An optimistic fair e-commerce protocol for large e-goods

Suppose two entities that do not trust each other want to exchange some arbitrary data over a public channel. A fair exchange protocol ensures that both parties get what they want or neither gets anything. In this paper, a fair e-commerce protocol for large e-goods is proposed and implemented. The proposed protocol provides a method for the fair exchange of e-money for e-products, and a method for verifying the contents of the exchanged items. The protocol is optimistic and efficient such that when none of the parties tries to cheat, only three messages are sufficient. In case of disputes, three more messages are needed. Furthermore, the customer remains anonymous after the transaction; thus, no information about the customers' shopping habits can be gathered through the protocol. The implementation results show that the protocol is efficient and secure and that only a small number of cryptographic operations is sufficient

Automatic Methods for Analyzing Non-repudiation Protocole with an Active Intruder

International audienceNon-repudiation protocols have an important role in many areas where secured transactions with proofs of participation are necessary. Formal methods are clever and without error, therefore using them for verifying such protocols is crucial. In this purpose, we show how to partially represent non-repudiation as a combination of authentications on the Fair Zhou-Gollmann protocol. After discussing the limitations of this method, we define a new one based on the handling of the knowledge of protocol participants. This second method is general and of natural use, as it consists in adding simple annotations in the protocol specification. It is very easy to implement in tools able to handle participants knowledge. We have implemented it in the AVISPA Tool and analyzed the optimistic Cederquist-Corin-Dashti protocol, discovering two attacks. This extension of the AVISPA Tool for handling non-repudiation opens a highway to the specification of many other properties, without any more change in the tool itself

A Blockchain-based Decentralized Electronic Marketplace for Computing Resources

AbstractWe propose a framework for building a decentralized electronic marketplace for computing resources. The idea is that anyone with spare capacities can offer them on this marketplace, opening up the cloud computing market to smaller players, thus creating a more competitive environment compared to today's market consisting of a few large providers. Trust is a crucial component in making an anonymized decentralized marketplace a reality. We develop protocols that enable participants to interact with each other in a fair way and show how these protocols can be implemented using smart contracts and blockchains. We discuss and evaluate our framework not only from a technical point of view, but also look at the wider context in terms of fair interactions and legal implications

Automatic Methods for Analyzing Non-Repudiation Protocols with an Active Intruder

Non-repudiation protocols have an important role in many areas where secured transactions with proofs of participation are necessary. Formal methods are clever and without error, therefore using them for verifying such protocols is crucial. In this purpose, we show how to partially represent non-repudiation as a combination of authentications on the Fair Zhou-Gollmann protocol. After discussing its limits, we define a new method based on the handling of the knowledge of protocol participants. This method is very general and is of natural use, as it consists in adding simple annotations, like for authentication problems. The method is very easy to implement in tools able to handle participants knowledge. We have implemented it in the AVISPA Tool and analyzed the optimistic Cederquist-Corin- Dashti protocol, discovering two unknown attacks. This extension of the AVISPA Tool for handling non-repudiation opens a highway to the specification of many other properties, without any more change in the tool itself

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

Instantaneous Decentralized Poker

We present efficient protocols for amortized secure multiparty computation with penalties and secure cash distribution, of which poker is a prime example. Our protocols have an initial phase where the parties interact with a cryptocurrency network, that then enables them to interact only among themselves over the course of playing many poker games in which money changes hands. The high efficiency of our protocols is achieved by harnessing the power of stateful contracts. Compared to the limited expressive power of Bitcoin scripts, stateful contracts enable richer forms of interaction between standard secure computation and a cryptocurrency. We formalize the stateful contract model and the security notions that our protocols accomplish, and provide proofs using the simulation paradigm. Moreover, we provide a reference implementation in Ethereum/Solidity for the stateful contracts that our protocols are based on. We also adopt our off-chain cash distribution protocols to the special case of stateful duplex micropayment channels, which are of independent interest. In comparison to Bitcoin based payment channels, our duplex channel implementation is more efficient and has additional features