Fair exchange in e-commerce and certified e-mail, new scenarios and protocols

We are witnessing a steady growth in the use of Internet in the electronic commerce field. This rise is promoting the migration from traditional processes and applications (paper based) to an electronic model. But the security of electronic transactions continues to pose an impediment to its implementation. Traditionally, most business transactions were conducted in person. Signing a contract required the meeting of all interested parties, the postman delivered certified mail in hand, and when paying for goods or services both customer and provider were present. When all parties are physically present, a transaction does not require a complex protocol. The participants acknowledge the presence of the other parties as assurance that they will receive their parts, whether a signature on a contract, or a receipt, etc. But with e-commerce growing in importance as sales and business channel, all these transactions have moved to its digital counterpart. Therefore we have digital signature of contracts, certified delivery of messages and electronic payment systems. With electronic transactions, the physical presence is not required,moreover, most of the times it is even impossible. The participants in a transaction can be thousands of kilometers away from each other, and they may not even be human participants, they can be machines. Thus, the security that the transaction will be executed without incident is not assured per se, we need additional security measures. To address this problem, fair exchange protocols were developed. In a fair exchange every party involved has an item that wants to exchange, but none of the participants is willing to give his item away unless he has an assurance he will receive the corresponding item from the other participants. Fair exchange has many applications, like digital signature of contracts, where the items to be exchanged are signatures on contracts, certified delivery of messages, where we exchange a message for evidence of receipt, or a payment process, where we exchange a payment (e-cash, e-check, visa, etc.) for digital goods or a receipt. The objective of this dissertation is the study of the fair exchange problem. In particular, it presents two new scenarios for digital contracting, the Atomic Multi- Two Party (AM2P) and the Agent Mediated Scenario (AMS), and proposes one optimistic contract signing protocol for each one. Moreover, it studies the efficiency of Multi-Party Contract Signing (MPCS) protocols from their architecture point of view, presenting a new lower bound for each architecture, in terms of minimum number of transactions needed. Regarding Certified Electronic Mail (CEM), this dissertation presents two optimistic CEMprotocols designed to be deployed on thecurrent e-mail infrastructure, therefore they assume the participation of multiple Mail Transfer Agents (MTAs). In one case, the protocol assumes untrusted MTAs whereas in the other one it assumes each User Agent (UA) trusts his own MTA. Regarding payment systems, this dissertation presents a secure and efficient electronic bearer bank check scheme allowing the electronic checks to be transferred fairly and anonymously.L’ús d’Internet en l’àmbit del comerç electrònic està experimentant un creixement estable. Aquest increment d’ús està promovent lamigració de processos tradicionals i aplicacions (basades en paper) cap a un model electrònic. Però la seguretat de les transaccions electròniques continua impedint la seva implantació. Tradicionalment, la majoria de les transaccions s’han dut a terme en persona. La firma d’un contracte requeria la presència de tots els firmants, el carter entrega les cartes certificades enmà, i quan es paga per un bé o servei ambdós venedor i comprador hi són presents. Quan totes les parts hi són presents, les transaccions no requereixen un protocol complex. Els participants assumeixen la presència de les altres parts com assegurança que rebran el que esperen d’elles, ja sigui la firma d’un contracte, un rebut d’entrega o un pagament. Però amb el creixement del comerç electrònic com a canal de venda i negoci, totes aquestes transaccions s’hanmogut al seu equivalent en el món electrònic. Així doncs tenim firma electrònica de contractes, enviament certificat de missatges, sistemes de pagament electrònic, etc. En les transaccions electròniques la presència física no és necessària, de fet, la majoria de vegades és fins it tot impossible. Els participants poden estar separats permilers de kilòmetres, i no és necessari que siguin humans, podrien sermàquines. Llavors, la seguretat de que la transacció s’executarà correctament no està assegurada per se, necessitem proporcionar mesures de seguretat addicionals. Per solucionar aquest problema, es van desenvolupar els protocols d’intercanvi equitatiu. En un intercanvi equitatiu totes les parts involucrades tenen un objecte que volen intercanviar, però cap de les parts implicades vol donar el seu objecte si no té la seguretat que rebrà els objectes de les altres parts. L’intercanvi equitatiu té multitud d’aplicacions, com la firma electrònica de contractes, on els elements a intercanviar son firmes de contractes, enviament certificat demissatges, on s’intercanvien unmissatge per una evidència de recepció, o un procés de pagament, on intercanviemun pagament (e-cash, visa, e-xec, etc.) per bens digitals o per un rebut. L’objectiu d’aquesta tesi és estudiar el problema de l’intercanvi equitatiu. En particular, la tesi presenta dos nous escenaris per a la firma electrònica de contractes, l’escenari multi-two party atòmic i l’escenari amb agents intermediaris, i proposa un protocol optimista per a cada un d’ells. A més, presenta un estudi de l’eficiència dels protocols de firma electrònica multi-part (Multi-Party Contract Signing (MPCS) protocols) des del punt de vista de la seva arquitectura, presentant una nova fita per a cada una, en termes de mínim nombre de transaccions necessàries. Pel que fa al correu electrònic certificat, aquesta tesi presenta dos protocols optimistes dissenyats per a ser desplegats damunt l’infraestructura actual de correu electrònic, per tant assumeix la participació demúltiples agents de transferència de correu. Un dels protocols assumeix que cap dels agents de transferència de correu participants és de confiança,mentre que l’altre assumeix que cada usuari confia en el seu propi agent. Pel que fa a sistemes de pagament, la tesi presenta un esquema de xec bancari al portador, eficient i segur, que garanteix que la transferència dels xecs es fa demanera anònima i equitativa

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

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

Fair and optimistic quantum contract signing

We present a fair and optimistic quantum contract signing protocol between two clients that requires no communication with the third trusted party during the exchange phase. We discuss its fairness and show that it is possible to design such a protocol for which the probability of a dishonest client to cheat becomes negligible, and scales as N^{-1/2}, where N is the number of messages exchanged between the clients. Our protocol is not based on the exchange of signed messages: its fairness is based on the laws of quantum mechanics. Thus, it is abuse-free, and the clients do not have to generate new keys for each message during the Exchange phase. We discuss a real-life scenario when the measurement errors and qubit state corruption due to noisy channels occur and argue that for real, good enough measurement apparatus and transmission channels, our protocol would still be fair. Our protocol could be implemented by today's technology, as it requires in essence the same type of apparatus as the one needed for BB84 cryptographic protocol. Finally, we briefly discuss two alternative versions of the protocol, one that uses only two states (based on B92 protocol) and the other that uses entangled pairs, and show that it is possible to generalize our protocol to an arbitrary number of clients.Comment: 11 pages, 2 figure

Hydra: Fast Isomorphic State Channels

State channels are an attractive layer-two solution for improving the throughput and latency of blockchains. They offer optimistic offchain settlement of payments and expedient offchain evolution of smart contracts between multiple parties without imposing any additional assumptions beyond those of the underlying blockchain. In the case of disputes, or if a party fails to respond, cryptographic evidence collected in the offchain channel is used to settle the last confirmed state onchain, such that in-progress contracts can be continued under mainchain consensus. A serious disadvantage present in current layer-two state channel protocols is that existing layer-one smart contract infrastructure and contract code cannot be reused offchain without change. In this paper, we introduce Hydra, an isomorphic multi-party state channel. Hydra simplifies offchain protocol and smart contract development by directly adopting the layer-one smart contract system, in this way allowing the same code to be used both on- and offchain. Taking advantage of the extended UTxO model, we develop a fast off-chain protocol for evolution of Hydra heads (our isomorphic state channels) that has smaller round complexity than all previous proposals and enables the state channel processing to advance on-demand, concurrently and asynchronously. We establish strong security properties for the protocol, and we present and evaluate extensive simulation results that demonstrate that Hydra approaches the physical limits of the network in terms of transaction confirmation time and throughput while keeping storage requirements at the lowest possible. Finally, our experimental methodology may be of independent interest in the general context of evaluating consensus protocols

Keeping Fairness Alive : Design and formal verification of optimistic fair exchange protocols

Fokkink, W.J. [Promotor]Pol, J.C. van de [Promotor

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

Fair electronic exchange using biometrics

Fair exchange between two parties can be defined as an instance of exchange such that either both parties obtain what they expected or neither one does. Protocols that facilitate such transactions are known as “fair exchange protocols”. We analyze one such protocol by Micali that demonstrates fair contract signing, where two parties exchange their commitments over an already negotiated contract. In this research we show that Micali’s protocol is not completely fair and demonstrate the possibilities for one party cheating by obtaining the other party’s commitment and not offer theirs. A revised version of this protocol by Bao provides superior fairness by handling the above mentioned weakness but fails to handle the possibility of a replay attack. Our proposed protocol improves on Bao’s protocol by addressing the weakness that leads to a replay attack. We also demonstrate a software implementation of our system which provides fair contract signing along with properties like user authentication achieved through the use of a fingerprint based authentication system and features like confidentiality, data-integrity and non-repudiation achieved through implementation of hybrid cryptography and digital signatures algorithms based on Elliptic Curve Cryptography

Decentralizing Trust with Resilient Group Signatures in Blockchains

Blockchains have the goal of promoting the decentralization of transactions in a P2Pbased internetworking model that does not depend on centralized trust parties. Along with research on better scalability, performance, consistency control, and security guarantees in their service planes, other challenges aimed at better trust decentralization and fairness models on the research community’s agenda today. Asymmetric cryptography and digital signatures are key components of blockchain systems. As a common flaw in different blockchains, public keys and verification of single-signed transactions are handled under the principle of trust centralization. In this dissertation, we propose a better fairness and trust decentralization model by proposing a service plane for blockchains that provides support for collective digital signatures and allowing transactions to be collaboratively authenticated and verified with groupbased witnessed guarantees. The proposed solution is achieved by using resilient group signatures from randomly and dynamically assigned groups. In our approach we use Threshold-Byzantine Fault Tolerant Digital Signatures to improve the resilience and robustness of blockchain systems while preserving their decentralization nature. We have designed and implemented a modular and portable cryptographic provider that supports operations expressed by smart contracts. Our system is designed to be a service plane agnostic and adaptable to the base service planes of different blockchains. Therefore, we envision our solution as a portable, adaptable and reusable plugin service plane for blockchains, as a way to provide authenticated group-signed transactions with decentralized auditing, fairness, and long-term security guarantees and to leverage a better decentralized trust model. We conducted our experimental evaluations in a cloudbased testbench with at least sixteen blockchain nodes distributed across four different data centers, using two different blockchains and observing the proposed benefits.As blockchains tem principal objetivo de promover a descentralização das transações numa rede P2P, baseada num modelo não dependente de uma autoridade centralizada. Em conjunto com maior escalabilidade, performance, controlos de consistência e garantias de segurança nos planos de serviço, outros desafios como a melhoria do modelo de descentralização e na equidade estão na agenda da comunidade científica. Criptografia assimétrica e as assinaturas digitais são a componente chave dos sistemas de blockchains. Porém, as blockchains, chaves públicas e verificações de transações assinadas estão sobre o princípio de confiança centralizada. Nesta dissertação, vamos propor uma solução que inclui melhores condições de equidade e descentralização de confiança, modelado por um plano de serviços para a blockchain que fornece suporte para assinaturas coletivas e permite que as transações sejam autenticadas colaborativamente e verificadas com garantias das testemunhadas. Isto será conseguido usando assinaturas resilientes para grupos formados de forma aleatória e dinamicamente. A nossa solução para melhorar a resiliência das blockchains e preservar a sua natureza descentralizada, irá ser baseada em assinaturas threshold à prova de falhas Bizantinas. Com esta finalidade, iremos desenhar e implementar um provedor criptográfico modelar e portável para suportar operações criptográficas que podem ser expressas por smart-contracts. O nosso sistema será desenhado de uma forma agnóstica e adaptável a diferentes planos de serviços. Assim, imaginamos a nossa solução como um plugin portável e adaptável para as blockchains, que oferece suporte para auditoria descentralizada, justiça, e garantias de longo termo para criar modelo melhor da descentralização da base de confiança. Iremos efetuar as avaliações experimentais na cloud, correndo o nosso plano de serviço com duas implementações de blockchain e pelo menos dezasseis nós distribuídos em quatro data centres, observando os benefícios da solução proposta