What a lovely hat

Anonymous credential schemes allow users to prove claims about themselves in a privacy-preserving manner. Put simply, a user can prove that she has an attribute value (e.g., she is a citizen) without revealing any other information about herself. Traditional schemes (including those with multiple credential issuers) operate under the assumption that each recognized issuer produces credentials for all user attributes. This assumption, however, is not practical: in the real world, no such...

Bitcoin secures over a trillion dollars in assets but remains largely absent from decentralized finance (DeFi) due to its restrictive scripting language. The emergence of BitVM, which enables verification of arbitrary off-chain computations via on-chain fraud proofs, opens the door to expressive Bitcoin-native applications without altering consensus rules. A key challenge for smart contracts executed on a public blockchain, however, is the privacy of data: for instance, bid privacy is...

In recent years, federated learning has gained significant momentum as a collaborative machine learning approach, particularly in the field of medicine. While the decentralized nature of federated learning boasts greater security guarantees compared to traditional machine learning methods, it is still susceptible to myriad attacks. Moreover, as federated learning becomes increasingly ubiquitous in medicine, its use for classification tasks is expected to increase; however, maintaining...

This paper proposes DIMSEPP, a decentralized identity management system that enhances privacy while preserving blockchain verifiability. The system cryptographically enforces data minimal disclosure principles by storing attribute commitments on-chain and validating them through zero-knowledge proofs, allowing users to demonstrate attribute validity without revealing sensitive values. The architecture maintains full compatibility with existing DID standards through standard document...

In timing-sensitive blockchain applications, such as decentralized finance (DeFi), achieving first-come-first-served (FCFS) transaction ordering among decentralized nodes is critical to prevent frontrunning attacks. Themis[CCS'23], a state-of-the-art decentralized FCFS ordering system, has become a key reference point for high-throughput fair ordering systems for real-world blockchain applications, such as rollup chains and decentralized sequencing, and has influenced the design of several...

Privacy is a growing concern for smart contracts on public ledgers. In recent years, we have seen several practical systems for privacy-preserving smart contracts, but they only target privacy of on-chain data, and rely on trusted off-chain parties with user data -- for instance, a decentralized finance application (e.g. exchange) relies on an off-chain matching engine to process client orders that get settled on-chain, where privacy only applies to the on-chain data. Privacy conscious...

Decentralized e-voting enables secure and transparent elections without relying on trusted authorities, with blockchain emerging as a popular platform. It has compelling applications in Decentralized Autonomous Organizations (DAOs), where governance relies on voting with blockchain-issued tokens. Quadratic voting (QV), a mechanism that mitigates the dominance of large token holders, has been adopted by many DAO elections to enhance fairness. However, current QV systems deployed in practice...

In the Social Internet of Vehicles (SIoV), effective data sharing is essential for applications including road safety, traffic management, and situational awareness. However, the decentralized and open nature of SIoV presents significant challenges in simultaneously ensuring data integrity, user privacy, and system accountability. This paper presents a protocol for secure and location-accurate traffic data sharing that fully preserves the anonymity and privacy of participating witnesses. The...

As generative models continue to evolve, verifying the authenticity, provenance, and integrity of digital media has become increasingly critical—particularly for domains like journalism, digital art, and scientific documentation. In this work, we present a decentralized verifiable media ecosystem for managing, verifying, and transacting authentic digital media using zero-knowledge proofs (ZKPs). Building on VIMz (Dziembowski et al., PETS'25), we extend the framework in three key...

Non-interactive publicly verifiable secret sharing (PVSS) schemes enable the decentralized (re-)sharing of secrets in adversarial environments, allowing anyone to verify the correctness of distributed shares. Such schemes are essential for large-scale decentralized applications, including committee-based systems that require both transparency and robustness. However, existing PVSS schemes rely on group-based cryptography, resulting them vulnerable to quantum attacks and limiting their...

Scalable data availability (DA) is essential for high-throughput, decentralized blockchains, enabling lightweight nodes to verify block availability without incurring the prohibitive costs of full data replication. Reed-Solomon (RS) code commitment schemes underpin modern DA protocols by ensuring that dispersed data fragments can be verified as part of a valid codeword, even in the presence of malicious block producers. However, state-of-the-art schemes such as FRIDA (Crypto'24), while...

Distributed Hash Tables (DHTs) are peer-to-peer protocols that serve as building blocks for more advanced applications. Recent examples, motivated by blockchains, include decentralized storage networks (e.g., IPFS), data availability sampling, or Ethereum's peer discovery protocol. In the blockchain context, DHTs are vulnerable to Sybil attacks, where an adversary compromises the network by joining with many malicious nodes. Mitigating such attacks requires restricting the adversary's...

A Threshold Fully Homomorphic Encryption (ThFHE) scheme enables the generation of a global public key and secret key shares for multiple parties, allowing any threshold of these parties to collaboratively decrypt a ciphertext without revealing their individual secret keys. By leveraging the homomorphic properties of FHE, this scheme supports the distributed computation of arbitrary functions across multiple parties. As distributed execution of cryptographic tasks becomes popular, the...

We present Bitcoin Thunderbolt, a novel off-chain protocol for asynchronous, secure transfer of Bitcoin UTXOs between uncoordinated users. Unlike prior solutions such as payment channels or the Lightning Network, Bitcoin Thunderbolt requires no prior trust, direct interaction, or continuous connectivity between sender and receiver. At its core, Bitcoin Thunderbolt employs a Byzantine fault-tolerant committee to manage threshold Schnorr signatures, enabling secure ownership delegation and...

Due to their widespread applications in decentralized and privacy preserving technologies, commitment schemes have become increasingly important cryptographic primitives. With a wide variety of applications, many new constructions have been proposed, each enjoying different features and security guarantees. In this paper, we systematize the designs, features, properties, and applications of vector commitments (VCs). We define vector, polynomial, and functional commitments and we discuss the...

Generalized secret sharing (GSS) enables flexible access control in distributed systems by allowing secrets to be shared across arbitrary monotone access structures. However, its adoption in transparent and trustless environments is hindered due to the reliance on trusted participants and secure communication channels. This reliance restricts GSS's ability to provide flexible control in the presence of adversaries. In this paper, we propose publicly verifiable generalized secret sharing...

Zero-knowledge range arguments are a fundamental cryptographic primitive that allows a prover to convince a verifier of the knowledge of a secret value lying within a predefined range. They have been utilized in diverse applications, such as confidential transactions, proofs of solvency and anonymous credentials. Range arguments with a transparent setup dispense with any trusted setup to eliminate security backdoor and enhance transparency. They are increasingly deployed in diverse...

Blockchain technology and smart contracts have revolutionized digital transactions by enabling trustless and decentralized exchanges of value. However, the inherent transparency and immutability of blockchains pose significant privacy challenges. On-chain data, while pseudonymous, is publicly visible and permanently recorded, potentially leading to the inadvertent disclosure of sensitive information. This issue is particularly pronounced in smart contract applications, where contract details...

In this paper, we present a ring referral scheme, by which a user can publicly prove her knowledge of a valid signature for a private message that is signed by one of an ad hoc set of authorized issuers, without revealing the signing issuer. Ring referral is a natural extension to traditional ring signature by allowing a prover to obtain a signature from a third-party signer. Our scheme is useful for diverse applications, such as certificate-hiding decentralized identity, privacy-enhancing...

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

Generalized secret sharing (GSS), which can offer more flexibility by accommodating diverse access structures and conditions, has been under-explored in distributed computing over the past decades. To address the gaps, we propose the publicly verifiable generalized secret sharing (PVGSS) scheme, enhancing the applicability of GSS in transparent systems. Public verifiability is a crucial property to gain trustworthiness for decentralized systems like blockchain. We begin by introducing two...

Threshold signatures have become a critical tool in cryptocurrency systems, offering enhanced security by distributing the signing process among multiple signers. In this work, we distribute this process between a client and a permissionless decentralized blockchain, and present novel protocols for ECDSA and EdDSA/Schnorr signatures in this setting. Typical threshold access architectures used by trusted custodians suffer from the honeypot problem, wherein the more assets the custodian holds,...

This work explores the application and efficient deployment of (standardized) post-quantum (PQ) digital signature algorithms in the blockchain environment. Specifically, we implement and evaluate four PQ signatures in the Ethereum Virtual Machine: W-OTS$^{+}$, XMSS, SPHINCS+, and MAYO. We focus on optimizing the gas costs of the verification algorithms as that is the signature schemes' only algorithm executed on-chain, thus incurring financial costs (transaction fees) for the users. Hence,...

Pairing-based arguments offer remarkably small proofs and space-efficient provers, but aggregating such proofs remains costly. Groth16 SNARKs and KZG polynomial commitments are prominent examples of this class of arguments. These arguments are widely deployed in decentralized systems, with millions of proofs generated per day. Recent folding schemes have greatly reduced the cost of proving incremental computations, such as batch proof verification. However, existing constructions require...

Succinct Non-interactive Arguments of Knowledge (SNARKs) can enable efficient verification of computation in many applications. However, generating SNARK proofs for large-scale tasks, such as verifiable machine learning or virtual machines, remains computationally expensive. A promising approach is to distribute the proof generation workload across multiple workers. A practical distributed SNARK protocol should have three properties: horizontal scalability with low overhead (linear...

Adaptor signatures (AS) extend the functionality of traditional digital signatures by enabling the generation of a pre-signature tied to an instance of a hard NP relation, which can later be turned (adapted) into a full signature upon revealing a corresponding witness. The recent work by Liu et al. [ASIACRYPT 2024] devised a generic AS scheme that can be used for any NP relation---which here we will refer to as universal adaptor signatures scheme, in short UAS---from any one-way function....

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,...

Distributed Key Generation (DKG) is a technique that enables the generation of threshold cryptography keys among a set of mutually untrusting nodes. DKG generates keys for a range of decentralized applications such as threshold signatures, multiparty computation, and Byzantine consensus. Over the past five years, research on DKG has focused on optimizing network communication protocols to improve overall system efficiency by reducing communication complexity. However, SOTA asynchronous...

We propose the first constructions of anonymous tokens with decentralized issuance. Namely, we consider a dynamic set of signers/issuers; a user can obtain a token from any subset of the signers, which is publicly verifiable and unlinkable to the issuance process. To realize this new primitive we formalize the notion of Blind Multi-Signatures (BMS), which allow a user to interact with multiple signers to obtain a (compact) signature; even if all the signers collude they are unable to link a...

Many applications rely on accumulators and authenticated dictionaries, from timestamping certificate transparency and memory checking to blockchains and privacy-preserving decentralized electronic money, while Merkle tree and its variants are efficient for arbitrary element membership proofs, non-membership proofs, i.e., universal accumulators, and key-based membership proofs may require trees up to 256 levels for 128 bits of security, assuming binary tree, which makes it inefficient in...

Users of decentralized finance (DeFi) applications face significant risks from adversarial actions that manipulate the order of transactions to extract value from users. Such actions---an adversarial form of what is called maximal-extractable value (MEV)---impact both individual outcomes and the stability of the DeFi ecosystem. MEV exploitation, moreover, is being institutionalized through an architectural paradigm known Proposer-Builder Separation (PBS). This work introduces a system...

Verifiable Random Functions (VRFs) are cryptographic primitives that generate unpredictable randomness along with proofs that are verifiable, a critical requirement for blockchain applications in decentralized finance, online gaming, and more. Existing VRF constructions often rely on centralized entities, creating security vulnerabilities. Distributed VRFs (DVRFs) offer a decentralized alternative but face challenges like large proof sizes or dependence on computationally expensive bilinear...

Cross-chain Decentralized Applications (dApps) are increasingly popular for their ability to handle complex tasks across various blockchains, extending beyond simple asset transfers or swaps. However, ensuring all dependent transactions execute correctly together, known as complete atomicity, remains a challenge. Existing works provide financial atomicity, protecting against monetary loss, but lack the ability to ensure correctness for complex tasks. In this paper, we introduce Avalon, a...

Threshold secret sharing enables distributing a message to $n$ parties such that no subset of fewer than $t$ parties can learn the message, whereas any subset of at least $t$ parties can recover the message. Despite being a fundamental primitive, secret sharing still suffers from one significant drawback, where its message reconstruction algorithm is computationally expensive for large privacy thresholds $t$. In this paper, we aim to address this significant drawback. We study general...

Self-sovereign identity (SSI) systems empower users to (anonymously) establish and verify their identity when accessing both digital and real-world resources, emerging as a promising privacy-preserving solution for user-centric identity management. Recent work by Maram et al. proposes the privacy-preserving Sybil-resistant decentralized SSI system CanDID (IEEE S&P 2021). While this is an important step, notable shortcomings undermine its efficacy. The two most significant among them being...

Decentralized payments have evolved from using pseudonymous identifiers to much more elaborate mechanisms to ensure privacy. They can shield the amounts in payments and achieve untraceability, e.g., decoy-based untraceable payments use decoys to obfuscate the actual asset sender or asset receiver. There are two types of decoy-based payments: full decoy set payments that use all other available users as decoys, e.g., Zerocoin, Zerocash, and ZCash, and user-defined decoy set payments where...

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...

Designing light clients to securely and efficiently read Proof-of-Work blockchains has been a foundational problem since the inception of blockchains. Nakamoto themselves, in the original Bitcoin paper, presented the first client protocol, i.e., the Simplified Payment Verification, which consumes an amount of bandwidth, computational, and storage resources that grows linearly in the system's lifetime $\mathcal{C}$. Today, the blockchain ecosystem is more mature and presents a variety of...

The Bitcoin ecosystem has continued to evolve beyond its initial promises of decentralization, transparency, and security. Recent advancements have notably been made with the integration of Layer-2 solutions, which address scalability issues by offloading transactions from the main blockchain. This facilitates faster and more cost-effective transactions while maintaining integrity. The advent of inscriptions and ordinal protocols has further broadened the spectrum of capabilities, enabling...

We study Sybil-Resilient Anonymous (SyRA) signatures, a cryptographic primitive that enables credentialed users to generate, on demand, unlinkable pseudonyms tied to any given context, and issue signatures on behalf of these pseudonyms. Concretely, SyRA allows a distributed issuer to turn any legacy identity or personhood identifier, possibly of low entropy, into a unique associated cryptographic key of high pseudoentropy, for use in generating signatures for any given context....

The advent of Web3 technologies promises unprecedented levels of user control and autonomy. However, this decentralization shifts the burden of security onto the users, making it crucial to understand their security behaviors and perceptions. To address this, our study introduces a comprehensive framework that identifies four core components of user interaction within the Web3 ecosystem: blockchain infrastructures, Web3-based Decentralized Applications (DApps), online communities, and...

Over the past decade, cryptocurrencies have garnered attention from academia and industry alike, fostering a diverse blockchain ecosystem and novel applications. The inception of bridges improved interoperability, enabling asset transfers across different blockchains to capitalize on their unique features. Despite their surge in popularity and the emergence of Decentralized Finance (DeFi), trustless bridge protocols remain inefficient, either relaying too much information (e.g.,...

The use of zero-knowledge Succinct Non-Interactive Arguments of Knowledge (zk-SNARK) and similar types of proofs has become increasingly popular as a solution for improving scalability, privacy, and interoperability of blockchain systems. However, even with the most advanced proving systems, verifying a single SNARK proof can require a significant amount of computational resources making it expensive to be performed on-chain. This becomes a noticeable bottleneck in scaling SNARK-based...

Decentralized applications (DApps), which are innovative blockchain-powered software systems designed to serve as the fundamental building blocks for the next generation of Internet services, have witnessed exponential growth in recent years. This paper thoroughly compares and analyzes two blockchain-based decentralized storage networks (DSNs), which are crucial foundations for DApp and blockchain ecosystems. The study examines their respective mechanisms for data persistence, strategies for...

Enhancing privacy on smart contract-enabled blockchains has garnered much attention in recent research. Zero-knowledge proofs (ZKPs) is one of the most popular approaches, however, they fail to provide full expressiveness and fine-grained privacy. To illustrate this, we underscore an underexplored type of Miner Extractable Value (MEV), called Residual Bids Extractable Value (RBEV). Residual bids highlight the vulnerability where unfulfilled bids inadvertently reveal traders’ unmet demands...

Suppose a prover, in possession of a large body of valuable evidence, wants to quickly convince a verifier by presenting only a small portion of the evidence. We define an Approximate Lower Bound Argument, or ALBA, which allows the prover to do just that: to succinctly prove knowledge of a large number of elements satisfying a predicate (or, more generally, elements of a sufficient total weight when a predicate is generalized to a weight function). The argument is approximate because...

Digital signature schemes with specific properties have recently seen various real-world applications with a strong emphasis on privacy-enhancing technologies. They have been extensively used to develop anonymous credentials schemes and to achieve an even more comprehensive range of functionalities in the decentralized web. Substantial work has been done to formalize different types of signatures where an allowable set of transformations can be applied to message-signature pairs to obtain...

We introduce Lanturn: a general purpose adaptive learning-based framework for measuring the cryptoeconomic security of composed decentralized-finance (DeFi) smart contracts. Lanturn discovers strategies comprising of concrete transactions for extracting economic value from smart contracts interacting with a particular transaction environment. We formulate the strategy discovery as a black-box optimization problem and leverage a novel adaptive learning-based algorithm to address it. Lanturn...

Decentralized Finance (DeFi) is a new paradigm in the creation, distribution, and utilization of financial services via the integration of blockchain technology. Our research conducts a comprehensive introduction and meticulous classification of various DeFi applications. Beyond that, we thoroughly analyze these risks from both technical and economic perspectives, spanning multiple layers. We point out research gaps and revenues, covering technical advancements, innovative economics, and...

Decentralized Finance (DeFi), a blockchain-based financial ecosystem, suffers from smart contract vulnerabilities that led to a loss exceeding 3.24 billion USD by April 2022. To address this, blockchain firms audit DeFi applications, a process known as DeFi auditing. Our research aims to comprehend the mechanism and efficacy of DeFi auditing. We discovered its ability to detect vulnerabilities in smart contract logic and interactivity with other DeFi entities, but also noted its limitations...

A linkable ring signature allows a user to sign anonymously on behalf of a group while ensuring that multiple signatures from the same user are detected. Applications such as privacy-preserving e-voting and e-cash can leverage linkable ring signatures to significantly improve privacy and anonymity guarantees. To scale to systems involving large numbers of users, short signatures with fast verification are a must. Concretely efficient ring signatures currently rely on a trusted authority...

Decentralized apps (DApps) often hold significant cryptocurrency assets. In order to manage these assets and coordinate joint investments, shareholders leverage the underlying smart contract functionality to realize a transparent, verifiable, and secure decision-making process. That is, DApps implement proposal-based voting. Permissionless blockchains, however, lead to a conflict between transparency and anonymity; potentially preventing free decision-making if individual votes and...

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...

With the growing number of decentralized finance (DeFi) applications, transaction fairness in blockchains has gained much research interest. As a broad concept in distributed systems and blockchains, fairness has been used in different contexts, varying from ones related to the liveness of the system to ones that focus on the received order of transactions. In this work, we revisit the fairness definitions and find that existing fairness definitions are not adapted to blockchains with...

Anonymous credentials (AC) have emerged as a promising privacy-preserving solu- tion for user-centric identity management. They allow users to authenticate in an anonymous and unlinkable way such that only required information (i.e., attributes) from their credentials are re- vealed. With the increasing push towards decentralized systems and identity, e.g., self-sovereign identity (SSI) and the concept of verifiable credentials, this also necessitates the need for suit- able AC systems. For...

A cryptographic accumulator is a succinct set commitment scheme with efficient (non-)membership proofs that typically supports updates (additions and deletions) on the accumulated set. When elements are added to or deleted from the set, an update message is issued. The collection of all the update messages essentially leaks the underlying accumulated set which in certain applications is not desirable. In this work, we define oblivious accumulators, a set commitment with concise membership...

Society appears to be on the verge of recognizing the need for control over sensitive data in modern web applications. Recently, many systems claim to give control to individuals, promising the preeminent goal of data sovereignty. However, despite recent attention, research and industry efforts are fragmented and lack a holistic system overview. In this paper, we provide the first transecting systematization of data sovereignty by drawing from a dispersed body of knowledge. We clarify the...

In the classical setting of differential privacy, a privacy-preserving query is performed on a private database, after which the query result is released to the analyst; a differentially private query ensures that the presence of a single database entry is protected from the analyst’s view. In this work, we contribute the first definitional framework for differential privacy in the trusted curator setting; clients submit private inputs to the trusted curator, which then computes individual...

A proxy signature enables a party to delegate her signing power to another. This is useful in practice to achieve goals related to robustness, crowd-sourcing, and workload sharing. Such applications, especially in the blockchain model, usually require delegation to satisfy several properties, including time bounds, anonymity, revocability, and policy enforcement. Despite the large amount of work on proxy signatures in the literature, none of the existing schemes satisfy all these properties;...

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...

Abstract—Corporate sexual harassment policies often prioritize liability mitigation over the creation of a corporate culture free of harassment. Victims of sexual harassment are often required to report claims individually to HR. This can create an environment of self-censorship when employees feel that they cannot trust HR to act as an unbiased mediator. This problem is compounded when corporations have a culture that is tolerant of certain types of harassment. Forcing employees to report...

We propose a new cryptographic primitive called "verifiably encrypted threshold key derivation" (vetKD) that extends identity-based encryption with a decentralized way of deriving decryption keys. We show how vetKD can be leveraged on modern blockchains to build scalable decentralized applications (or "dapps") for a variety of purposes, including preventing front-running attacks on decentralized finance (DeFi) platforms, end-to-end encryption for decentralized messaging and social networks...

Integrating traditional Internet (web2) identities with blockchain (web3) identities presents considerable obstacles. Conventional solutions typically employ a mapping strategy, linking web2 identities directly to specific blockchain addresses. However, this method can lead to complications such as fragmentation of identifiers across disparate networks. To address these challenges, we propose a novel scheme, Address Abstraction (AA), that circumvents the need for direct mapping. AA scheme...

Recent years have seen the emergence of practical advanced cryptographic tools that not only protect data privacy and authenticity, but also allow for jointly processing data from different institutions without sacrificing privacy. The ability to do so has enabled implementations a number of traditional and decentralized financial applications that would have required sacrificing privacy or trusting a third party. The main catalyst of this revolution was the advent of decentralized...

Anonymous authentication primitives, e.g., group or ring signatures, allow one to realize privacy-preserving data collection applications, as they strike a balance between authenticity of data being collected and privacy of data providers. At PKC 2021, Diaz and Lehmann defined group signatures with User-Controlled Linkability (UCL) and provided an instantiation based on BBS+ signatures. In a nutshell, a signer of a UCL group signature scheme can link any of her signatures: linking evidence...

Cross-chain communication is instrumental in unleashing the full potential of blockchain technologies, as it allows users and developers to exploit the unique design features and the profit opportunities of different existing blockchains. The majority of interoperability solutions are provided by centralized exchanges and bridge protocols based on a trusted majority, both introducing undesirable trust assumptions compared to native blockchain assets. Hence, increasing attention has been...

As digital tokens on blockchains such as non-fungible tokens (NFTs) increase in popularity and scale, the existing interfaces (ERC-721, ERC-20, and many more) are being exposed for being expensive and not scalable. As a result, tokens are being forced to be implemented on alternative blockchains where it is cheaper but less secure. To offer a solution without making security tradeoffs, we propose using joint cryptographic accumulators (e.g. joint Merkle trees). We propose a method of...

Distributed key generation (DKG) allows bootstrapping threshold cryptosystems without relying on a trusted party, nowadays enabling fully decentralized applications in blockchains and multiparty computation (MPC). While we have recently seen new advancements for asynchronous DKG (ADKG) protocols, their performance remains the bottleneck for many applications, with only one protocol being implemented (DYX+ ADKG, IEEE S&P 2022). DYX+ ADKG relies on the Decisional Composite Residuosity...

Security and privacy issues with centralized exchange services have motivated the design of atomic swap protocols for decentralized trading across currencies. These protocols follow a standard blueprint similar to the 2-phase commit in databases: (i) both users first lock their coins under a certain (cryptographic) condition and a timeout; (ii-a) the coins are swapped if the condition is fulfilled; or (ii-b) coins are released after the timeout. The quest for these protocols is to minimize...

Time-lock puzzles (TLP) are a fascinating type of cryptographic problem that is easy to generate, but takes a certain time to solve, even when arbitrary parallel speedup is allowed. TLPs have wide-ranging applications including fairness, round efficient computation, and more. To reduce the effort needed to solve large numbers of TLPs, prior work has proposed batching techniques to reduce the cost of solving. However, these proposals either require: (1) a trusted setup or (2) the puzzle size...

Distributed Key Generation (DKG) is a technique to bootstrap threshold cryptosystems without a trusted party. DKG is an essential building block to many decentralized protocols such as randomness beacons, threshold signatures, Byzantine consensus, and multiparty computation. While significant progress has been made recently, existing asynchronous DKG constructions are inefficient when the reconstruction threshold is larger than one-third of the total nodes. In this paper, we present a simple...

Restricting access to certified users is not only desirable for many blockchain applications, it is also legally mandated for decentralized finance (DeFi) applications to counter malicious actors. Existing solutions, however, are either (i) non-private, i.e., they reveal the link between users and their wallets to the authority granting credentials, or (ii) they introduce additional trust assumptions by relying on a decentralized oracle to verify anonymous credentials (ACs). To remove...

Traditional blockchain systems execute program state transitions on-chain, requiring each network node participating in state-machine replication to re-compute every step of the program when validating transactions. This limits both scalability and privacy. Recently, Bowe et al. introduced a primitive called decentralized private computation (DPC) and provided an instantiation called ZEXE, which allows users to execute arbitrary computations off-chain without revealing the program logic to...

Vector commitments (VC) are a cryptographic primitive that allow one to commit to a vector and then “open” some of its positions efficiently. Vector commitments are increasingly recognized as a central tool to scale highly decentralized networks of large size and whose content is dynamic. In this work, we examine the demands on the properties that an ideal vector commitment should satisfy in the light of the emerging plethora of practical applications and propose new constructions that...

Digital ledger technologies supporting smart contracts usually does not ensure any privacy for user transactions or state. Most solutions to this problem either use private network setups, centralized parties, hardware enclaves, or cryptographic primitives, which are novel, complex, and computationally expensive. This paper looks into an alternative way of implementing smart contracts. Our construction of a protocol for smart contracts employs an overlay protocol design pattern for...

Blockchain oracles are systems that connect blockchains with the outside world by interfacing with external data providers. They provide decentralized applications with the external information needed for smart contract execution. In this paper, we focus on decentralized price oracles, which are distributed systems that provide exchange rates of digital assets to smart contracts. They are the cornerstone of the safety of some decentralized finance applications such as stable coins or...

Over the past years, the interest in Blockchain technology and its applications has tremendously increased. This increase of interest was however accompanied by serious threats that raised concerns over user data privacy. Prominent examples include transaction traceability and identification of senders, receivers, and transaction amounts. This resulted in a multitude of privacy-preserving techniques that offer different guarantees in terms of trust, decentralization, and traceability....

In the wake of the Covid-19 pandemic, countries and organizations started looking towards technology to curb the spread of the disease, for instance, conducting contact tracing with smartphones. Many contact tracing applications are on the market, built on different technology, such as Bluetooth, GPS, Sound, and QR code scanning systems. The use of sound is an area that has potential for further exploration; currently, only NOVID is utilizing this technology. On top of that, there is a need...

Smart contracts are a new form of software that will revolutionize how software is written, IT systems are maintained, and applications and whole businesses are built. Smart contracts are composable and autonomous pieces of software that run on decentralized blockchains, which makes them tamperproof and unstoppable. In this paper, we describe the Internet Computer (IC), which is a radical new design of blockchain that unleashes the full potential of smart contracts, overcoming the...

The rise of blockchain technology has boosted interest in privacy-enhancing technologies, in particular, anonymous transaction authentication. Permissionless blockchains realize transaction anonymity through one-time pseudonyms, whereas permissioned blockchains leverage anonymous credentials. Earlier solutions of anonymous credentials assume a single issuer; as a result, they hide the identity of users but still reveal the identity of the issuer. A countermeasure is delegatable credentials,...

We revisit decentralized random beacons with a focus on practical distributed applications. Decentralized random beacons (Beaver and So, Eurocrypt 1993) provide the functionality for $n$ parties to generate an unpredictable sequence of bits in a way that cannot be biased, which is useful for any decentralized protocol requiring trusted randomness. Existing beacon constructions are highly inefficient in practical settings where protocol parties need to rejoin after crashes or disconnections,...

The preponderance of smart devices, such as smartphones, has boosted the development and use of mobile applications (apps) in the recent years. This prevalence induces a large volume of mobile app usage data. The analysis of such information could lead to a better understanding of users' behaviours in using the apps they have installed, even more if these data can be coupled with a given context (location, time, date, sociological data...). However, mobile and apps usage data are very...

With the advent of the Internet of Things (IoT), e-health has become one of the main topics of research. Due to the sensitivity of patient information, patient privacy seems challenging. Nowadays, patient data is usually stored in the cloud in healthcare programs, making it difficult for users to have enough control over their data. The recent increment in announced cases of security and surveillance breaches compromising patients' privacy call into question the conventional model, in which...

Repeated Modular Squaring is a versatile computational operation that has led to practical constructions of timed-cryptographic primitives like time-lock puzzles (TLP) and verifiable delay functions (VDF) that have a fast growing list of applications. While there is a huge interest for timed-cryptographic primitives in the blockchains area, we find two real-world concerns that need immediate attention towards their large-scale practical adoption: Firstly, the requirement to constantly...

Decentralized finance (DeFi) refers to interoperable smart contracts running on distributed ledgers offering financial services beyond payments. Recently, there has been an explosion of DeFi applications centered on Ethereum, with close to a hundred billion USD in total assets deposited as of September 2021. These applications provide financial services such as asset management, trading, and lending. The wide adoption of DeFi has raised important concerns, and among them is the key issue of...

Many decentralized applications require a common source of randomness that cannot be biased or predicted by any single party. Randomness beacons provide such a functionality, allowing parties to periodically obtain fresh random outputs and verify that they are computed correctly. In this work, we propose Mt. Random, a multi-tiered randomness beacon that combines Publicly Verifiable Secret Sharing (PVSS) and (Threshold) Verifiable Random Function (VRF) techniques in order to provide...

The Bitcoin blockchain was the first publicly verifiable, and distributed ledger, where it is possible for everyone to download and check the full history of all data records from the genesis block. These properties lead to the emergence of new types of applications and the redesign of traditional systems that no longer respond to current business needs (e.g., transparency, protection against censorship, decentralization). One particular application is the use of blockchain technology to...

A ring signature allows a party to sign messages anonymously on behalf of a group, which is called ring. Traceable ring signatures are a variant of ring signatures that limits the anonymity guarantees, enforcing that a member can sign anonymously at most one message per tag. Namely, if a party signs two different messages for the same tag, it will be de-anomymized. This property is very useful in decentralized platforms to allow members to anonymously endorse statements in a...

We propose OmniLytics, a blockchain-based secure data trading marketplace for machine learning applications. Utilizing OmniLytics, many distributed data owners can contribute their private data to collectively train an ML model requested by some model owners, and receive compensation for data contribution. OmniLytics enables such model training while simultaneously providing 1) model security against curious data owners; 2) data security against the curious model and data owners; 3)...

Payment channel networks (PCNs) mitigate the scalability issues of current decentralized cryptocurrencies. They allow for arbitrarily many payments between users connected through a path of intermediate payment channels, while requiring interacting with the blockchain only to open and close the channels. Unfortunately, PCNs are (i) tailored to payments, excluding more complex smart contract functionalities, such as the oracle-enabling Discreet Log Contracts and (ii) their need for active...

Timed-release encryption can encrypt a message to a future time such that it can only be decrypted after that time. Potential applications include sealed bid auctions, scheduled confidential transactions, and digital time capsules. To enable such applications as decentralized smart contracts, we explore how to use timed-release encryption on blockchains. Practical constructions in literature rely on a trusted server (or servers in a threshold setting), which periodically publishes an...

The proliferation of the Internet of Things (IoT) technology has made ubiquitous computing a reality by broadening Internet connectivity across diverse application domains, thus bridging billions of devices and human beings as well for information collection, data processing, and decision-making. In recent years, IoT technology and its applications in various industrial sectors have grown exponentially. Most existing industrial IoT (IIoT) implementations, however, are still relying on a...

Decentralized ledgers that support rich applications suffer from three limitations. First, applications are provisioned tiny execution environments with limited running time, minimal stack size, and restrictive instruction sets. Second, applications must reveal their state transition, enabling miner frontrunning attacks and consensus instability. Third, applications offer weak guarantees of correctness and safety. We design, implement, and evaluate Leo, a new programming language designed...

Reliable and verifiable public randomness is not only an essential building block in various cryptographic primitives, but also is a critical component in many distributed and decentralized protocols, e.g., blockchain sharding. A 'good' randomness generator should preserve several distinctive properties, such as public-verifiability, bias-resistance, unpredictability, and availability. However, it is a challenging task to generate such good randomness. For instance, a dishonest party may...

Suppose that $n$ players want to elect a random leader and they communicate by posting messages to a common broadcast channel. This problem is called leader election, and it is fundamental to the distributed systems and cryptography literature. Recently, it has attracted renewed interests due to its promised applications in decentralized environments. In a game theoretically fair leader election protocol, roughly speaking, we want that even majority coalitions cannot increase its own chance...

Contact tracing is an important mitigation tool for national health services to fight epidemics such as COVID-19. While many of the existing approaches for automated contact tracing focus on privacy-preserving decentralized solutions, the use of blockchain in these applications is often suggested for the transparency and immutability of the data being collected. We present CovidBloc, a contact tracing system that implements the COVID 19 exposure database on Hyperledger Fabric Blockchain...

The Covid-19 pandemic created various new challenges for our societies. Quickly discovering new infections using automated contact tracing without endangering privacy of the general public is one of these. Most discussions concerning architectures for contact tracing applications revolved around centralized against decentralized approaches. In contrast, the system proposed in this work builds on the idea of message-based contact tracing to inform users of their risk. Our main contribution is...

Nowadays, virtually all products and services offered by financial institutions are backed by technology. While the frontend banking services seem to be simple, the core-banking backend systems and architecture are complex and often based on legacy technologies. Customer-facing applications and services are evolving rapidly, yet they have data dependencies on core banking systems running on ancient technology standards. While those legacy systems are preferred for their stability,...

We initiate the study of multi-party functional encryption (MPFE) which unifies and abstracts out various notions of functional encryption which support distributed ciphertexts or secret keys, such as multi-input FE, multi-client FE, decentralized multi-client FE, multi-authority FE, dynamic decentralized FE, adhoc multi-input FE and such others. Using our framework, we identify several gaps in the literature and provide some constructions to fill these: 1) Multi-Authority ABE with Inner...

Asset custody is a core financial service in which the custodian holds in-safekeeping assets on behalf of the client. Although traditional custody service is typically endorsed by centralized authorities, decentralized custody scheme has become technically feasible since the emergence of digital assets, and furthermore, it is greatly needed by new applications such as blockchain and DeFi (Decentralized Finance). In this work, we propose a framework of decentralized asset custody scheme that...