[Show abstract][Hide abstract] ABSTRACT: The fair exchange problem is key to trading electronic items in systems of mutually untrusted parties. In modern variants of such systems,
each party is equipped with a security module. The security modules trust each other but can only communicate by exchanging
messages through their untrusted host parties, that could drop those messages.
We describe a synchronous algorithm that ensures deterministic fair exchange if a majority of parties are honest, which is optimal in terms of resilience. If there is no honest majority, our algorithm
degrades gracefully: it ensures that the probability of unfairness can be made arbitrarily low.
Our algorithm uses, as an underlying building block, an early-stopping subprotocol that solves, in a general omission failure
model, a specific variant of consensus we call biased consensus. Interestingly, this modular approach combines concepts from both cryptography and distributed computing, to derive new results
on the classical fair exchange problem.
[Show abstract][Hide abstract] ABSTRACT: We consider the problem of secure multi-party computation (SMC) in a new model where individual processes contain a tamper-proof security module. Security modules can be trusted by other processes and can establish secure channels between each other. However, their availability is restricted by their host, i.e., a corrupted party can stop the computation of its own security module as well as drop any message sent by or to its security module. In this model we show that SMC is solvable if and only if a majority of processes is correct. We prove this by relating SMC to the problem of Uniform Interactive Consistency among security modules (a variant of the Byzantine Generals Problem from the area of fault-tolerance). The obtained solutions to SMC for the first time allow to compute any function securely with a complexity which is polynomial only in the number of processes (i.e., the complexity does not depend on the function which is computed). We conclude that adding secure hardware does not improve the resilience of SMC but can effectively improve the efficiency.
Sicherheit 2005: Sicherheit - Schutz und Zuverlässigkeit, Beiträge der 2. Jahrestagung des Fachbereichs Sicherheit der Gesellschaft für Informatik e.v. (GI), 5.-8. April 2005 in Regensburg; 01/2005
[Show abstract][Hide abstract] ABSTRACT: Publish/subscribe (pub/sub) is considered a valuable middleware architecture that proliferates loose coupling and leverages reconfigurability and evolution. Up to now, existing pub/sub middleware was optimized for static systems where users as well as the underlying system structure was rather fixed. We study the question whether existing pub/sub middleware can be extended to support mobile and location-dependent applications.We first analyze the requirements of such applications and distinguish two orthogonal forms of mobility: the system-centric physical mobility and an application-centric logical mobility (where users are aware that they are changing location). For logical-mobility we introduce location-dependent subscriptions as a suitable means to exploit the power of the event-based paradigm in mobile applications. Briefly spoken, a location-dependent subscription offers to express interest in all events which are related to a user's current location. We present efficient implementations for both forms of mobility within the content-based pub/sub middleware Rebeca. Our solutions draw much of their efficiency from the refined routing capabilities (namely, covering and merging) of the Rebeca system.
[Show abstract][Hide abstract] ABSTRACT: The tolerance theory by Arora and Kulkarni views a fault-tolerant program as the composition of a fault-intolerant program and fault tolerance components called detectors and correctors.At its core, the theory assumes that the correctness specifications under consideration are fusion closed.In general, fusion closure of specifications can be achieved by adding history variables to the program. However, addition of history variables causes an exponential growth of the state space of the program.To redress this problem, we present a method which can be used to add history information to a program in a way that (in a certain sense) minimizes the additional states. Hence, automated methods that add fault tolerance can now be efficiently applied to environments with not fusion closed specifications.
[Show abstract][Hide abstract] ABSTRACT: Self-stabilizing systems can automatically recover from arbitrary state perturbations in finite time. They are therefore well-suited for dynamic, failure prone environments. Spanning-tree construction in distributed systems is a fundamental task which forms the basis for many other network algorithms (like token circulation or routing).This paper surveys self-stabilizing algorithms that construct a spanning tree within a network of processing entities. Lower bounds and related work are also discussed.
[Show abstract][Hide abstract] ABSTRACT: this article is structured as follows. We state our system assumptions, give some informal definitions of fair exchange and discuss special properties of what is exchanged in Section 2. In the same section, we examine how items can be specified and how their validity can be checked. Subsequently, in Section 3, we describe several approaches to rigorously formalize the fair exchange problem and discuss under which assumptions it can be solved. In Section 4, we introduce a generalizing framework which allows us to implement several different fair exchange protocols. Then we can select the appropriate protocol depending on the properties of the exchanged items and on the required fairness level. After providing an extensive review of the work relevant to the context of fair exchange in Section 5, we conclude our paper with a summary and a discussion of our approach in Section 6
[Show abstract][Hide abstract] ABSTRACT: This paper proposes that the healthcare domain can serve as an archetypical field of research in pervasive computing. We present this area from a technological perspective, arguing that it provides a wide range of possible applications of pervasive computing technology. We further recognize that pervasive computing technology is likely to create concerns about the security of healthcare systems, due to increased data aggregation, ubiquitous access, and increasing dependency on technical solutions. But we also justify why the same technology can help building more robust, more dependable systems that increase the quality of healthcare. We identify building blocks that are necessary to achieve this goal: a pervasive middleware, appropriate handling of exceptional situations, and dependability assertions for small devices.
Security in Pervasive Computing, First International Conference, Boppard, Germany, March 12-14, 2003, Revised Papers; 01/2003
[Show abstract][Hide abstract] ABSTRACT: We present a method,of combining,a self-stabilizing algorithm with a hierarchical structure to construct a self-stabilizing algorithm with improved stabilization time complexity and fault-containment features. As a case study, a self-stabilizing spanning-tree algorithm is presented which in favorable settings has logarithmic stabilization time complexity.
[Show abstract][Hide abstract] ABSTRACT: Because of its loose coupling between event producers and consumers, publish/subscribe (pub/sub) middleware has many advantages when implementing systems for spontaneous, ad-hoc, pervasive applications. One main aspect of such applications is device mobility, but unfortunately, most of the current pub/sub systems do not adequately support mobile clients. Mobility has two orthogonal aspects: physical mobility is concerned with location transparency (i.e., roaming clients) while logical mobility deals with location awareness (i.e., subscriptions are automatically adapted to a client's current location). To efficiently support mobility, it is necessary to adequately deal with the uncertainty introduced by client movement. This paper sketches how this is done in the existing pub/sub middleware REBECA and shows how to increase the efficiency of logical mobility by adapting the implementation of physical mobility. The paper closes with a list of open research issues related to the use of pub/sub middleware in the context of mobile and pervasive computing.
International Middleware Conference, Workshop Proceedings, June 16-20, 2003, Rio de Janeiro, Brazil; 01/2003
[Show abstract][Hide abstract] ABSTRACT: Mobile commerce over the Internet always includes the exchange of electronic goods. Fair exchange protocols establish fairness and ensure that both participants can engage in the exchange without the risk of suffering a disadvantage (e.g., losing their money without receiving anything for it). In general, fair exchange protocols require the continuous availability of an external trusted third party (TTP), a dedicated site which is trusted by both participants. Implementations of TTPs for fair exchange have been proposed to be based on carefully secured Internet hosts in order to establish trust. In this paper we present solutions to the fair exchange problem in mobile environments, where customers frequently disconnect from the network and thus continuous availability of the external TTP is not given. Our approach utilizes tamper-poof hardware on the customer's side partly taking over the duties of the TTP. Besides supporting disconnected operations our approach also allows the proper handling of time-sensitive items (i.e., items which lose value over time), a feature which previous protocols lack.
Mobile Networks and Applications 09/2002; · 1.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Distinguishing trace-based system properties into safety properties on the one hand and liveness properties on the other has proven very useful for specifying and validating concurrent and fault-tolerant systems. We study the adequacy of these abstractions, especially the liveness property abstraction, in the context of secure systems for two different scenarios: (1) Denial-of-service attacks and (2) brute-force attacks on secret keys. We argue that in both cases the concept of a liveness property needs to be adapted. We show how this can be done and relate the resulting concepts to related work in the areas of concurrency theory and fault-tolerance.
[Show abstract][Hide abstract] ABSTRACT: Event-based systems are developed and used as a coordination model to integrate components in loosely coupled systems. Research and product development focused so far on efficiency issues but neglected methodological support to build such systems. In this paper, we present the modular design and implementation of an event system which supports scopes and event mappings, two new and powerful structuring methods that facilitate engineering and coordination of components in event-based systems. The approach is based on a trace-based specification method adapted from temporal logic.
Proceedings of the 2002 ACM Symposium on Applied Computing (SAC), March 10-14, 2002, Madrid, Spain; 01/2002
[Show abstract][Hide abstract] ABSTRACT: We present an evaluation of advanced routing algorithms for content-based publish/subscribe systems that focuses on the inherent characteristics of routing algorithms (routing table sizes and filter forwarding overhead) instead of system-specific parameters (CPU load etc.). The evaluation is based on a working prototype instead of simulations and compares several routing algorithms to each other. Moreover, the effects of locality among the interests of the consumers are investigated. The results offer new insights into the behavior of content-based routing algorithms. Firstly, advanced routing algorithms can be considered mandatory in large-scale publish/subscribe systems. Secondly, the use of advertisements considerably improves scalability. Thirdly, advanced routing algorithms operate efficiently in more dynamic environments than was previously thought. Finally, the good behavior of the algorithms improves even if the interests of the consumers are not evenly distributed, which can be expected in practice.
10th International Workshop on Modeling, Analysis, and Simulation of Computer and Telecommunication Systems (MASCOTS 2002), 11-16 October 2002, Fort Worth, Texas, USA; 01/2002
[Show abstract][Hide abstract] ABSTRACT: This paper investigates the amount of information about failures needed to solve the predicate detection problem in asynchronous systems with crash failures. In particular, we show that predicate detection cannot be solved with traditional failure detectors, which are only functions of failures. In analogy to the definition of failure detectors, we define a failure detection sequencer, which can be regarded as a generalization of a failure detector. More specifically, our failure detection sequencer ¿ outputs information about failures and about the final state of the crashed process. We show that ¿ is necessary and sufficient to solve predicate detection. Moreover, ¿ can be implemented in synchronous systems. Finally, we relate sequencers to perfect failure detectors and characterize the amount of knowledge about failures they additionally offer.
Distributed Computing, 16th International Conference, DISC 2002, Toulouse, France, October 28-30, 2002 Proceedings; 01/2002
[Show abstract][Hide abstract] ABSTRACT: Fair exchange protocols ensure that the participating parties, customer and vendor, can engage in electronic commerce transactions without the risk of suffering a disadvantage. This means that neither of them delivers his digital item without receiving the other party's item. In general, fair exchange cannot be solved without the help of a trusted third party (TTP), a dedicated computer which is trusted by both participants. Trust can be established by carefully securing the TTP or even better by introducing tamper-proof hardware. However, if the communication to the TTP is unreliable or disrupted, then the exchange cannot be performed in a timely fashion or not at all. Up to now, this has been a problem especially for the exchange of time-sensitive items, i.e., items which lose value over time. We present a novel approach to perform fair exchange using tamper-poof hardware on the customer's side. More specifically, co-located to the customer's machine we use a smart card which partially takes over the role of the TTP. The challenge of designing protocols in this environment lies in the fact that the communication between the smart card and the vendor is under control of the customer. Our approach has the following benefits: It supports the exchange in mobile environments where customers frequently experience a disconnection from the network. Furthermore, our approach is the first to handle time-sensitive items properly.
Electronic Commerce, Second International Workshop, WELCOM 2001 Heidelberg, Germany, November 16-17, 2001, Proceedings; 01/2001
[Show abstract][Hide abstract] ABSTRACT: In an asynchronous system, where processes can crash, perfect predicate detection for general predicates is difficult to achieve.
A general predicate thereby is of the form α ∧ ß, whereα and ß refer to a normal process variable and to the operational state
of that process, respectively. Indeed, the accuracy of predicate detection largely depends on the quality of failure detection.
In this paper, we investigate the predicate detection semantics that are achievable for general predicates using either failure
detector classes □◊Ρ, ◊Ρ, or Ρ. For this purpose, we introduce weaker variants of the predicate detection problem, which we
call stabilizing and infinitely often accurate. We show that perfect predicate detection is impossible using the aforementioned failure detectors. Rather, ◊Ρ and Ρ only
allow stabilizing predicate detection. Consequently, we explore alternative approaches to perfect predicate detection: introducing
a stronger failure detector, called ordered perfect, or restricting the general nature of predicates.
[Show abstract][Hide abstract] ABSTRACT: Mobile agents have been advocated to support electronic commerce over the Internet. While being a promising paradigm, many intricate problems need to be solved to make this vision reality. The problem of fair exchange be- tween two agents is one such fundamental problem. Informally speaking, this means to exchange two electronic items in such a way that neither agent suf- fers a disadvantage. We study the problem of fair exchange in the mobile agent paradigm. We show that while existing protocols for fair exchange can be sub- stantially simplified in the context of mobile agents, there are still many problems related to security which remain difficult to solve. We propo se three increasingly flexible solutions to the fair exchange problem and show how t o implement them using existing agent technology. The basis for ensuring the security properties of fair exchange is a tamper-proof hardware device called a t rusted processing environment.
Agent Systems, Mobile Agents, and Applications, Second International Symposium on Agent Systems and Applications and Fourth International Symposium on Mobile Agents, ASA/MA 2000, Zürich, Switzerland, September 13-15, 2000, Proceedings; 01/2000