promoting direct and Integrity-Driven Editing (IDE) for Accountable DACs, which results in better integrity accountability, traceability, and system simplicity. We further incorporate an Edit-bound Conformity Check that...

This note examines a nuance in the methods employed for counting the adversarial online complexity in the security proofs of duplex-based modes, with a focus on authenticated encryption. A recent study by Gilbert et al., reveals an attack on a broad class of duplex-based authenticated encryption modes. In particular, their approach to quantifying the adversarial online complexity, which capture realistic attack scenarios, includes certain queries in the count which are not in the security...

Auctions, a long-standing method of trading goods and services, are a promising use case for decentralized finance. However, due to the inherent transparency property of blockchains, current sealed-bid auction implementations on smart contracts requires a bidder to send at least two transactions to the underlying blockchain: a bidder must first commit their bid in the first transaction during the bidding period and reveal their bid in the second transaction once the revealing period starts....

Proxy re-encryption (PRE) allows a proxy to transform a ciphertext intended for Alice (delegator) to another ciphertext intended for Bob (delegatee) without revealing the underlying message. Recently, a new variant of PRE, namely fine-grained PRE (FPRE), was proposed in [Zhou et al., Asiacrypt 2023]. Generally, FPRE is designed for a function family F: each re-encryption key rk_{A→B}^f is associated with a function f ∈ F, and with rk_{A→B}^f, a proxy can transform Alice's ciphertext...

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...

We introduce the notion of traceable mixnets. In a traditional mixnet, multiple mix-servers jointly permute and decrypt a list of ciphertexts to produce a list of plaintexts, along with a proof of correctness, such that the association between individual ciphertexts and plaintexts remains completely hidden. However, in many applications, the privacy-utility tradeoff requires answering some specific queries about this association, without revealing any information beyond the query result. We...

The Automatic Certificate Management Environment protocol (ACME) has significantly contributed to the widespread use of digital certificates in safeguarding the authenticity and privacy of Internet data. These certificates are required for implementing the Transport Layer Security (TLS) protocol. However, it is well known that the cryptographic algorithms employed in these certificates will become insecure with the emergence of quantum computers. This study assesses the challenges in...

Encrypted search schemes have been proposed to address growing privacy concerns. However, several leakage-abuse attacks have highlighted the shortcomings of these schemes. The literature remains vague about the consequences of these attacks for real-world applications: are these attacks dangerous in practice? Is it safe to use these schemes? Do we even need countermeasures? This paper introduces a novel mathematical model for attackers' knowledge using statistical estimators. Our model...

Secure computation enables mutually distrusting parties to jointly compute a function on their secret inputs, while revealing nothing beyond the function output. A long-running challenge is understanding the required communication complexity of such protocols---in particular, when communication can be sublinear in the circuit representation size of the desired function. Significant advances have been made affirmatively answering this question within the two-party setting, based on a...

Fully homomorphic encryption (FHE) is an advanced cryptography technique to allow computations (i.e., addition and multiplication) over encrypted data. After years of effort, the performance of FHE has been significantly improved and it has moved from theory to practice. The transciphering framework is another important technique in FHE to address the issue of ciphertext expansion and reduce the client-side computational overhead. To apply the transciphering framework to the CKKS FHE scheme,...

Privacy-preserving blueprint schemes (Kohlweiss et al., EUROCRYPT'23) offer a mechanism for safeguarding user's privacy while allowing for specific legitimate controls by a designated auditor agent. These schemes enable users to create escrows encrypting the result of evaluating a function $y=P(t,x)$, with $P$ being publicly known, $t$ a secret used during the auditor's key generation, and $x$ the user's private input. Crucially, escrows only disclose the blueprinting result $y=P(t,x)$...

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...

In their 2022 study, Kuang et al. introduced the Multivariable Polynomial Public Key (MPPK) cryptography, a quantum-safe public key cryptosystem leveraging the mutual inversion relationship between multiplication and division. MPPK employs multiplication for key pair construction and division for decryption, generating public multivariate polynomials. Kuang and Perepechaenko expanded the cryptosystem into the Homomorphic Polynomial Public Key (HPPK), transforming product polynomials over...

A $t$-out-of-$n$ threshold decryption system assigns key shares to $n$ parties so that any $t$ of them can decrypt a well-formed ciphertext. Existing threshold decryption systems are not secure when these parties are rational actors: an adversary can offer to pay the parties for their key shares. The problem is that a quorum of $t$ parties, working together, can sell the adversary a decryption key that reveals nothing about the identity of the traitor parties. This provides a risk-free...

Private Set Intersection (PSI) is a well-studied secure two-party computation problem in which a client and a server want to compute the intersection of their input sets without revealing additional information to the other party. With this work, we present nested Cuckoo hashing, a novel hashing approach that can be combined with additively homomorphic encryption (AHE) to construct an efficient PSI protocol for unbalanced input sets. We formally prove the security of our protocol against...

Private information retrieval (PIR) allows a client to read data from a server, without revealing which information they are interested in. A PIR is doubly efficient if the server runtime is, after a one-time pre-processing, sublinear in the database size. A recent breakthrough result from Lin, Mook, and Wichs [STOC’23] proposed the first-doubly efficient PIR with (online) server computation poly-logarithmic in the size of the database, assuming the hardness of the standard Ring-LWE...

Committing security has gained considerable attention in the field of authenticated encryption (AE). This can be traced back to a line of recent attacks, which entail that AE schemes used in practice should not only provide confidentiality and authenticity, but also committing security. Roughly speaking, a committing AE scheme guarantees that ciphertexts will decrypt only for one key. Despite the recent research effort in this area, the finalists of the NIST lightweight cryptography...

As concerns are increasingly raised about data privacy, encrypted database management system (DBMS) based on fully homomorphic encryption (FHE) attracts increasing research attention, as FHE permits DBMS to be directly outsourced to cloud servers without revealing any plaintext data. However, the real-world deployment of FHE-based DBMS faces two main challenges: i) high computational latency, and ii) lack of elastic query processing capability, both of which stem from the inherent...

Tiptoe is a private web search engine that allows clients to search over hundreds of millions of documents, while revealing no information about their search query to the search engine’s servers. Tiptoe’s privacy guarantee is based on cryptography alone; it does not require hardware enclaves or non-colluding servers. Tiptoe uses semantic embeddings to reduce the problem of private full-text search to private nearest-neighbor search. Then, Tiptoe implements private nearest-neighbor search...

Proxy re-encryption (PRE) allows a proxy with a re-encryption key to translate a ciphertext intended for Alice (delegator) to another ciphertext intended for Bob (delegatee) without revealing the underlying message. However, with PRE, Bob can obtain the whole message from the re-encrypted ciphertext, and Alice cannot take flexible control of the extent of the message transmitted to Bob. In this paper, we propose a new variant of PRE, called Fine-Grained PRE (FPRE), to support...

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...

Real-world healthcare data sharing is instrumental in constructing broader-based and larger clinical data sets that may improve clinical decision-making research and outcomes. Stakeholders are frequently reluctant to share their data without guaranteed patient privacy, proper protection of their data sets, and control over the usage of their data. Fully homomorphic encryption (FHE) is a cryptographic capability that can address these issues by enabling computation on encrypted data without...

Order-preserving encryption (OPE) allows efficient comparison operations over encrypted data and thus is popular in encrypted databases. However, most existing OPE schemes are vulnerable to inference attacks as they leak plaintext frequency. To this end, some frequency-hiding order-preserving encryption (FH-OPE) schemes are proposed and claim to prevent the leakage of frequency. FH-OPE schemes are considered an important step towards mitigating inference attacks. Unfortunately, there are...

In public key encryption (PKE), anonymity is essential to ensure privacy by preventing the ciphertext from revealing the recipient’s identity. However, the literature has addressed the anonymity of PKE under different attack scenarios to a limited extent. Benhamouda et al. (TCC 2020) introduced the first formal definition of anonymity for PKE under corruption, and Huang et al. (ASIACRYPT 2022) made further extensions and provided a generic framework. In this paper, we introduce a new...

Circuit-based Private Set Intersection (circuit-PSI) refers to cryptographic protocols that let two parties with input set $X$ and $Y$ compute a function $f$ over the intersection set $X \cap Y$, without revealing any other information. The research efforts for circuit-PSI mainly focus on the case where input set sizes $|X|$ and $|Y|$ are similar so far, and they scale poorly for extremely unbalanced set sizes $|X| \gg |Y|$. Recently, Lepoint \textit{et al.} (ASIACRYPT'21) proposed the first...

Given $\ell$ parties with sets $X_1, \dots, X_\ell$ of size $n$, we would like to securely compute the intersection $\cap_{i=1}^\ell X_i$, if it is larger than $n-t$ for some threshold $t$, without revealing any other additional information. It has previously been shown (Ghosh and Simkin, Crypto 2019) that this function can be securely computed with a communication complexity that only depends on $t$ and in particular does not depend on $n$. For small values of $t$, this results in protocols...

Existing proofs of adaptive security (e.g., in settings in which decryption keys are adaptively revealed) often rely on guessing arguments. Such guessing arguments can be simple (and, e.g., just involve guessing which keys are revealed), or more complex "partitioning'' arguments. Since guessing directly and negatively impacts the loss of the corresponding security reduction, this leads to black-box lower bounds for a number of cryptographic scenarios that involve adaptive security. In...

Topology-Hiding Computation (THC) enables parties to securely compute a function on an incomplete network without revealing the network topology. It is known that secure computation on a complete network can be based on oblivious transfer (OT), even if a majority of the participating parties are corrupt. In contrast, THC in the dishonest majority setting is only known from assumptions that imply (additively) homomorphic encryption, such as Quadratic Residuosity, Decisional Diffie-Hellman,...

Updatable encryption is a useful primitive that enables key rotation for storing data on an untrusted storage provider without the leaking anything about the plaintext or the key. In this work, we make two contributions. Firstly, we extend updatable encryption to the public-key setting, providing its security model and three different efficient constructions. Using a public-key updatable encryption scheme, a user can receive messages directly in the cloud from multiple senders without...

Attribute-Based Encryption (ABE) is a very attractive primitive to limit access according to specific rights. While very powerful instantiations have been offered, under various computational assumptions, they rely on either classical or post-quantum problems, and are quite intricate to implement, generally resulting in poor efficiency; the construction we offer results in a powerful efficiency gap with respect to existing solutions. With the threat of quantum computers, post-quantum...

Encrypted search algorithms (ESAs) enable private search on encrypted data and can be constructed from a variety of cryptographic primitives. All known sub-linear ESA algorithms leak information and, therefore, the design of leakage attacks is an important way to ascertain whether a given leakage profile is exploitable in practice. Recently, Oya and Kerschbaum (Usenix '22) presented an attack called IHOP that targets the query equality pattern---which reveals if and when two queries...

The Ascon authenticated encryption scheme has recently been selected as winner of the NIST Lightweight Cryptography competition. Despite its fame, however, there is no known overall generic security treatment of its mode: most importantly, all earlier related generic security results only use the key to initialize the state and do not take into account key blinding internally and at the end. In this work we present a thorough security analysis of the Ascon mode: we consider multi-user and...

The Supersingular Isogeny Diffie-Hellman (SIDH) protocol has been the main and most efficient isogeny-based encryption protocol, until a series of breakthroughs led to a polynomial-time key-recovery attack. While some countermeasures have been proposed, the resulting schemes are significantly slower and larger than the original SIDH. In this work, we propose a new countermeasure technique that leads to significantly more efficient and compact protocols. To do so, we introduce the...

With the increased use of data and communication through the internet and the abundant misuse of personal data by many organizations, people are more sensitive about their privacy. Privacy-preserving computation is becoming increasingly important in this era. Functional encryption allows a user to evaluate a function on encrypted data without revealing sensitive information. Most implementations of functional encryption schemes are too time-consuming for practical use. Mera et al. first...

In two-message authenticated key exchange (AKE), it is necessary for the initiator to keep a round state after sending the first round-message, because he/she has to derive his/her session key after receiving the second round-message. Up to now almost all two-message AKEs constructed from public-key encryption (PKE) only achieve weak security which does not allow the adversary obtaining the round state. How to support state reveal to obtain a better security called IND-AA security has been...

A whistleblower is a person who leaks sensitive information on a prominent individual or organisation engaging in an unlawful or immoral activity. Whistleblowing has the potential to mitigate corruption and fraud by identifying the misuse of capital. In extreme cases whistleblowing can also raise awareness about unethical practices to individuals by highlighting dangerous working conditions. Obtaining and sharing the sensitive information associated with whistleblowing can carry great...

Motivated by the privacy requirements in network intrusion detection and DNS policy checking, we have developed a suite of protocols and algorithms for regular expression matching with enhanced privacy: - A new regular expression matching algorithm that is oblivious to the input strings, of which the complexity is only $O(mn)$ where $m$ and $n$ are the length of strings and the regular expression respectively. It is achieved by exploiting the structure of the Thompson nondeterministic...

Witness Encryption is a holy grail of cryptography that remains elusive. It asks that a secret is only revealed when a particular computational problem is solved. Modern smart contracts and blockchains make assumptions of “honest majority”, which allow for a social implementation of Witness Encryption. The core idea is to make use of a partially trusted committee to carry out the responsibilities mandated by these functionalities – such as keeping the secret private, and then releasing it...

Online auctions have a steadily growing market size, creating billions of US dollars in sales value every year. To ensure fairness and auditability while preserving the bidder's privacy is the main challenge of an auction scheme. At the same time, utility driven blockchain technology is picking up the pace, offering transparency and data integrity to many applications. In this paper, we present a blockchain-based first price sealed-bid auction scheme. Our scheme offers privacy and public...

Recent work on dynamic structured and searchable symmetric encryption has focused on achieving the notion of forward-privacy. This is mainly motivated by the claim that forward-privacy protects against adaptive file injection attacks (Zhang, Katz, Papamanthou, Usenix Security, 2016). In this work, we revisit the notion of forward-privacy in several respects. First, we observe that forward-privacy does not necessarily guarantee security against adaptive file injection attacks if a scheme...

Garbled circuits are a fundamental cryptographic primitive that allows two or more parties to securely evaluate an arbitrary Boolean circuit without revealing any information beyond the output using a constant number of communication rounds. Garbled circuits have been introduced by Yao (FOCS’86) and generalized to the multi-party setting by Beaver, Micali and Rogaway (STOC’90). Since then, several works have improved their efficiency by providing different garbling schemes and several...

Gradient Boosting Decision Tree (GBDT) and its variants are widely used in industry, due to their strong interpretability. Secure multi-party computation allows multiple data owners to compute a function jointly while keeping their input private. In this work, we present Squirrel, a two-party GBDT training framework on a vertically split dataset, where two data owners each hold different features of the same data samples. Squirrel is private against semi-honest adversaries, and no sensitive...

We revisit the problem of {\em reusable} non-interactive secure computation (NISC). A standard NISC protocol for a sender-receiver functionality $f$ enables the receiver to encrypt its input $x$ such that any sender, on input $y$, can send back a message revealing only $f(x,y)$. Security should hold even when either party can be malicious. A {\em reusable} NISC protocol has the additional feature that the receiver's message can be safely reused for computing multiple outputs $f(x,y_i)$. Here...

Secure computation enables mutually distrusting parties to jointly compute a function on their secret inputs, while revealing nothing beyond the function output. A long-running challenge is understanding the required communication complexity of such protocols---in particular, when communication can be sublinear in the circuit representation size of the desired function. For certain functions, such as Private Information Retrieval (PIR), this question extends to even sublinearity in the input...