Cryptography's most common use is secure communication---e.g. Alice can use encryption to hide the contents of the messages she sends to Bob (confidentiality) and can use signatures to assure Bob she sent these messages (authenticity). While one typically considers stateless security guarantees---for example a channel that Alice can use to send messages securely to Bob---one can also consider stateful ones---e.g. an interactive conversation between Alice, Bob and their friends where...

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 the rapidly evolving Web3 ecosystem, transparent auditing has emerged as a critical component for both applications and users. However, there is a significant gap in understanding how users perceive this new form of auditing and its implications for Web3 security. Utilizing a mixed-methods approach that incorporates a case study, user interviews, and social media data analysis, our study leverages a risk perception model to comprehensively explore Web3 users' perceptions regarding...

We study the problem of generating public unbiased randomness in a distributed manner within the recent You Only Speak Once (YOSO) framework for stateless multiparty computation, introduced by Gentry et al. in CRYPTO 2021. Such protocols are resilient to adaptive denial-of-service attacks and are, by their stateless nature, especially attractive in permissionless environments. While most works in the YOSO setting focus on independent random corruptions, we consider YOSO protocols with...

Security of blockchain technologies primarily relies on decentralization making them resilient against a subset of entities being taken down or corrupt. Blockchain scaling, crucial to decentralisation, has been addressed by architectural changes: i.e., the load of the nodes is reduced by parallelisation, called sharding or by taking computation load off the main blockchain via rollups. Both sharding and rollups have limitations in terms of decentralization and security. A crucial component...

Web3 applications, such as on-chain games, NFT minting, and leader elections necessitate access to unbiased, unpredictable, and publicly verifiable randomness. Despite its broad use cases and huge demand, there is a notable absence of comprehensive treatments of on-chain verifiable randomness services. To bridge this, we offer an extensive formal analysis of on-chain verifiable randomness services. We present the $first$ formalization of on-chain verifiable randomness in the...

The elegant paradigm of Anamorphic Encryption (Persiano et al., Eurocrypt 2022) considers the question of establishing a private communication in a world controlled by a dictator. The challenge is to allow two users, sharing some secret anamorphic key, to exchange covert messages without the dictator noticing, even when the latter has full access to the regular secret keys. Over the last year several works considered this question and proposed constructions, novel extensions and...

Verifiable Random Functions (VRFs) play a pivotal role in Proof of Stake (PoS) blockchain due to their applications in secret leader election protocols. However, the original definition by Micali, Rabin and Vadhan is by itself insufficient for such applications. The primary concern is that adversaries may craft VRF key pairs with skewed output distribution, allowing them to unfairly increase their winning chances. To address this issue David, Gaži, Kiayias and Russel (2017/573) proposed a...

We introduce a new cryptographic primitive, called Sybil-Resilient Anonymous (SyRA) signatures, which enable users to generate, on demand, unlinkable pseudonyms tied to any given context, and issue signatures on behalf of these pseudonyms. Concretely, given a personhood relation, an issuer (who may be a distributed entity) enables users to prove their personhood and extract an associated long-term key, which can then be used to issue signatures for any given context and message....

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...

Byzantine Agreement (BA) allows a set of $n$ parties to agree on a value even when up to $t$ of the parties involved are corrupted. While previous works have shown that, for $\ell$-bit inputs, BA can be achieved with the optimal communication complexity $\mathcal{O}(\ell n)$ for sufficiently large $\ell$, BA only ensures that honest parties agree on a meaningful output when they hold the same input, rendering the primitive inadequate for many real-world applications. This gave rise to...

Secure multi-party computation (MPC) allows a set of $n$ parties to jointly compute a function over their private inputs. The seminal works of Ben-Or, Canetti and Goldreich [STOC '93] and Ben-Or, Kelmer and Rabin [PODC '94] settled the feasibility of MPC over asynchronous networks. Despite the significant line of work devoted to improving the communication complexity, current protocols with information-theoretic security and optimal resilience $t<n/3$ communicate $\Omega(n^4C)$ field...

Account recovery enables users to regain access to their accounts when they lose their authentication credentials. While account recovery is well established and extensively studied in the Web2 (traditional web) context, Web3 account recovery presents unique challenges. In Web3, accounts rely on a (cryptographically secure) private-public key pair as their credential, which is not expected to be shared with a single entity like a server owing to security concerns. This makes account recovery...

Auction and voting schemes play a crucial role in the Web3 ecosystem. Yet currently deployed implementations either lack privacy or require at least two rounds, hindering usability and security. We introduce Cicada, a general framework for using linearly homomorphic time-lock puzzles (HTLPs) to enable provably secure, non-interactive private auction and voting protocols. We instantiate our framework with an efficient new HTLP construction and novel packing techniques that enable succinct...

Web3 applications based on blockchains regularly need access to randomness that is unbiased, unpredictable, and publicly verifiable. For Web3 gaming applications, this becomes a crucial selling point to attract more users by providing credibility to the "random reward" distribution feature. A verifiable random function (VRF) protocol satisfies these requirements naturally, and there is a tremendous rise in the use of VRF services. As most blockchains cannot maintain the secret keys required...

Broadcast protocols enable a set of $n$ parties to agree on the input of a designated sender, even facing attacks by malicious parties. In the honest-majority setting, a fruitful line of work harnessed randomization and cryptography to achieve low-communication broadcast protocols with sub-quadratic total communication and with "balanced" sub-linear communication cost per party. However, comparatively little is known in the dishonest-majority setting. Here, the most...

Account Abstraction is a powerful feature that will transform today Web3 onboarding UX. This notes describes an EVM (Ethereum Virtual Machine) implementation of the well known secp256r1 and ed25519 curves optimized for the specificities of the EVM environment. Our optimizations rely on EVM dedicated XYZZ elliptic coordinates system, hacked precomputations, and assembly tricks to cut from more than 1M to 200K/62K (with or withoutprecomputations)

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...

We prove adaptive security of a simple three-round threshold Schnorr signature scheme, which we call Sparkle+. The standard notion of security for threshold signatures considers a static adversary – one who must declare which parties are corrupt at the beginning of the protocol. The stronger adaptive adversary can at any time corrupt parties and learn their state. This notion is natural and practical, yet not proven to be met by most schemes in the literature. In this paper, we...

Can an adversary hack into our computer and steal sensitive data such as cryptographic keys? This question is almost as old as the Internet and significant effort has been spent on designing mechanisms to prevent and detect hacking attacks. Once quantum computers arrive, will the situation remain the same or can we hope to live in a better world? We first consider ubiquitous side-channel attacks, which aim to leak side information on secret system components, studied in the...

Blockchain has been broadly recognized as a breakthrough technology of the world. Web3, recently, is emerging as a buzzword, indicating the next generation of Internet based on Blockchain, envisioning the Internet of Money to store and transfer value. However, when people want a comprehensive view throughout advancements in the Blockchain space, there is a missing in the academic domain and scientific publications regarding distributed ledger technology (DLT) classification and taxonomy for...

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...

A single-leader election (SLE) is a way to elect one leader randomly among the parties in a distributed system. If the leader is secret (i.e., unpredictable) then it is called a secret single leader election (SSLE). In this paper, we model the security of SLE in the universally composable (UC) model. Our model is adaptable to various unpredictability levels for leaders that an SLE aims to provide. We construct an SLE protocol that we call semi-anonymous single leader election (SASLE). We...

Messages in large-scale networks such as blockchain systems are typically disseminated using flooding protocols, in which parties send the message to a random set of peers until it reaches all parties. Optimizing the communication complexity of such protocols and, in particular, the per-party communication complexity is of primary interest since nodes in a network are often subject to bandwidth constraints. Previous flooding protocols incur a per-party communication complexity of...

BLS signatures have fast aggregated signature verification but slow individual signature verification. We propose a three part optimisation that dramatically reduces CPU time in large distributed system using BLS signatures: First, public keys should be given on both source groups $\mathbb{G}_1$ and $\mathbb{G}_2$, with a proof-of-possession check for correctness. Second, aggregated BLS signatures should carry their particular aggregate public key in $\mathbb{G}_2$, so that verifiers can do...

We present mining-based techniques to reduce the size of various cryptographic outputs without loss of security. Our approach can be generalized for multiple primitives, such as cryptographic key generation, signing, hashing and encryption schemes, by introducing a brute-forcing step to provers/senders aiming at compressing submitted cryptographic material. Interestingly, mining can result in record-size cryptographic outputs, and we show that 5%-12% shorter hash digests and signatures...

Distributed key generation (DKG) protocols are an essential building block for threshold cryptosystems. Many DKG protocols tolerate up to $t_s<n/2$ corruptions assuming a well-behaved synchronous network, but become insecure as soon as the network delay becomes unstable. On the other hand, solutions in the asynchronous model operate under arbitrary network conditions, but only tolerate $t_a<n/3$ corruptions, even when the network is well-behaved. In this work, we ask whether one can...

In this paper we introduce dCommon - auditable and programmable MPC as a service for solving multichain governance coordination problems throughout DeFi and Web3; Along with its on-chain part Common Interest Protocol (CIP) - an autonomous and immutable registry smart contract suite. CIP enables arbitrary business logic for off-chain computations using dCommon’s network/subnetworks with Ethereum smart contracts. In Stakehouse, CIP facilitates a trustless recovery of signing keys and key...

A major challenge for blockchain interoperability is having an on-chain light client protocol that is both efficient and secure. We present a protocol that provides short proofs about the state of a decentralised consensus protocol while being able to detect misbehaving parties. To do this naively, a verifier would need to maintain an updated list of all participants' public keys which makes the corresponding proofs long. In general, existing solutions either lack accountability or are not...