Trusted content-based publish/subscribe trees

Publish/Subscribe systems hold strong assumptions of the expected behaviour of clients and routers, as it is assumed they all abide by the matching and routing protocols. Assumptions of implicit trust between the components of the publish/subscribe infrastructure are acceptable where the underlying event distribution service is under the control of a single or multiple co-operating administrative entities and contracts between clients and these authorities exist, however there are application contexts where these presumptions do not hold. In such environments, such as ad hoc networks, there is the possibility of selfish and malicious behaviour that can lead to disruption of the routing and matching algorithms. The most commonly researched approach to security in publish/subscribe systems is role-based access control (RBAC). RBAC is suitable for ensuring confidentiality, but due to the assumption of strong identities associated with well defined roles and the absence of monitoring systems to allow for adaptable policies in response to the changing behaviour of clients, it is not appropriate for environments where: identities can not be assigned to roles in the absence of a trusted administrative entity; long-lived identities of entities do not exist; and where the threat model consists of highly adaptable malicious and selfish entities. Motivated by recent work in the application of trust and reputation to Peer-to-Peer networks, where past behaviour is used to generate trust opinions that inform future transactions, we propose an approach where the publish/subscribe infrastructure is constructed and re-configured with respect to the trust preferences of clients and routers. In this thesis, we show how Publish/Subscribe trees (PSTs) can be constructed with respect to the trust preferences of publishers and subscribers, and the overhead costs of event dissemination. Using social welfare theory, it is shown that individual trust preferences over clients and routers, which are informed by a variety of trust sources, can be aggregated to give a social preference over the set of feasible PSTs. By combining this and the existing work on PST overheads, the Maximum Trust PST with Overhead Budget problem is defined and is shown to be in NP-complete. An exhaustive search algorithm is proposed that is shown to be suitable only for very small problem sizes. To improve scalability, a faster tabu search algorithm is presented, which is shown to scale to larger problem instances and gives good approximations of the optimal solutions. The research contributions of this work are: the use of social welfare theory to provide a mechanism to establish the trustworthiness of PSTs; the finding that individual trust is not interpersonal comparable as is considered to be the case in much of the trust literature; the Maximum Trust PST with Overhead Budget problem; and algorithms to solve this problem

Society-oriented cryptographic techniques for information protection

Groups play an important role in our modern world. They are more reliable and more trustworthy than individuals. This is the reason why, in an organisation, crucial decisions are left to a group of people rather than to an individual. Cryptography supports group activity by offering a wide range of cryptographic operations which can only be successfully executed if a well-defined group of people agrees to co-operate. This thesis looks at two fundamental cryptographic tools that are useful for the management of secret information. The first part looks in detail at secret sharing schemes. The second part focuses on society-oriented cryptographic systems, which are the application of secret sharing schemes in cryptography. The outline of thesis is as follows

Security and privacy for large ad-hoc networks

Ph.DDOCTOR OF PHILOSOPH

Verifiable Secret Sharing as Secure Computation

We present a stronger notion of verifiable secret sharing and exhibit a protocol implementing it. We show that our new notion is preferable to the old ones whenever verifiable secret sharing is used as a tool within larger protocols, rather than being a goal in itself. 1 Introduction Secret Sharing and Verifiable Secret Sharing (VSS for short) are fundamental notions and tools for secure cryptographic design. Despite the centrality and the maturity of this concept (almost 10 years passed from its original introduction), we shall advocate that a stronger and better definition of a VSS is needed. The intuitive notion of a VSS. As first introduced by Chor, Goldwasser, Micali and Awerbuch in [3], a VSS protocol consists of a two-stage protocol. Informally, there are n players, t of which may be bad and deviate from their prescribed instructions. One of the players, the dealer, possesses a value s as a secret input. In the first stage, the dealer commits to a unique value v (no matter w..

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

Secret sharing using artificial neural network

Secret sharing is a fundamental notion for secure cryptographic design. In a secret sharing scheme, a set of participants shares a secret among them such that only pre-specified subsets of these shares can get together to recover the secret. This dissertation introduces a neural network approach to solve the problem of secret sharing for any given access structure. Other approaches have been used to solve this problem. However, the yet known approaches result in exponential increase in the amount of data that every participant need to keep. This amount is measured by the secret sharing scheme information rate. This work is intended to solve the problem with better information rate