Stephan Dübler, Federico Badaloni, Pedro Moreno-Sánchez, Clara Schneidewind
The heterogeneity of the blockchain landscape has motivated the design of blockchain protocols tailored to specific blockchains and applications that, hence, require custom security proofs. We observe that many blockchain protocols share common security and functionality goals, which can be captured by an atomic transfer graph (ATG) describing the structure of desired transfers. Based on this observation, we contribute a framework for generating secure-by-design protocols that realize these goals. The resulting protocols build upon Conditional Timelock Contracts (CTLCs), a novel minimal smart contract functionality that can be implemented in a large variety of cryptocurrencies with a restricted scripting language (e.g., Bitcoin), and payment channels. We show how ATGs, in addition to enabling novel applications, capture the security and functionality goals of existing applications, including many examples from payment channel networks and complex multi-party cross-currency swaps among Ethereum-style cryptocurrencies. Our framework is the first to provide generic and provably secure protocols for all these use cases while matching or improving the performance of existing use-case-specific protocols.
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cs.CR
Blockchain Technology Applications and Security
Innovative Microfluidic and Catalytic Techniques Innovation
Smart vehicle applications play a crucial role in intelligent transportation systems, enabling sensor-equipped vehicles to establish dynamic networks for efficient collection, sharing, and aggregation. This significantly enhances road security and efficiency by transmitting crucial information to traffic authorities. However, partial research on certificateless aggregation signcryption (CLAS) scheme reveals an intriguing phenomenon where each proposed scheme consistently exhibits numerous security vulnerabilities, particularly susceptible to public key replacement attacks. To address these challenges, this paper proposes a blockchain-assisted certificateless aggregation sign-cryption scheme (BACLAS), leveraging blockchain technology to securely store users’ public key on a distributed ledger and prevent public key replacement attacks effectively. Furthermore, it is a provably secure communication scheme for real-world Vehicle-to-Infrastructure (V2I) communication while addressing practical security concerns. The BACLAS scheme ensures security in terms of existential unforgeability against adaptive chosen message attacks (EUF-CMA) and indistinguishability against adaptive chosen ciphertext attacks (IND-CCA2) based on the hardness assumption of the elliptic curve discrete logarithm problem and computational Diffie-Hellman problem in the random oracle model. The proposed scheme effectively reduces computational costs and time consumption, resulting in a significant reduction of the computation burden ranging from 50.02% to 88.28% compared to other competitive schemes. Moreover, it successfully addresses the key-escrow problem, thereby achieving enhanced security properties.
The sealed-bid auction enables bidders to secretly send their bids to the auctioneer, which compares all bids and publishes the winning one on the bid-opening day. This type of auction is friendly for protecting the bid privacy, and sufficiently fair for all bidders if the auctioneer acts faithfully. Unfortunately, the auctioneer may not always be trustworthy. The auctioneer has the ability to deliberately leak any bid information to a part of bidders for raising the final winning price based on the investigation. Meanwhile, the auctioneer can appoint any bidder as the winner, as long as the bidder accepts a higher winning price than the current highest bid. Since bidders cannot obtain any bid information from others, to the best of our knowledge, it is difficult to prevent bid leakage from the auctioneer, and support bidders to verify the bid comparison results without disclosing the winning bid, simultaneously. To alleviate these problems, we first construct a homomorphic encryption(HE)-based bid comparison circuit. All bidders can directly compute a cipher of the winning bid by using this circuit; hence, the winning bid does not need to be exposed to all bidders. Then, we propose a blockchain-based sealed-bid scheme (BSS) by integrating the circuit with commitment and zero-knowledge proof. The auctioneer only obtains the commitments of bids before the bid-opening day, and he has to prove that the winner's bid is the same as the plaintext of the bidders' computed cipher. Thus, the auctioneer can neither leak the bid information nor publish a higher winning price during in the auction. Detailed performance analysis shows that the computational complexity of BSS is linear with the binary length of bids.
The prosperity of blockchain technique has spawned numerous types of cryptocurrencies. However, a secure and privacy-preserving cryptocurrency exchange protocol that can support atomicity and unlinkability, and effectively resist various attacks is still missed in the literature. This paper proposes SPCEX, a secure and privacy-preserving cryptocurrency exchange protocol, which supports atomicity and unlinkability, as well as resists collusion attacks, front-running attacks and Sybil attacks without relying on any trust execution environments (TEEs). SPCEX employs a mixing protocol to conceal trader involvement and preserve their identity privacy. It applies a privacy-preserving comparison algorithm based on an improved homomorphic re-encryption scheme to match buy and sell orders secretly, thus resisting front-running attacks. In addition, we construct atomic transactions with smart contracts to protect tokens of honest traders. A collateral deposit mechanism is employed to deter collusive parties and penalize abnormal behaviors, which discourages collusion attacks and Sybil attacks. We analyze the security and privacy of SPCEX, and formally prove the security of the improved homomorphic re-encryption scheme. Through proof-of-concept implementation, we demonstrate its validity and reliability, and show its advanced performance by comparing it with a cutting-edge scheme.
Erkan Tairi, Pedro Moreno-Sánchez, Clara Schneidewind
The scalability and interoperability challenges in current cryptocurrencies have motivated the design of cryptographic protocols that enable efficient applications on top and across widely used cryptocurrencies such as Bitcoin or Ethereum. Examples of such protocols include (virtual) payment channels, atomic swaps, oracle-based contracts, deterministic wallets, and coin mixing services. Many of these protocols are built upon minimal core functionalities supported by a wide range of cryptocurrencies. Most prominently, adaptor signatures (AS) have emerged as a powerful tool for constructing blockchain protocols that are (mostly) agnostic to the specific logic of the underlying cryptocurrency. Even though AS-based protocols are built upon the same cryptographic principles, there exists no modular and faithful way for reasoning about their security. Instead, all the works analyzing such protocols focus on reproving how adaptor signatures are used to cryptographically link transactions while considering highly simplified blockchain models that do not capture security-relevant aspects of transaction execution in blockchain-based consensus.
Sourav Das, Philippe Camacho, Zhuolun Xiang, J Sierra Nieto · 6 authors
Threshold signatures protect the signing key by sharing it among a group of signers so that an adversary must corrupt a threshold number of signers to be able to forge signatures. Existing threshold signatures with succinct signatures and constant verification times do not work if signers have different weights. Such weighted settings are seeing increasing importance in decentralized systems, especially in the Proof-of-Stake blockchains. This paper presents a new paradigm for threshold signatures for pairing and discrete logarithm-based cryptosystems. Our scheme has a compact verification key consisting of only 7 group elements, and a signature consisting of 8 group elements. Verifying the signature requires 8 exponentiations and 8 bilinear pairings. Our scheme supports arbitrary weight distributions among signers and arbitrary thresholds. It requires non-interactive preprocessing after a universal powers-of-tau setup. We prove the security of our scheme in the Algebraic Group Model and implement it using Golang. Our evaluation shows that our scheme achieves a comparable signature size and verification time to a standard (unweighted) threshold signature. Compared to existing multisignature schemes, our scheme has a much smaller public verification key.
Qianqian Pan, Jun Wu, Jianhua Li, Wu Yang · 5 authors
As a promising technology, intelligent reflecting surface (IRS) enables future communications and networks to realize programmable data transmissions. Due to the untrustworthiness of the communication environment and the selfishness of wireless devices, secure and intelligent IRS resource management is still an open issue. In this paper, we aim to implement IRS resource scheduling with properties of security, intelligence, efficiency, and fairness. To realize the above goals, we propose the blockchain and multi-agent learning empowered incentive scheduling system for tamper-proof and undeniable IRS resource management. To overcome the low throughout and intensive computation issues of blockchain, we devise a hybrid framework combining traditional Satoshi-style and directed acyclic graph blockchain for IRS resource scheduling. Due to the storage limitation of wireless devices, an intelligent blockchain storage reduction mechanism is proposed, where a multi-dimensional multi-hierarchy feature-based scheme is designed to determine block storage priority. Based on this storage priority and device states, the selection of storage-reduction devices is formulated as a cooperative multi-agent decision problem. Then, a multi-agent deep reinforcement learning-driven scheme is proposed to determine reduction strategies. To facilitate IRS providers/subscribers participating in the proposed system and maintain the efficiency of resource scheduling, an auction-based incentive mechanism is devised. In this mechanism, we propose the IRS resource allocation scheme and the payment scheme to achieve economic robustness and high efficiency. Finally, security analysis and experiment analysis indicate the feasibility and effectiveness of the proposed IRS resource scheduling in intelligent reconfigurable networks.
Despite the great potential and flexibility of smart contract-enabled blockchains, building privacy-preserving applications using these platforms remains an open question. Existing solutions fall short since they ask end users to coordinate and perform the computation off-chain themselves. While such an approach reduces the burden of the miners of the system, it largely limits the ability of lightweight users to enjoy privacy since performing the actual computation on their own and attesting to its correctness is expensive even with state-of-the-art proof systems.To address this limitation, we propose smartFHE, a framework to support private smart contracts using fully homomorphic encryption (FHE). To the best of our knowledge, smartFHE is the first to use FHE in the blockchain model; moreover, it is the first to support arbitrary privacy-preserving applications for lightweight users under the same computation-on-demand model pioneered by Ethereum. smartFHE does not overload the user since miners are instead responsible for performing the private computation. This is achieved by employing FHE so miners can compute over encrypted data and account balances. Users are only responsible for proving well-formedness of their private inputs using efficient zero-knowledge proof systems (ZKPs). We formulate a notion for a privacy-preserving smart contract (PPSC) scheme and show a concrete instantiation of our smartFHE framework. We address challenges resulting from using FHE in the blockchain setting—including concurrency and dealing with leveled schemes. We also show how to choose suitable FHE and ZKP schemes to instantiate our framework, since naively choosing these will lead to poor performance in practice. We formally prove correctness and security of our construction. Finally, we conduct experiments to evaluate its efficiency, including comparisons with a state-of-the-art scheme and testing several private smart contract applications. We have open-sourced our (highly optimized) ZKP library, which could be of independent interest.
As one of the crucial features of the blockchain technique, immutability plays the most important role in winning the so-called praise of the “trust machine” for blockchain. However, there are two sides to everything. The property of immutability of blockchain is applied maliciously sometimes, say publishing harmful or even dangerous data and hindering authorities’ law enforcement. To address this issue, authorized redactability of blockchain was introduced to support block modification without lowering the fundamental basis of security and trust that is cherished on the blockchain. During the past years, several techniques of redactable blockchain were proposed, mainly based on the well-known chameleon hashing. Different from existing methodologies, we propose a new redactable blockchain scheme for permissioned settings in this article. We first employ the trapdoor verifiable delay function to attach a time-lapse proof to each block. Moreover, the trapdoor is used to quickly construct a chain fork to redact blocks that are authorized to alter. Our proposal does not need to rollback irrelevant blocks. As a remarkable and unique feature, our proposal realizes the property of strong synchronization of redaction, which means that all nodes in the blockchain will have identical views on the chain even after some blocks are altered. Security analysis shows that the consistency of the chain is guaranteed, and the long-range attack can be resisted effectively. The performance comparison shows that our method is feasible and practical.
Sharding scales throughput by splitting blockchain nodes into parallel groups. However, different shards' independent and random scheduling for cross-shard transactions results in numerous conflicts and aborts, since cross-shard transactions from different shards may access the same account. A deterministic ordering can eliminate conflicts by determining a global order for transactions before processing, as proved in the database field. Unfortunately, due to the intertwining of the Byzantine environment and information isolation among shards, there is no trusted party able to predetermine such an order for cross-shard transactions. To tackle this challenge, this paper proposes Prophet, a conflict-free sharding blockchain based on Byzantine-tolerant deterministic ordering. It first depends on untrusted self-organizing coalitions of nodes from different shards to pre-execute cross-shard transactions for prerequisite information about ordering. It then determines a trusted global order based on stateless ordering and post-verification for pre-executed results, through shard cooperation. Following the order, the shards thus orderly execute and commit transactions without conflicts. Prophet orchestrates the pre-execution, ordering, and execution processes in the sharding consensus for minimal overhead. We rigorously prove the determinism and serializability of transactions under the Byzantine and sharded environment. An evaluation of our prototype shows that Prophet improves the throughput by $3.11\times$ and achieves nearly no aborts on 1 million Ethereum transactions compared with state-of-the-art sharding.
Consensus algorithms that function in permissionless blockchain systems must randomly select new block proposers in a decentralised environment. Our contribution is a new blockchain consensus algorithm called Proof-of-Publicly Verifiable Randomness (PoPVR). It may be used in blockchain design to make permissionless blockchain systems function as pseudo-random number generators and to use the results for decentralised consensus. The method employs verifiable random functions to embed pseudo-random number seeds in the blockchain that are confidential, tamper-resistant, unpredictable, collision-resistant, and publicly verifiable. PoPVR does not require large-scale computation, as is the case with Proof-of-Work and is not vulnerable to the exclusion of less wealthy stakeholders from the consensus process inherent in stake-based alternatives. It aims to promote fairness of participation in the consensus process by all participants and functions transparently using only open-source algorithms. PoPVR may also be useful in blockchain systems where asset values cannot be directly compared, for example, logistical systems, intellectual property records and the direct trading of commodities and services. PoPVR scales well with complexity linear in the number of transactions per block.
Security and a decentralized system are identical unique features of Blockchain. In recent times, blockchain-based cryptocurrency has become mainstream, but the growth and value of transactions and application services remain volatile. Among all these applications, finding a fast consensus in a large-scale blockchain network frequently requires extreme energy for huge computations and storing the complete blockchain for verification. These problems prevent further commercialization. Here, we present a solution to this problem. In this paper, we introduce a revised blockchain consensus algorithm, PDPoS, to address the scalability and transaction efficiency limitations. The symmetry in between Proof of Stake (PoS) and Delegated Proof of Stake (DPoS) is PoS. However, their ways of working are dissimilar. Here, we review the existing consensus algorithms, such as Proof of work (PoW), PoS and DPoS, as they are directly relating to our proposed work: PDPoS. We highligh Delegated Proof of Stake (DPoS)–based crypto-currencies, as they have much higher transactions per second (TPS) than PoW-based currencies. Then, we describe our proposed works and the working steps of the proposed PDPoS. Simulation results of the proposed PDPoS with two layers result in improved efficiency. We used TPS as the evolution criteria for showing that the proposed PDPoS is more efficient than DPoS. This makes the proposed work more relevant to the large-scale blockchain network as it is more efficient and requires less energy consumption.
Maxime Buser, Rafael Dowsley, Muhammed F. Esgin, Clémentine Gritti · 12 authors
Blockchain technology provides efficient and secure solutions to various online activities by utilizing a wide range of cryptographic tools. In this article, we survey the existing literature on post-quantum secure digital signatures that possess exotic advanced features and that are crucial cryptographic tools used in the blockchain ecosystem for (1) account management, (2) consensus efficiency, (3) empowering scriptless blockchain, and (4) privacy. The exotic signatures that we particularly focus on in this work are the following: multi-/aggregate, threshold, adaptor, blind, and ring signatures. Herein the term ”exotic” refers to signatures with properties that are not just beyond the norm for signatures, e.g., unforgeability, but also imbue new forms of functionalities. Our treatment of such exotic signatures includes discussions on existing challenges and future research directions in the post-quantum space. We hope that this article will help to foster further research to make post-quantum cryptography more accessible so that blockchain systems can be made ready in advance of the approaching quantum threats.
Blockchain is a Byzantine fault tolerant (BFT) system wherein decentralized nodes execute consensus protocols to drive the agreement process on new blocks added to a distributed ledger. Generally, two-round communications among [Formula: see text] nodes are required to tolerate up to [Formula: see text] faults in BFT-based consensus networks. This communication pattern corresponds to the worse-case scenario of consensus achievement, even under asynchronous network conditions. Nevertheless, it is not uncommon for a network to operate under better conditions, where a consensus can be reached with a lower communication cost. Hence, with the addition of a faster optimistic path toward an agreement, the idea of dual-mode consensus has been proposed as a promising approach to enhance the performance of asynchronous BFT protocols. However, this opportunity is not completely exploited by existing dual-mode protocols as the fast path can be followed only in a nonfaulty and synchronous network. This article presents a novel dual-mode protocol consisting of fast and backup subprotocols. To create different consensus committees for fast and backup-mode operations, the network contains both active and passive nodes. A consensus can be expedited through a fast-mode operation when majority of the active nodes can communicate synchronously. Under non-ideal conditions, the backup protocol takes over the agreement process from its fast-mode counterpart without starting over the suspended round. The safety and liveness of the proposed protocol are guaranteed with lower communication costs, which balance the trade-off between protocol efficiency and availability.
As a promising solution to blockchain scalability, sharding divides blockchain nodes into small groups called shards, splitting the workload. Existing works for sharding, however, are limited by cross-shard transactions, since they need to split each cross-shard transaction into multiple sub-transactions, each of which costs a consensus round to commit. In this paper, we introduce PYRAMID, a novel sharding system based on the idea of layered sharding. In PYRAMID, the nodes with better hardware are allowed to participate in multiple shards and store the blockchains of these shards thus they can validate and execute the cross-shard transactions without splitting. Next, to commit the cross-shard transactions with consistency among the related shards, we design a cooperative cross-shard consensus based on collective signature-based inter-shard collaboration. Furthermore, we present an optimization framework to compute an optimal layered sharding strategy maximizing the transaction throughput with the constraint of system security and node resource. Finally, we implement a prototype for PYRAMID based on Ethereum and the experimental results reveal the efficiency of PYRAMID in terms of performance and scalability, especially in workloads with a high percentage of cross-shard transactions. PYRAMID improves the throughput by up to 3.2 times compared with the state-of-the-art works and achieves about 3821 transaction per seconds for 20 shards.
Shan Wang, Ming Yang, Bryan Pearson, Tingjian Ge · 6 authors
Attacks on consensus protocols against a blockchain system are often caused by inner malicious nodes, and inject valid but malicious transactions or blocks to the blockchain by exploiting the consensus protocol. Much attention is paid to attacks such as the 51% attack on the Proof-of-Work (PoW) and long range attack on Proof-of-Stake (PoS) on the consensus protocol in a public blockchain, where the attack cost is high. There is no much systematic work on the attacks on the consensus protocol in a permissioned blockchain. In this paper, we perform a holistic security study of the “execute-order-validate” paradigm used by a permissioned blockchain system such as Hyperledger Fabric. We first systematically present the consensus protocol in the execute-order-validate blockchain paradigm and abstract the consensus protocol as Proof-of-Policy (PoP). We then analyze the chaincode deployment process of Fabric and show it can be exploited to deploy malicious chaincode to launch collusion attacks against PoP. The collusion attacks do not incur high computational cost or monetary cost like attacks on PoW and PoS. The scale of a permissioned blockchain system is often limited, and there is no built-in penalty for such attacks. Therefore, the risk of those collusion attacks is high compared with those against public blockchain systems. We build a Fabric test network to validate the attacks. A large-scale analysis is performed on 7036 Fabric projects on GitHub to evaluate the attack generality.
Directed acyclic graph (DAG) blockchain is a new paradigm of blockchain that solves the poor scalability and low throughput caused by a single-chain structure for IoT applications in the blockchain. However, for DAG-structured blockchains, it is difficult to determine the order of blocks. Additionally, there is also lack of secure and efficient ways to generate blocks in parallel. To address these problems, we propose an efficient DAG blockchain architecture. First, a novel heaviest chain rule based on the block weight is proposed to guide the selection of the parent and uncle blocks for a new block, which realize that appending is ordering. Second, a tree-based gossip protocol (TBGP) is proposed to improve consensus efficiency by reducing message redundancy. Furthermore, federated learning is used to select nodes constructing the tree-based gossip network (TBGN). Finally, compared to the random gossip protocol (RGP), the simulation results indicate TBGP can effectively reduce communication redundancy and improve consensus efficiency.
Francesco D’Amato, Joachim Neu, Ertem Nusret Tas, David Tse
The LMD GHOST consensus protocol is a critical component of proof-of-stake Ethereum. In its current form, this protocol is brittle, as evidenced by recent attacks and patching attempts. We propose Goldfish, a new protocol that satisfies key properties required of a drop-in replacement for LMD GHOST: Goldfish is secure in the sleepy model, assuming a majority of the validators follows the protocol. Goldfish is reorg resilient so that honestly produced blocks are guaranteed inclusion in the ledger, and it supports fast confirmation with expected confirmation latency independent of the desired security level. Subsampling validators can improve the communication efficiency of Goldfish, and Goldfish is composable with finality/accountability gadgets. Crucially, Goldfish is structurally similar to LMD GHOST, providing a credible path to adoption in Ethereum. Attacks on LMD GHOST exploit lack of coordination among honest validators, typically provided by a locking mechanism in classical BFT protocols. However, locking requires votes from a quorum of all participants and is not compatible with fluctuating participation. Goldfish is powered by a novel coordination mechanism to synchronize the honest validators' actions. Experiments with our prototype implementation of Goldfish suggest practicality.
Existing centralized cryptocurrency exchanges designed as trusted central entities are vulnerable to theft of funds, while decentralized exchanges that make all orders public are vulnerable to price manipulation, such as frontrunning attacks. To overcome the flaws, we propose a secure decentralized exchange that supports matching trade orders and atomic cross-chain settlements, and facilitates price discovery. The order matching protocol achieves privacy-preserving yet efficient design by using secure multi-party computation and a trusted execution environment. The atomic cross-chain settlement protocol based on adaptor signatures reduces the number of on-chain transactions while retaining the desirable properties like confidentiality and indistinguishability of these transactions. Security and performance analysis illustrates that our exchange can help traders swap across different blockchains in a privacy-preserving manner without compromising efficiency.
Directed Acyclic Graph (DAG) is a popular approach to achieve scalability of blockchain networks. Due to its high efficiency in data communication and great scalability, DAG has been widely adopted in many applications such as Internet of Things (IoT) and Decentralized Finance (DeFi). DAG-based blockchain, nevertheless, faces the key challenge of transaction inclusion collision due to the high concurrency and the network delay. Particularly, the transaction inclusion collision in DAG-based blockchain leads to the revenue and throughput dilemmas, which would greatly degrade the system performance. In this paper, we propose "TIPS", the Transaction Inclusion Protocol with Signaling, which broadcasts a signal indicating the transactions in the block. We show that with the prompt broadcast of a signal, TIPS substantially reduces the transaction collision and thus resolves these dilemmas. Moreover, we show that TIPS can defend against both the denial-of-service and the delay-of-service attacks. We also conduct intensive experiments to demonstrate the superior performance of the proposed protocol.
Cross-chain technology has become an important research direction of blockchain, which realizes value transfer and intercommunication between different blockchain systems. The latency and the throughput of the existing cross-chain systems hinder further expansion due to the limitation of synchronous consensuses. In order to address the low performance and limited reliability of cross-chain systems, this paper proposes a relay chain-based cross-chain system, called ARC, with the asynchronous consensus, which ensures the global consensuses and improves the robustness and scalability of the cross-chain systems. We also conduct a security analysis to show the security property of ARC when deployed in adversarial scenarios.
Η Τεχνολογία βρίσκεται διαχρονικά σε διαλεκτική σχέση με τη ζωή και την Οικονομία. Ο αέναος κύκλος της οικονομικής άνθισης και ύφεσης τροφοδοτεί και τροφοδοτείται διαρκώς από τη σχέση αυτή. Παγιωμένα πρότυπα, διαδικασίες και θεσμοί τίθενται περιοδικά υπό κρίση και παράγουν νέες μορφές λειτουργίας και αλληλεπίδρασης σε τεχνολογικό, οικονομικό και κοινωνικό επίπεδο. Μια από τις πρόσφατες σχετικά τεχνολογικές εξελίξεις, αυτή του Blockchain, έρχεται να εισάγει μια νέα φιλοσοφία στις συναλλαγές, στην οικονομική και εν γένει κοινωνική αλληλεπίδραση. Η δυνατότητα δυο δυνητικά μη έμπιστων αλληλοεπιδρώντων μερών να συναλλάσσονται μεταξύ τους με αξιοπιστία χωρίς την αναγκαιότητα της παρεμβολής τρίτου έμπιστου μέρους είναι επαναστατική. Καθίσταται πλέον σαφής η ανάγκη ενδελεχούς μελέτης των μηχανισμών της και η σε βάθος έρευνα της επίδρασής της στην σύγχρονη οικονομία με ποσοτικά κριτήρια.Η ιδέα των «κατανεμημένων καθολικών» (distributed ledgers) και των «αλυσίδων αναμφισβήτητων μεταβάσεων» (Blockchain) είναι σχετικά νέα [1],[2],[3], ωστόσο η καταλυτική της επίδραση στην ανθρώπινη δραστηριότητα είναι πρωτοφανής και οι πιθανές της επιπτώσεις της τώρα αρχίζουν να γίνονται αντιληπτές [4],[5],[6]. Υπό το πρίσμα και την πρωτοφανή αύξηση της δημοτικότητας και της αξίας των κρυπτογραφικών νομισμάτων, οι τεχνολογίες Blockchain τυγχάνουν ραγδαία αυξανόμενης σημαντικότητας στην παγκόσμια δραστηριότητα. Έχουν καταλυτική επίπτωση σε υπολογιστικό, οικονομικό, διαχειριστικό και διοικητικό επίπεδο [7]-[15]. Κάθε πτυχή, τόσο της της ανθρώπινης δραστηριότητας, όσο και της δραστηριότητας πρακτόρων ευφυούς λογισμικού αλλά και αντικειμένων στο πλαίσιο του διαδικτύου των πραγμάτων [6],[7], η οποία χαρακτηρίζεται από την αλληλεπίδραση και την ανταλλαγή πληροφορίας, υπόκειται δυνητικά στην επαναστατική αυτή διαδικασία [8]-[13]. Η μοντέρνα οικονομία και διοίκηση (management / administration) καλείται ήδη να αφομοιώσει την τεχνολογική αυτή εξέλιξη [4],[5],[7]-[15]. Το Blockchain προκαλεί «αναστάτωση» ανάλογη των μεγάλων τεχνολογικών και διανοητικών επιτευγμάτων όπως οι τηλεπικοινωνίες, το συμβόλαιο, το χρήμα. Η υιοθέτηση μοντέλων Blockchain στην παραγωγική και σύγχρονη διοικητική διαδικασία προϋποθέτει την κατανόηση και αποδοχή τους.Η εισαγωγή ενός νέου τεκμηρίου σε ένα πραγματικό ή εικονικό περιβάλλον είναι μια δυνητικά απαιτητική διαδικασία από πλευράς υπολογιστικού ή και επικοινωνιακού φόρτου και κόστους, αλλά, μόλις συμβεί αυτό, η επαλήθευση και αποδοχή ενός τεκμηρίου ως «αντικειμενικά πραγματικού» είναι στη συνέχεια αυταπόδεικτη και δεν απαιτεί ιδιαίτερη προσπάθεια.Σε φιλοσοφικό επίπεδο έρχεται να εισάγει ένα νέο προσδιορισμό της «αντικειμενικής», υπό την έννοια της «δύσκολα αμφισβητήσιμης», συλλογικής αλήθειας [16]-[20], τόσο σε φυσικά περιβάλλοντα αλληλεπίδρασης και συναλλαγής, όσο και σε περιβάλλοντα εικονικής, επαυξημένης και μικτής πραγματικότητας. Πρόκειται για μια επαναστατική διαδικασία, την οποία το σύγχρονο οικονομικό περιβάλλον καλείται να αφομοιώσει και να αξιοποιήσει ταχύτατα. Η παρούσα Διατριβή εξετάζει την εφαρμογή της ιδέας των κατανεμημένων αλυσίδων αναμφισβήτητων μεταβάσεων στις συναλλαγές και εν γένει στη σύγχρονη οικονομική λειτουργία, σε περιβάλλοντα πεπερασμένης πολυπλοκότητας.Υπό το πρίσμα αυτό, διερευνά τη δυνητική επίδραση κατανεμημένων μοντέλων του τύπου Blockchain στην εγκαθίδρυση μιας οικουμενικής αντίληψης της έννοιας του έγκυρου, χωρίς την απαίτηση τρίτων έμπιστων μερών, την επίπτωση και το κόστος εφαρμογής που αυτό έχει στο σύγχρονη οικονομία και διοίκηση.ΕΡΕΥΝΗΤΙΚΟ ΕΝΔΙΑΦΕΡΟΝ - ΠΡΩΤΟΤΥΠΙΑΗ ιδέα του καθολικού είναι θεμελιώδης στη σύγχρονη λογιστική και χρηματοοικονομική επιστήμη. Η ακεραιότητα των καταγραφών των συναλλαγών εδράζεται σε σημαντικό βαθμό στο διπλογραφικό σύστημα καταχωρήσεων [21]. Η ιδέα της ύπαρξης «ακριβούς αντίγραφου» της πληροφορίας της συναλλαγής είναι κεντρική, και διασφαλίζει τη συναλλαγή από την ύπαρξη κακόβουλου συναλλασσόμενου μέρους. Κατά αναλογία, η ύπαρξη ενός διαφανούς, υπό την έννοια του δημοσίως διαθέσιμου, προσβάσιμου και διατηρήσιμου από τον καθένα αντιγράφου του καθολικού των συναλλαγών αποτελεί βασικό συστατικό της τεχνολογίας Blockchain. Η ίδια η αξιοπιστία και η ευρωστία του Blockchain στηρίζεται επί του παρόντος στην ιδέα της ακριβούς αναπαραγωγής του καθολικού των συναλλαγών σε πολλαπλά αντίγραφα. Όπως γίνεται άμεσα αντιληπτό, προϊόντος του χρόνου, παραλλαγές της αλυσίδας αναμένεται να εμφανιστούν στο σύστημα τόσο εξαιτίας τεχνικών αδυναμιών των δικτύων επικοινωνιών (καθυστέρηση διάδοσης) όσο και εξαιτίας της δυνητικής ύπαρξης κακόβουλων μερών. Η ύπαρξη γενικά αποδεκτών κανόνων συναίνεσης ως προς την εγκυρότητα του καθολικού είναι θεμελιώδης, η δε επίτευξη συναίνεσης με πλήρως κατανεμημένο τρόπο συνιστά ανοικτό ερευνητικό πεδίο.Στην παρούσα εργασία επανεξετάζουμε και αμφισβητούμε την αναγκαιότητα της «απόλυτης και ακριβούς» μονολιθικής αντιγραφής του συνόλου του καθολικού των συναλλαγών, και αντιπροτείνουμε τον επιμερισμό της αντιγραφής στο επίπεδο της ελάχιστης συναλλακτικής αλληλεπίδρασης, αυτή της μονομερούς συναλλαγής. Με τον τρόπο αυτό πετυχαίνουμε:Α. να φέρουμε τη λειτουργικότητα του Blockchain στο ελάχιστο δυνατό φυσικό επίπεδο, μέσα σε κάθε μικρο-συσκευή του Internet των πραγμάτων.Β. να δομήσουμε και να υποστηρίξουμε πολύπλοκους μηχανισμούς κατανεμημένης συναίνεσης και αντικειμενικότητας συλλογικά, σε συστήματα ατόμων πεπερασμένων δυνατοτήτων. Η χρήση του πλήρως κατανεμημένου Blockchain ως βασικού μέσου για τη διαμόρφωση και την εγκαθίδρυση μιας γενικά αποδεκτής και υπό την οπτική αυτή αντικειμενικής αλήθειας και συναλλακτικής πίστης, συνιστά καινοτόμα θεώρηση, η οποία αγγίζει την ίδια τη φύση της ανθρώπινης λειτουργίας και αλλάζει άρδην τους κανόνες της οικονομικής και όχι μόνο αλληλεπίδρασης και συναλλαγής: μόλις ένας απλός κανόνας μετάβασης γίνει γενικά αποδεκτός (αξίωμα) τότε, το σύνολο της αλήθειας ενός περιβάλλοντος μπορεί να χτιστεί και να συντηρηθεί συλλογικά, βασισμένο επάνω σε αυτό τον απλό κανόνα. Η παρούσα εργασία αναλύει την αιτιότητα της διαδικασίας επίτευξης συναίνεσης σε συστήματα ομότιμων ατόμων και μελετά διεξοδικά τόσο την εφικτότητα της εφαρμογής της όσο και τις παραμέτρους που υπεισέρχονται σε αυτή. Αποδεικνύει στην πράξη ότι ένα μικρό σύνολο πρωταρχικών μεταβατικών συναλλακτικών κανόνων είναι αρκετό για εγκαθιδρυθεί και να επιτηρηθεί συλλογικά η έννοια της έγκυρης συναλλαγής και εν γένει της εγκυρότητας μέσα σε ένα σύστημα.Βιώνουμε την απαρχή της μετάβασης από την «πίστη στους θεσμούς» στην «πίστη στην αποδείξιμη διαδικασία». Η μετάβαση αυτή απαιτεί αυξημένες δεξιότητες σε ατομικό επίπεδο. Προσδίδει δε στο άτομο και την κοινωνία απαράμιλλες δυνατότητες αυτόνομης λειτουργίας, και επιβάλλει πλήρη σεβασμό στους κοινά αποδεκτούς κανόνες του συνόλου. Η σύγχρονη οικονομική και διοικητική λειτουργία έχει ήδη ξεκινήσει να ερευνά και να αφομοιώνει την τεχνολογική αυτή εξέλιξη [8]-[14].Η Συναίνεση σε ένα σύστημα είναι θεμελιώδους σημασίας. Βασίζεται στην αμοιβαία αποδεκτή απόδειξη μιας ιδιότητας και αποτελεί τη βάση κάθε συναλλαγής. Η ιδιότητα αυτή μπορεί να ποικίλει ανάλογα με τις απαιτήσεις κάθε πεδίου εφαρμογής. Μερικές από τις διαδεδομένες «κλάσεις αποδείξεων» αποτελούν οι: απόδειξη έργου (Proof of Work – PoW), μεριδίου (Proof of Stake-PoS), γνησιότητας (Proof of Originality-PoO), ιδιοκτησίας (Proof of Ownership – PoOw), συνέπειας (Proof of consistency-PoC), κλπ.. Ενδιαφέρον παρουσιάζει επίσης η κατηγορία της «μη διαδραστικής απόδειξης – απόδειξης έργου (Non Interactive proof of PoW)» η οποία επιτρέπει την διεξαγωγή χρηματικών συναλλαγών σε περιβάλλοντα μειωμένης παρουσίας ή και περιστασιακής απουσίας δικτύου (off-grid). Μια από τις αδυναμίες των κυρίαρχων σήμερα αρχιτεκτονικών Blockchain είναι η εφαρμογή τους σε πλήρως κατανεμημένα περιβάλλοντα αυτόνομων, ομότιμων κόμβων (ατόμων). Η παρούσα διατριβή θεραπεύει αυτή την αδυναμία, εισάγοντας την έννοια της «προσωπικής και επαληθεύσιμης διασυνδεδεμένης αλυσίδας» ως μέσο επίτευξης συλλογικής συναίνεσης σε ένα σύστημα. Μελετάει την ανάπτυξη εξελικτικών μεθόδων επίτευξης συλλογικής συναίνεσης και συναλλακτικής πίστης, αξιοποιώντας καλά εδραιωμένες μεθόδους υπό το πρίσμα της θεωρίας των πολύ-Γράφων και των Παιγνίων. Μελετά και διευρύνει τη θεώρηση του Blockchain όχι ως μιας απλής «αποκεντρωμένης» ως μιας «πλήρως κατανεμημένης» δομής.Οι κλασσικές αρχιτεκτονικές Blockchain βασίζονται στην ύπαρξη πολλαπλών ακριβών αντιγράφων του συνόλου του καθολικού σε πολλαπλούς κόμβους στο σύστημα (exact database replication). Η παρούσα εργασία εισάγει την ιδέα του πλήρως κατανεμημένου καθολικού, υπό τη μορφή υποκειμενικών διασυνδεδεμένων αλυσίδων γεγονότων οι οποίες τηρούνται τοπικά σε κάθε κόμβο. Με τον τρόπο αυτό, κάθε κόμβος αποκτά λειτουργική αυτοτέλεια και αποτελεί μοναδικό «άτομο» του συστήματος.Αποδεικνύει στην πράξη, ότι με τη χρήση των αρχών των τεχνολογιών Blockchain είναι δυνατή και υπολογιστικά εφικτή η επίτευξη σύνθετης συναίνεσης σε συστήματα τα οποία αποτελούνται από άτομα πεπερασμένων δυνατοτήτων. Με βάση τα ευρήματα της παρούσας εργασίας αποδεικνύεται εμφατικά ότι άτομα «τετριμμένων δυνατοτήτων» είναι δυνατόν να συνεργαστούν προκειμένου να εξυπηρετήσουν διαδικασίες επίτευξης σύνθετης και πολύπλοκης συναίνεσης, η οποία ξεπερνά κατά πολύ τις υπολογιστικές δυνατότητες του μεμονωμένου ατόμου. Αυτή είναι μια θεμελιώδης και καινοτόμα θεώρηση, η οποία εισάγει και προάγει την αντίληψή μας επί του Blockchain ως μιας πλήρως κατανεμημένης, συλλογικής οργανωτικής δομής η οποία διαρθρώνεται «από κάτω προς τα πάνω», σέβεται την αυτοτέλεια του ατόμου και εδράζει στην ελάχιστη κοινή πρωτογενή ικανότητά του να διακρίνει το «ὄν» από το «οὐκ ὄν», το υφιστάμενο από το μη υφιστάμενο. Εισάγει και μελετά την έννοια της «υποκειμενικής αλυσίδας γεγονότων» και αποδεικνύει ότι εισάγοντας ένα απλό και καθολικό λειτουργικό κανόνα εγκυρότητας σε ατομικό επίπεδο, κάθε σύστημα μπορεί αυτοδύναμα να ορίσει και να επιτηρήσει σύνθετους κανόνες συναίνεσης. Μπορεί να αναπτύσσει και να υποστηρίζει σύνθετους μηχανισμούς ελέγχου εγκυρότ