We will review some of the major results in random graphs and some of the more challenging open problems. We will cover algorithmic and structural questions. We will touch on newer models, including those related to the WWW.

Random graphs

Intermittently connected mobile networks are sparse wireless networks where most of the time there does not exist a complete path from the source to the destination. These networks fall into the general category of Delay Tolerant Networks. There are many real networks that follow this paradigm, for example, wildlife tracking sensor networks, military networks, inter-planetary networks, etc. In this context, conventional routing schemes would fail.To deal with such networks researchers have suggested to use flooding-based routing schemes. While flooding-based schemes have a high probability of delivery, they waste a lot of energy and suffer from severe contention, which can significantly degrade their performance. Furthermore, proposed efforts to significantly reduce the overhead of flooding-based schemes have often be plagued by large delays. With this in mind, we introduce a new routing scheme, called Spray and Wait, that "sprays" a number of copies into the network, and then "waits" till one of these nodes meets the destination.Using theory and simulations we show that Spray and Wait outperforms all existing schemes with respect to both average message delivery delay and number of transmissions per message delivered; its overall performance is close to the optimal scheme. Furthermore, it is highly scalable retaining good performance under a large range of scenarios, unlike other schemes. Finally, it is simple to implement and to optimize in order to achieve given performance goals in practice.

/pdf/spray-and-wait-an-efficient-routing-scheme-for-50seq3wqos.pdf

Spray and wait: an efficient routing scheme for intermittently connected mobile networks

Wireless vehicular communication has the potential to enable a host of new applications, the most important of which are a class of safety applications that can prevent collisions and save thousands of lives. The automotive industry is working to develop the dedicated short-range communication (DSRC) technology, for use in vehicle-to-vehicle and vehicle-to-roadside communication. The effectiveness of this technology is highly dependent on cooperative standards for interoperability. This paper explains the content and status of the DSRC standards being developed for deployment in the United States. Included in the discussion are the IEEE 802.11p amendment for wireless access in vehicular environments (WAVE), the IEEE 1609.2, 1609.3, and 1609.4 standards for Security, Network Services and Multi-Channel Operation, the SAE J2735 Message Set Dictionary, and the emerging SAE J2945.1 Communication Minimum Performance Requirements standard. The paper shows how these standards fit together to provide a comprehensive solution for DSRC. Most of the key standards are either recently published or expected to be completed in the coming year. A reader will gain a thorough understanding of DSRC technology for vehicular communication, including insights into why specific technical solutions are being adopted, and key challenges remaining for successful DSRC deployment. The U.S. Department of Transportation is planning to decide in 2013 whether to require DSRC equipment in new vehicles.

Dedicated Short-Range Communications (DSRC) Standards in the United States

In this paper, we define and explore proofs of retrievability (PORs). A POR scheme enables an archive or back-up service (prover) to produce a concise proof that a user (verifier) can retrieve a target file F, that is, that the archive retains and reliably transmits file data sufficient for the user to recover F in its entirety.A POR may be viewed as a kind of cryptographic proof of knowledge (POK), but one specially designed to handle a large file (or bitstring) F. We explore POR protocols here in which the communication costs, number of memory accesses for the prover, and storage requirements of the user (verifier) are small parameters essentially independent of the length of F. In addition to proposing new, practical POR constructions, we explore implementation considerations and optimizations that bear on previously explored, related schemes.In a POR, unlike a POK, neither the prover nor the verifier need actually have knowledge of F. PORs give rise to a new and unusual security definition whose formulation is another contribution of our work.We view PORs as an important tool for semi-trusted online archives. Existing cryptographic techniques help users ensure the privacy and integrity of files they retrieve. It is also natural, however, for users to want to verify that archives do not delete or modify files prior to retrieval. The goal of a POR is to accomplish these checks without users having to download the files themselves. A POR can also provide quality-of-service guarantees, i.e., show that a file is retrievable within a certain time bound.

PORs: Proofs of Retrievability for Large Files

Vehicular networks are very likely to be deployed in the coming years and thus become the most relevant form of mobile ad hoc networks. In this paper, we address the security of these networks. We provide a detailed threat analysis and devise an appropriate security architecture. We also describe some major design decisions still to be made, which in some cases have more than mere technical implications. We provide a set of security protocols, we show that they protect privacy and we analyze their robustness and efficiency.

/pdf/securing-vehicular-ad-hoc-networks-4sgegi8hgk.pdf

Securing vehicular ad hoc networks

While federated identity management separates service provisioning from identity provisioning, the identity provider is usually operated at the home organization of the identities. We address the challenge of outsourcing the entire identity provider with its user database to an untrusted external provider in a secure and privacy-preserving way. With this type of outsourcing, the home organization is no longer required to operate high availability infrastructure for access management. Instead, the home organization only needs to frequently attest that the identity data in the outsourced database is still up to date, a task that is much less demanding than providing access decisions whenever a user wants to make use of a service. In this paper we present Occasio, a concept that permits secure outsourcing of identity and access management to untrusted external providers. Occasio builds on concepts of outsourcing databases and particularly on Merkle Hash Trees. We show that Occasio matches all security requirements for operation in an untrusted environment. Furthermore, we demonstrate that Occasio can be easily integrated into the SAML standard. We present results of a performance evaluation that shows that Occasio behaves well in terms of overhead. Finally, we show that with Occasio identity data of different home organizations can be `aggregated' without being linkable by someone other than the services that are granted to do so by the user.

OCCASIO: An operable concept for confidential and secure identity outsourcing

This thesis is that a peer-to-peer network's overlay topology should adapt to match the demand graph of the peer-to-peer network. In order to assess the effectiveness of various adaptation mechanisms, a comparison with an optimal topology for a given demand graph would be helpful. However, several related optimization/decision problems have been shown to be NP-hard. The contributions of this paper are threefold: i) NP-hardness results related to assessing overlay adaptation strategies were briefly surveyed, ii) a specific optimization problem and metric was presented, and iii) experimental results indicating the potential of optimizing the overlay topology were provided. Finally, in the spirit of a 'challenge paper' various open issues were stated and discussed.

On the challenge of assessing overlay topology adaptation mechanisms

Sharing data within a closed user group is the basis for lots of applications today in both personal and professional life. The risk that the storage provider or an attacker illegitimately inspects or tampers with the shared data can be mitigated by employing client-side cryptography. In this work, we estimate the resource consumption that secure data sharing based on client-side cryptography requires in terms of computation time and network traffic volume. We therefore go beyond asymptotical analyses and state the absolute resource consumption for different secure data sharing protocols and client devices under realistic sharing models. These sharing models are extracted from traces of real-world collaboration platforms running in production. To the best of our knowledge, this is the first time that such a characterization of sharing models is published. Furthermore, we provide a clearly defined resource consumption estimation model. Our results show that for users who just up-and download data, the execution time of required cryptographic operations is typically up to a few seconds. The results also show that group owners have to deal with significantly higher computation times and network traffic when a user is removed from a sharing group with a few hundred or more members, given that basic secure data sharing protocols are used as they are in place today. A further finding is that computation time and network traffic volume are considerably lowered by extending the secure data sharing protocol with group key management approaches, at the price of slightly raised computation times for smaller groups.

On the Resource Consumption of Secure Data Sharing

The logical hierarchies of Web sites (i.e. Web site taxonomies) are obvious to humans, because humans can distinguish different menu levels and their relationships. But such accurate information about the logical structure is not yet available to machines. Many applications would benefit if Web site taxonomies could be mined from menus, but it was an almost unsolvable problem in the past. While a tag newly introduced in HTML5 and novel mining methods allow to distinguish menus from other contents today, it has not yet been researched, how the underlying taxonomies can be extracted, given the menus. In this paper we present the first detailed analysis of the problem and introduce rule-based concepts for addressing each identified sub problem. We report on a large-scale study on mining hierarchical menus of 350 randomly selected domains. Our methods allow extracting Web site taxonomy information that was not available before with high precision and high recall.

/pdf/mining-taxonomies-from-web-menus-rule-based-concepts-and-28mi5issod.pdf

Mining taxonomies from web menus: rule-based concepts and algorithms

Over the past several years, there has been significant interest and progress in using wireless communication technologies for vehicular environments in order to increase traffic safety and efficiency. Due to the fact that these systems are still under development and large-scale tests based on real hardware are difficult to manage, simulations are a widely-used and cost-efficient method to explore such scenarios. Furthermore, simulations provide a possibility to look at specific aspects individually and to identify major influencing effects out of a wide range of configurations. In this context, we use the HP XC4000 for an extensive and detailed sensitivity analysis in order to evaluate the robustness and performance of communication protocols as well as to capture the complex characteristics of such systems in terms of an empirical model.

Hannes Hartenstein

Papers

OCCASIO: An operable concept for confidential and secure identity outsourcing

On the challenge of assessing overlay topology adaptation mechanisms

On the Resource Consumption of Secure Data Sharing

Mining taxonomies from web menus: rule-based concepts and algorithms

Simulative Analysis of Vehicle-to-X Communication considering Traffic Safety and Efficiency