Towards practicalization of blockchain-based decentralized applications

Blockchain can be defined as an immutable ledger for recording transactions, maintained in a distributed network of mutually untrusting peers. Blockchain technology has been widely applied to various fields beyond its initial usage of cryptocurrency. However, blockchain itself is insufficient to meet all the desired security or efficiency requirements for diversified application scenarios. This dissertation focuses on two core functionalities that blockchain provides, i.e., robust storage and reliable computation. Three concrete application scenarios including Internet of Things (IoT), cybersecurity management (CSM), and peer-to-peer (P2P) content delivery network (CDN) are utilized to elaborate the general design principles for these two main functionalities. Among them, the IoT and CSM applications involve the design of blockchain-based robust storage and management while the P2P CDN requires reliable computation. Such general design principles derived from disparate application scenarios have the potential to realize practicalization of many other blockchain-enabled decentralized applications. In the IoT application, blockchain-based decentralized data management is capable of handling faulty nodes, as designed in the cybersecurity application. But an important issue lies in the interaction between external network and blockchain network, i.e., external clients must rely on a relay node to communicate with the full nodes in the blockchain. Compromization of such relay nodes may result in a security breach and even a blockage of IoT sensors from the network. Therefore, a censorship-resistant blockchain-based decentralized IoT management system is proposed. Experimental results from proof-of-concept implementation and deployment in a real distributed environment show the feasibility and effectiveness in achieving censorship resistance. The CSM application incorporates blockchain to provide robust storage of historical cybersecurity data so that with a certain level of cyber intelligence, a defender can determine if a network has been compromised and to what extent. The CSM functions can be categorized into three classes: Network-centric (N-CSM), Tools-centric (T-CSM) and Application-centric (A-CSM). The cyber intelligence identifies new attackers, victims, or defense capabilities. Moreover, a decentralized storage network (DSN) is integrated to reduce on-chain storage costs without undermining its robustness. Experiments with the prototype implementation and real-world cyber datasets show that the blockchain-based CSM solution is effective and efficient. The P2P CDN application explores and utilizes the functionality of reliable computation that blockchain empowers. Particularly, P2P CDN is promising to provide benefits including cost-saving and scalable peak-demand handling compared with centralized CDNs. However, reliable P2P delivery requires proper enforcement of delivery fairness. Unfortunately, most existing studies on delivery fairness are based on non-cooperative game-theoretic assumptions that are arguably unrealistic in the ad-hoc P2P setting. To address this issue, an expressive security requirement for desired fair P2P content delivery is defined and two efficient approaches based on blockchain for P2P downloading and P2P streaming are proposed. The proposed system guarantees the fairness for each party even when all others collude to arbitrarily misbehave and achieves asymptotically optimal on-chain costs and optimal delivery communication

Optimistic fair exchange

A fair exchange guarantees that a participant only reveals its items (such as signatures, payments, or data) if it receives the expected items in exchange. Efficient fair exchange requires a so-called third party, which is assumed to be correct. Optimistic fair exchange involves this third party only if needed, i.e., if the participants cheat or disagree. In Part I, we prove lower bounds on the message and time complexity of two particular instances of fair exchange in varying models, namely contract signing (fair exchange of two signatures under a contract) and certified mail (fair exchange of data for a receipt). We show that all given bounds are tight by describing provably time- and message-optimal protocols for all considered models and instances. In Part II, we have a closer look at formalizing the security of fair exchange. We introduce a new formal notion of security (including secrecy) for reactive distributed systems. We illustrate this new formalism by a specification of certified mail as an alternative to the traditional specification given in Part I. In Part III, we describe protocols for generic and optimistic fair exchange of arbitrary items. These protocols are embedded into the SEMPER Fair Exchange Layer, which is a central part of the SEMPER Framework for Secure Electronic Commerce.Ein Austausch ist fair, wenn eine Partei die angebotenen Güter, wie zum Beispiel digitale Signaturen, Zahlungen oder Daten, nur abgibt, wenn sie die erwarteten Güter im Tausch erhält. Ohne eine als korrekt angenommene dritte Partei, welche eine mit einem Notar vergleichbare Rolle übernimmt, ist fairer Austausch nicht effizient möglich. Ein fairer Austausch heißt optimistisch, falls diese dritte Partei nur in Problemfällen am Protokoll teilnimmt. In Teil I werden beweisbar zeit- und nachrichtenoptimale Protokolle für die Spezialfälle ';elektronische Vertragsunterzeichnung" (fairer Austausch zweier Signaturen; engl. contract signing) und ';elektronisches Einschreiben" (fairer Austausch von Daten gegen eine Quittung; engl. certified mail) von fairem Austausch vorgestellt. Teil II beschreibt einen neuen Integritäts- und Geheimhaltungsbegriff für reaktive Systeme. Dieser basiert auf einer Vergleichsrelation ';so sicher wie", welche die Sicherheit zweier Systeme vergleicht. Ein verteiltes, reaktives System wird dann als sicher bezeichnet, wenn es so sicher wie ein idealisiertes System (engl. trusted host) für diesen Dienst ist. Mit diesem Formalismus geben wir eine alternative Sicherheitsdefinition von ';elektronischem Einschreiben" an, deren Semantik im Gegensatz zu der in Teil I beschriebenen Definition nun unabhängig vom erbrachten Dienst ist. Teil III beschreibt ein Design und optimistische Protokolle für generischen fairen Austausch von zwei beliebigen Gütern und den darauf aufbauenden SEMPER Fair Exchange Layer. Dieser ist ein wesentlicher Baustein des SEMPER Framework for Secure Electronic Commerce

Optimistic fair exchange

A fair exchange guarantees that a participant only reveals its items (such as signatures, payments, or data) if it receives the expected items in exchange. Efficient fair exchange requires a so-called third party, which is assumed to be correct. Optimistic fair exchange involves this third party only if needed, i.e., if the participants cheat or disagree. In Part I, we prove lower bounds on the message and time complexity of two particular instances of fair exchange in varying models, namely contract signing (fair exchange of two signatures under a contract) and certified mail (fair exchange of data for a receipt). We show that all given bounds are tight by describing provably time- and message-optimal protocols for all considered models and instances. In Part II, we have a closer look at formalizing the security of fair exchange. We introduce a new formal notion of security (including secrecy) for reactive distributed systems. We illustrate this new formalism by a specification of certified mail as an alternative to the traditional specification given in Part I. In Part III, we describe protocols for generic and optimistic fair exchange of arbitrary items. These protocols are embedded into the SEMPER Fair Exchange Layer, which is a central part of the SEMPER Framework for Secure Electronic Commerce.Ein Austausch ist fair, wenn eine Partei die angebotenen Güter, wie zum Beispiel digitale Signaturen, Zahlungen oder Daten, nur abgibt, wenn sie die erwarteten Güter im Tausch erhält. Ohne eine als korrekt angenommene dritte Partei, welche eine mit einem Notar vergleichbare Rolle übernimmt, ist fairer Austausch nicht effizient möglich. Ein fairer Austausch heißt optimistisch, falls diese dritte Partei nur in Problemfällen am Protokoll teilnimmt. In Teil I werden beweisbar zeit- und nachrichtenoptimale Protokolle für die Spezialfälle \u27;elektronische Vertragsunterzeichnung" (fairer Austausch zweier Signaturen; engl. contract signing) und \u27;elektronisches Einschreiben" (fairer Austausch von Daten gegen eine Quittung; engl. certified mail) von fairem Austausch vorgestellt. Teil II beschreibt einen neuen Integritäts- und Geheimhaltungsbegriff für reaktive Systeme. Dieser basiert auf einer Vergleichsrelation \u27;so sicher wie", welche die Sicherheit zweier Systeme vergleicht. Ein verteiltes, reaktives System wird dann als sicher bezeichnet, wenn es so sicher wie ein idealisiertes System (engl. trusted host) für diesen Dienst ist. Mit diesem Formalismus geben wir eine alternative Sicherheitsdefinition von \u27;elektronischem Einschreiben" an, deren Semantik im Gegensatz zu der in Teil I beschriebenen Definition nun unabhängig vom erbrachten Dienst ist. Teil III beschreibt ein Design und optimistische Protokolle für generischen fairen Austausch von zwei beliebigen Gütern und den darauf aufbauenden SEMPER Fair Exchange Layer. Dieser ist ein wesentlicher Baustein des SEMPER Framework for Secure Electronic Commerce

Efficient Verifiable Escrow and Fair Exchange with Trusted Hardware

At the heart of many fair exchange problems is verifiable escrow: a sender encrypts some value using the public key of a trusted party (called the recovery agent), and then must convince the receiver of the ciphertext that the corresponding plaintext satisfies some property (e.g., it contains the sender\u27s signature on a contract). Previous solutions to this problem are interactive, and often rely on communication-intensive cut-and-choose zero-knowledge proofs. In this paper, we provide a solution that uses generic trusted hardware to create an efficient, non-interactive verifiable escrow scheme. Our solution allows the protocol to use a set of recovery agents with a threshold access structure, the \emph{verifiable group escrow} notion which was informally introduced by Camenisch and Damgard and which is formalized here. Finally, this paper shows how this new non-interactive verifiable escrow scheme can be used to create an efficient optimistic protocol for fair exchange of signatures

Lower Bounds for Off-Chain Protocols: Exploring the Limits of Plasma

Blockchain is a disruptive new technology introduced around a decade ago. It can be viewed as a method for recording timestamped transactions in a public database. Most of blockchain protocols do not scale well, i.e., they cannot process quickly large amounts of transactions. A natural idea to deal with this problem is to use the blockchain only as a timestamping service, i.e., to hash several transactions $\mathit{tx}_1,\ldots,\mathit{tx}_m$ into one short string, and just put this string on the blockchain, while at the same time posting the hashed transactions $\mathit{tx}_1,\ldots,\mathit{tx}_m$ to some public place on the Internet (``off-chain\u27\u27). In this way the transactions $\mathit{tx}_i$ remain timestamped, but the amount of data put on the blockchain is greatly reduced. This idea was introduced in 2017 under the name \emph{Plasma} by Poon and Buterin. Shortly after this proposal, several variants of Plasma have been proposed. They are typically built on top of the Ethereum blockchain, as they strongly rely on so-called \emph{smart contracts} (in order to resolve disputes between the users if some of them start cheating). Plasmas are an example of so-called \emph{off-chain protocols}. In this work we initiate the study of the inherent limitations of Plasma protocols. More concretely, we show that in every Plasma system the adversary can either (a) force the honest parties to communicate a lot with the blockchain, even though they did not intend to (this is traditionally called \emph{mass exit}); or (b) an honest party that wants to leave the system needs to quickly communicate large amounts of data to the blockchain. What makes these attacks particularly hard to handle in real life is that these attacks do not have so-called \emph{uniquely attributable faults}, i.e.~the smart contract cannot determine which party is malicious, and hence cannot force it to pay the fees for the blockchain interaction. An important implication of our result is that the benefits of two of the most prominent Plasma types, called \emph{Plasma Cash} and \emph{Fungible Plasma}, cannot be achieved simultaneously. Besides of the direct implications on real-life cryptocurrency research, we believe that this work may open up a new line of theoretical research, as, up to our knowledge, this is the first work that provides an impossibility result in the area of off-chain protocols

Beyond subjective and objective in statistics

We argue that the words "objectivity" and "subjectivity" in statistics discourse are used in a mostly unhelpful way, and we propose to replace each of them with broader collections of attributes, with objectivity replaced by transparency, consensus, impartiality, and correspondence to observable reality, and subjectivity replaced by awareness of multiple perspectives and context dependence. The advantage of these reformulations is that the replacement terms do not oppose each other. Instead of debating over whether a given statistical method is subjective or objective (or normatively debating the relative merits of subjectivity and objectivity in statistical practice), we can recognize desirable attributes such as transparency and acknowledgment of multiple perspectives as complementary goals. We demonstrate the implications of our proposal with recent applied examples from pharmacology, election polling, and socioeconomic stratification.Comment: 35 page