<div> Mixnet protocols are used in electronic voting protocols to mix the ballot box before the tally, to preserve ballots privacy and unlinkabiliy. Whereas proving security properties of the other components of the electronic voting protocols has globally already been done in several logical frameworks and tools, proofs of mixnets remain a real challenge to handle. In this paper we focus on the quite recent CCSA logic, which enables handling of computational security proofs with first-order logics facilities. We enrich the logic to be able to deal with zero-knowledge proofs and rewinding techniques, and provide the first complete proof of Terelius-Wikström mixnet protocol. </div>
Chhavi Yadav, Evan Monroe Laufer, Dan Boneh, Kamalika Chaudhuri
In principle, explanations are intended as a way to increase trust in machine learning models and are often obligated by regulations. However, many circumstances where these are demanded are adversarial in nature, meaning the involved parties have misaligned interests and are incentivized to manipulate explanations for their purpose. As a result, explainability methods fail to be operational in such settings despite the demand \cite{bordt2022post}. In this paper, we take a step towards operationalizing explanations in adversarial scenarios with Zero-Knowledge Proofs (ZKPs), a cryptographic primitive. Specifically we explore ZKP-amenable versions of the popular explainability algorithm LIME and evaluate their performance on Neural Networks and Random Forests. Our code is publicly available at https://github.com/emlaufer/ExpProof.
This thesis explores the implementation of a Self-Sovereign Identity (SSI) system using Ethereum and Decentralized Identifiers (DIDs). The project focuses on leveraging blockchain technology to create a secure and decentralized framework for digital identity management, incorporating Verifiable Credentials (VCs) and Verifiable Presentations (VPs). Key components include DID document management, secure user authentication, and user-friendly interface. What makes this system different is the integration with existing wallets, privacy and user control through selective disclosure, allowing users to share only necessary information, and key rotation. It also uses EIP-712 signatures for secure and structured data signing, which allows users to clearly see and understand what they are signing while the cryptography is securely handled by the wallet. Future work will focus on adding more wallet support, improving data storage, and enhancing system scalability and security.
Ben Biedermann, Matthew Scerri, Victoria Kozlova, Joshua Ellul
Web3’s decentralised infrastructure has upended the standardised approach to digital identity established by protocols like OpenID Connect. Web2 and Web3 currently operate in silos, with Web2 leveraging selective disclosure JSON web tokens (SD-JWTs) and Web3 dApps being reliant on on-chain data and sometimes clinging to centralised system data. This fragmentation hinders user esxperience and the interconnectedness of the digital world. This article explores the integration of Web3 within the OpenID Connect framework, scrutinising established authentication protocols for their adaptability to decentralised identities. The research examines the interplay between OpenID Connect and decentralised identity concepts, the limitations of the existing protocols like OpenID Connect for verifiable credential issuance, OpenID Connect framework for verifiable presentations, and self-issued OpenID provider. As a result, a novel privacy-preserving digital identity bridge is proposed, which aims to answer the research question of whether authentication protocols should inherently support Web3 functionalities and the mechanisms for their integration. Through a Decentralised Autonomous Organisation (DAO) use case, the findings indicate that a privacy-centric bridge can mitigate the existing fragmentation by aggregating different identities to provide a better user experience. While the digital identity bridge demonstrates a possible approach to harmonise digital identity across platforms for their use in Web3, the bridging is unidirectional and limits root trust of credentials. The bridge’s dependence on centralised systems may further fuel the debate on (de)centralised identities.
As an emerging paradigm in digital identity, Decentralized Identity (DID) appears advantages over traditional identity management methods in a variety of aspects, e.g., enhancing user-centric online services and ensuring complete user autonomy and control. Verifiable Credential (VC) techniques are used to facilitate decentralized DID-based access control across multiple entities. However, existing DID schemes generally rely on a distributed public key infrastructure that also causes challenges, such as context information deduction, key exposure, and issuer data leakage. To address the issues above, this paper proposes a issuer-hiding and privacy-preserving DID multi-party authentication model with a signature-less VC scheme, named SLVC-DIDA, for the first time. Our proposed scheme avoids the dependence on signing keys by employing hashing and issuer membership proofs, which supports universal zero-knowledge multi-party DID authentications, eliminating additional technical integrations. We adopt a novel zero-knowledge circuit to maintain the anonymity of the issuer set, thereby enabling public verification while safeguarding the privacy of identity attributes via a Merkle tree-based VC list. Furthermore, by eliminating reliance on a Public Key Infrastructure (PKI), SLVC-DIDA enables decentralized and self-sovereign DID authentication. Our experiments further evaluate the effectiveness and practicality of SLVC-DIDA.
ObjectiveByzantine Fault-Tolerant (BFT) consensus algorithms in blockchain systems were confronted with two fundamental challenges that significantly impacted their practical implementation: inefficient view changes resulting from dishonest primary nodes and excessive communication overhead during consensus processes.MethodsA novel Dynamic Dual-Master Practical Byzantine Fault Tolerance (DM-PBFT) algorithm was developed to simultaneously overcome these limitations through three key technical innovations. The DM-PBFT architecture was constructed with two tightly integrated components: an optimized consensus process and an intelligent view-change mechanism. To address the critical issue of communication overhead, the algorithm incorporated a dual-primary node framework (designated as B1 and B2) combined with Verifiable Secret Sharing (VSS) to streamline the consensus workflow. In this carefully designed process: (1) client A initiated transactions by sending requests to primary node B1; (2) B1 subsequently broadcast these requests along with A's unique identifier to the network; (3) the secondary primary node B2 distributed cryptographic subkeys and corresponding commitment values to all consensus nodes in set B; (4) each node independently verified the received subkeys using the commitment values to ensure cryptographic integrity; (5) upon successful verification, nodes computed request approval digests incorporating their subkeys and transmitted these digests back to B1; (6) after collecting sufficient subkeys, B1 performed efficient key recovery and compared the results with B2's master key; (7) finally, verification values were broadcast network-wide and persistently stored in all nodes' state databases to complete the consensus process.For optimizing view-change efficiency, a sophisticated reputation evaluation system was implemented based on an enhanced PeerTrust model. This system incorporated multidimensional confidence factors that dynamically synthesized both local interaction history and global reputation metrics through comprehensive analysis of inter-node verification relationships. The algorithm strategically organized nodes into three distinct operational tiers (primary layer, consensus layer, and non-consensus layer), each with specialized reputation evaluation methodologies. View transitions were executed through continuous hierarchical scoring during consensus rounds, with the system automatically adjusting node classifications based on their evolving reputation scores.The view-change protocol incorporated three carefully designed failure recovery procedures: (1) When primary node B1 was identified as Byzantine, the top-ranked consensus node was automatically promoted to replace it while maintaining operational continuity through B2's consistent subkey management; (2) For failures of B2, a similar replacement protocol was activated with additional safeguards to ensure complete subkey redistribution; (3) In the rare case of simultaneous failure of both primary nodes, the two highest-ranked consensus nodes were promoted to form a new primary pair, with the system automatically reinitializing the consensus process. Within the consensus layer, Byzantine nodes were systematically identified through continuous monitoring and temporarily marked rather than immediately replaced. Replacement was only triggered when the concentration of marked Byzantine nodes reached precisely one-third of the consensus layer's capacity, at which point a corresponding number of top-performing nodes from the non-consensus layer were promoted. This threshold-based approach strategically minimized unnecessary view changes while maintaining rigorous fault tolerance guarantees.Through rigorous algorithmic analysis, DM-PBFT was formally proven to satisfy all critical BFT properties: (1) Request messages and subkeys were cryptographically secured through the combined use of advanced signature algorithms and commitment schemes; (2) Consistency was guaranteed through mathematical proof showing all honest nodes would agree on the same sequence of requests; (3) Liveness was ensured by demonstrating the system would always progress within bounded time, even during view changes; (4) Termination was mathematically verified through analysis of the reputation-based view-change protocol. Communication complexity analysis established that DM-PBFT achieved O(n) complexity, representing a significant improvement over PBFT's O(n²) scaling. Time complexity analysis, conducted under realistic asynchronous network models, confirmed the algorithm's superior temporal efficiency compared to existing approaches.ResultsAn extensive experimental evaluation was conducted to validate DM-PBFT's performance across multiple critical dimensions. The testing framework systematically compared DM-PBFT against three established benchmarks (PBFT, reputation grouping, and HotStuff) while examining consensus latency, communication overhead, throughput, and resilience to network dynamics under various operational conditions.In controlled latency testing with network sizes scaling to 500 nodes, DM-PBFT demonstrated remarkable performance, achieving consensus latency of just 0.46176 seconds. This represented a 400× improvement over conventional PBFT (186.44476s), a 100× improvement over reputation grouping (46.09653s), and a 56× improvement over HotStuff (25.8702s). Detailed analysis revealed that while all tested algorithms exhibited increased latency with network growth, DM-PBFT's hierarchical architecture maintained superior scalability, with latency increasing at a sub-linear rate compared to the polynomial growth observed in other approaches.Communication overhead measurements showed that DM-PBFT achieved stable performance after reaching network stratification thresholds, consistently maintaining O(n√n) complexity. In practical terms, this translated to a 62.4% reduction in bandwidth consumption compared to traditional PBFT implementations and measurable improvements over HotStuff's message efficiency. The communication overhead grew gradually with network size until reaching stratification points, after which it remained effectively constant regardless of additional node joins.Throughput benchmarking produced equally impressive results, with DM-PBFT sustaining 3,500 transactions per second (TPS) in large-scale configurations compared to PBFT's 1,200 TPS. While matching HotStuff's peak throughput, DM-PBFT demonstrated significantly better scalability, with throughput improvement rates exceeding those of the reputation grouping approach by substantial margins after crossing stratification thresholds.Additional experiments examined DM-PBFT's resilience under challenging network conditions. Node churn tests confirmed that the frequency of node joins/leaves only impacted performance through net changes in total node count, not through the rate of such changes. Network bandwidth fluctuation tests demonstrated the reputation system's effectiveness in automatically compensating for variable connection quality, with the algorithm maintaining stable performance across diverse bandwidth conditions after several consensus rounds of adaptation.The comprehensive experimental results collectively established DM-PBFT's advantages across three fundamental metrics: (1) Exceptional latency characteristics, delivering 400× improvements at scale; (2) Efficient bandwidth utilization, achieving 62.4% reductions compared to conventional approaches; (3) Consistently high throughput maintenance even under adversarial conditions with Byzantine node concentrations up to 30%. The view-change process demonstrated particular efficiency, completing in just 1.2 seconds compared to PBFT's 12.8 seconds - a 90.6% reduction that proved critical for practical deployment scenarios.ConclusionsThe DM-PBFT algorithm represented a significant theoretical and practical advancement in Byzantine fault-tolerant consensus mechanisms through its novel integration of dual-primary node management, verifiable secret sharing, and dynamic reputation-based stratification. The solution demonstrated particular effectiveness for large-scale consortium blockchain implementations, successfully achieving the dual objectives of sub-linear communication growth and rapid fault recovery without compromising security guarantees.The algorithm's architectural innovations, especially its stratified node management framework and intelligent threshold-based view-change protocol, established a new foundation for next-generation consensus protocol design in increasingly complex and adversarial network environments. Future research directions were identified to further enhance the algorithm's practical utility, including: (1) Optimization for real-world deployment scenarios with heterogeneous hardware; (2) Development of cross-shard coordination mechanisms for sharded blockchain architectures; (3) Enhanced security analysis under sophisticated adaptive adversary models; (4) Integration with emerging cryptographic techniques such as zero-knowledge proofs for additional privacy preservation. These advancements promised to extend DM-PBFT's applicability to an even broader range of production blockchain environments while maintaining its fundamental advantages in efficiency, security, and scalability.
A novel identity management concept known as decentralized identity (or self-sovereign identity) has drawn significant interest and extensive development within both academic and industrial circles. Decentralized oracles empower users to demonstrate the origin of data accessed through TLS from a specific website and verify statements concerning that data. This system operates without the reliance on trusted hardware or requiring modifications on the server side. In DECO (CCS 2020), the first decentralized oracle within TLS 1.2 was introduced. It also investigates the methodology of optionally proving statements about this data in a zero-knowledge setting, preserving the confidentiality of the data itself. Furthermore, a separate study proposed an attribute-based anonymous credential system incorporating a commitment scheme (ASIACRYPT 2020), introducing show proofs to verify a set of attributes in a credential to verifiers without disclosing attribute specifics. Building upon these research findings, this thesis presents a series of optimizations and expansions aimed at advancing self-sovereign identity solutions. First, we present DIDO and DIDO+ (decentralized identification oracle), which expands DECO to TLS 1.3. In DIDO, we address several unresolved challenges, encompassing the incorporation of X25519 key exchange, the creation of a round-optimal three-party key exchange, the structuring of a 2PC system for TLS 1.3 key scheduling, and the optimization of circuit design for 2PC protocols. Furthermore, we expand upon DIDO to create DIDO+, which introduces a protocol named selective disclosure. This protocol facilitates the extraction of particular plaintext substrings from websites, enhancing the functionality and flexibility of the system. Our implementation is verified against real-world websites, and a security proof is furnished to validate its integrity. Next, we introduce DEVS (decentralized verification service), the first generic decentralized verification service built on a decentralized oracle. DEVS enhances the traditional single-verifier oracle by incorporating a multi-verifier mechanism. Our aim is to streamline the protocol for reduced communication and computation costs while enabling reusable proofs and maintaining security. DEVS comprises three key components: a reconstructed decentralized oracle to bolster data trust, secure storage for share retention, and a verification process tailored to the needs of verifiers and authorities. Considering 10 verifiers, the reconstruction reduces the running time (approximately 89.1% or 89.6%) in WAN settings and communication bandwidth (approximately 89.1% or 87.8%) of TLS 1.2 or 1.3, it provides a significant improvement when working with multiple verifiers. Lastly, we introduce a unified attribute-based anonymous credential system where users consistently receive credentials in a standardized format from the issuer. This system allows users to opt for efficient multi-use or single-use show proofs, offering a more user-centric perspective compared to current schemes. Technically, we suggest an interactive method for the credential issuance protocol utilizing two-party computation with additive homomorphic encryption. This approach maintains crucial security features such as impersonation resilience, anonymity, and unlinkability. In addition to the interactive protocol, we develop show proofs tailored for efficient single-use credentials that uphold user anonymity throughout the process.
Eclipse attack is a major threat to the blockchain network layer, wherein an attacker isolates a target node by monopolizing all its connections, cutting it off from the rest of the network.Despite the attack's demonstrated effectiveness in Bitcoin (Usenix'15, SP'20, Usenix'21, CCS'21, SP'23) and partially in Ethereum (NDSS'23, SP'23), its applicability to a wider range of blockchain systems remains uncertain.In this paper, we investigate eclipse attacks against Monero, a blockchain system known for its strong anonymity and pioneering the use of Dandelion++ (the state-of-the-art blockchain network layer protocol for transaction privacy protection).Our analysis of Monero's connection management mechanism reveals that existing eclipse attacks are surprisingly ineffective against Monero.We accordingly introduce the first practical eclipse attack against Monero by proposing a connection reset approach, which forces the target node to drop all benign connections and reconnect with malicious nodes.Specifically, we outline two methods for executing such an attack.The first one exploits the private transaction mechanisms, while the second method leverages the differences in propagation between stem transactions and fluff transactions under Dandelion++.Our attack is not only applicable to Monero but to all blockchain systems utilizing Dandelion++ and similar connection management strategies.We conduct experiments on the Monero mainnet.Evaluation results confirm the feasibility of our attack.Unlike existing eclipse attacks, our connection reset-based approach does not require restarting the target node, significantly accelerating the attack process and making it more controllable.We also provide countermeasures to mitigate the proposed eclipse attack while minimizing the impact on Monero.In addition, we have ethically reported our investigation to Monero official team.
The group signature with designated traceability (GSdT) is a kind of group signatures (GS) which aim to restrict the opening authority of the group manager; by setting an access structure over openers' attributes at the signing, a signer is able to control openers who can open the signature.A generic construction of GSdT was given when the notion was introduced, then a pairing-based construction and a symmetric-key-based one were presented.Nonetheless, it remains open whether a post-quantum GSdT with full anonymity can be truly constructed.In this paper, we give a lattice-based GSdT scheme that has full anonymity for the first time.In our construction, the lattice-based ciphertext-policy attribute-based encryption (CP-ABE) by Tsabary and the lattice-based group signatures (GS) by Libert et al. are employed.The CP-ABE is based on the Regev public-key encryption, while the GS uses a non-interactive zero-knowledge proof to prove the correctness of the encryption in the signing process.Based on the compatibility, we combine and modify them to build up a GSdT scheme.
Microservices commonly rely on bearer tokens such as JSON Web Tokens (JWTs). Although fast, these tokens reveal user claims to every service and can be reused if stolen. This thesis designs and evaluates a privacy-preserving alternative based on zero-knowledge proofs (ZKPs) that verifies each request under a zero-trust model without disclosing identity attributes. The authentication prototype service is built with two implementations (TypeScript/NestJS and Rust) and compared with a JWT baseline in privacy, access control correctness, and performance. Across six concurrency levels (1, 10, 15, 20, 25, 30) over 15-second runs, CPU and memory usage were sampled every 250 ms, and means were reported across the concurrency points. The workflow achieves zero attribute disclosure while keeping verification practical: verification adds 1.8 ms per microservice hop with a native Rust verifier and ∼130 ms with a NestJS verifier. Proof generation is the main computational cost (∼0.7s in NestJS; ∼1.4s in Rust), with NestJS demonstrating superior performance due to efficient WASM witness generation and reuse of artefacts. These measurements were taken on the same host with an otherwise idle machine to control variance. Formal statistical significance would require multiple independent repetitions; the present analysis reports central tendencies and observed stability. Contributions are: (i) a working ZKP-based authentication workflow suitable for microservices, (ii) a dual-stack prototype and comparative evaluation against JWT, and (iii) evidence that privacy can be improved without prohibitive verification overhead.
Domain-specific languages (DSLs) express requirements or designs through visual abstractions.To support complex development tasks such as code generation, testing and analysis, DSLs need semantic foundations.This paper introduces such a semantic framework for DSLs based on graph rewriting.We apply our framework to a DSL for defining multi-party dynamic role-based access control policies for smart contracts.Role-based access control models (RBACMs) express constraints on who can access which resources.Dynamic RBACMs allow a dynamic role membership.Access control policies, in particular for smart contracts, can involve multiple parties such as members of different groups or organisations, combining complex logical and dynamic constraints, and hence are hard to design, understand, validate and test at code level.Our diagrammatic notation supports complex authorisation patterns, including alternatives and multiplicities, to address nuanced access control requirements.Defining the operational semantics for RBACMs by graph rewriting, we let the Groove model checker produce traces for actions where access is granted or denied and generate tests for smart contracts in the Digital Asset Modelling Language (DAML).We validate dynamic access control scenarios generated by ChatGPT for use as test cases or advising users at runtime.Such scenarios represent business workflows interleaved with operations to add or remove role members.They are expressed as Groove control programs and are also verified by its model checker.
Die zunehmende Verbreitung verteilter Datenspeicher wie dem InterPlanetary File System erfordert robuste Sicherheitsmechanismen, um den Schutz sensibler Daten in dezentralen Umgebungen zu gewährleisten. Diese Masterarbeit präsentiert ein auf Smart Contracts basierendes Authentifizierungs- und Zugangskontrollsystem, das den Ethereum Attestation Service nutzt, um Rollen und Berechtigungen als nicht übertragbare Attestierungen zu verwalten. Durch die Integration von IPFS und clientseitiger Verschlüsselung wird ein hybrides Modell entwickelt, das öffentliche Datenspeicher durch granular definierte Zugriffskontrollbedingungen absichert. Die Arbeit evaluiert das System in einem praxisnahen Anwendungsfall einer Decentralized Autonomous Organization und demonstriert die Skalierbarkeit durch die Nutzung von Layer-2-Blockchains wie Scroll. Die Analyse der Kosteneffizienz von Smart Contract-Interaktionen zeigt, dass das Framework eine dezentrale, interoperable Verwaltung von Zugriffsrechten ermöglicht und dabei Transaktionskosten durch die Nutzung von Layer-2-Netzwerken signifikant reduziert. Zudem wird die Machbarkeit einer rollenbasierten Zugriffskontrolle ohne zentrale Instanz nachgewiesen, die gleichzeitig die Integrität und Verfügbarkeit der Daten gewährleistet. Die Arbeit leistet einen Beitrag zur Erforschung blockchainbasierter Sicherheitslösungen und bietet eine Blaupause für Organisationen, die dezentrale Speicherlösungen in offenen Ökosystemen einsetzen. Zukünftige Arbeiten könnten die Integration weiterer dezentraler Identitätsmodelle oder die Erweiterung um attributbasierte Zugriffskontrollmechanismen untersuchen, um die Privatsphäre und Flexibilität des Systems weiter zu optimieren.
As network applications rapidly evolve toward mobile and ubiquitous scenarios, identity authentication protocols face heightened demands for privacy protection and computational efficiency while maintaining security.Traditional authentication schemes often struggle to achieve an effective balance between privacy preservation, computational complexity, and security during design, with performance bottlenecks becoming increasingly prominent in resource-constrained environments.To address these challenges, this study proposes an optimized algebraic curve identity authentication protocol incorporating zero-knowledge proofs.Building upon Elliptic Curve Cryptography (ECC) as its cryptographic foundation, the protocol leverages ECC's inherent advantages of shorter key lengths and higher computational efficiency for equivalent security levels.Simultaneously, it integrates zero-knowledge proof mechanisms to minimize the exposure of user identity information during authentication.Through systematic optimization of the key generation mechanism, zero-knowledge proof interaction flow, and identity verification logic, the proposed protocol effectively reduces computational and communication overhead while ensuring identity anonymity and authentication integrity.Experimental results demonstrate that compared to traditional ECC authentication protocols and classical zero-knowledge proof schemes, the optimized protocol exhibits significant advantages in key generation time, authentication response latency, and communication load.It effectively resists common security threats such as replay attacks and forgery attacks, making it suitable for resource-constrained network environments and privacy-sensitive applications.
In decentralized governance, existing token-based voting mechanisms employed by Decentralized Autonomous Organizations (DAOs) often suffer from centralization risks, as a small number of large token holders can dominate decision-making. While reputation-based voting has emerged as an alternative that reflects members’ historical contributions, it is also prone to ossification, where early contributors accumulate disproportionate influence over time. To address these challenges, we propose a novel hybrid dynamic voting mechanism that combines tokens and reputation in the voting power. Furthermore, we incorporate an exponential decay mechanism into the reputation component to ensure that voting power dynamically reflects recent and active participation rather than legacy status. We also utilize Gini and Nakamoto coefficients to evaluate the proposed mechanism. To validate our proposed mechanism, we design some computational experiments, and the experimental results show that the proposed hybrid mechanism achieves better fairness and decentralization, while allowing new contributors to more quickly gain meaningful influence. This work provides insights into designing adaptive, fair, and secure governance mechanisms for DAOs.
With the growing integration of cloud computing and the increasing adoption of Internet-of-Things (IoT) devices, ensuring the integrity and privacy of data has become critical in digital systems. Data integrity is fundamental to maintain the completeness and reliability of data throughout the data lifecycle. Its importance is particularly evident in domains such as healthcare, where accurate diagnoses rely on trustworthy data. However, as systems evolve and become more complex, traditional centralised solutions often lack transparency and resilience, while resource-constrained devices make it more difficult to guarantee security and privacy. This thesis addresses these challenges by proposing a framework that integrates Distributed Ledger Technology (DLT) to support privacy-preserving data sharing and strengthen trust among system stakeholders. Based on this framework, an architecture was designed with three main modules: a middleware integrator for service interoperability, an authorisation manager for fine-grained access control, and a data integrity validator leveraging metadata anchored on a distributed ledger to ensure compliance with the General Data Protection Regulation. A proof of concept was designed and implemented using IoT devices, healthcare data, and low-resource hardware. Experimental results demonstrate that the proposed solution enables efficient data sharing and integrity validation with minimal overhead on the system. The DLT layer validated the integrity of shared data through a metadata model while preserving user privacy. Furthermore, the access control mechanism supported scalable and granular authorisation policies, and the middleware facilitated interoperability across heterogeneous stakeholders. This work contributes to new insights into the security of digital systems and provides responsible entities with a trustworthy approach for sharing data among diverse entities.
Jan 1, 2025·Proceedings of the 4th International Conference on Information Technology, Civil Innovation, Science, and Management, ICITSM 2025, 28-29 April 2025, Tiruchengode, Tamil Nadu, India, Part I
G. Sowmya Bala, P. S. G. Aruna Sri, Satyanarayana Korada, Suneel Gone
Traditional ticketing systems are at risk of fraud, counterfeiting, and issues concerning scalability. In this research, we investigate the application of blockchain technology towards the revolutionary concept of event tickets. We analyze how fundamental attributes of blockchain technology, such as