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

A survey on cloud computing security

This literature review has focused on smart governance as an emerging domain of study that attracts significant scientific and policy attention. More specifically, this paper aims to provide more insight in the definitions of and relationships between smart governance and concepts such as smart and electronic government, in the context of smart cities. The literature review shows that smart government can be considered as a basis for developing smart governance, through the application of emergent information and communication technologies (ICT) for governing. Smart governance as the intelligent use of ICT to improve decision-making through better collaboration among different stakeholders, including government and citizens, can be strongly related to government approaches. In this case ICT-based tools, such as social media, and openness can be factors that increase citizen engagement and support the development of new governance models for smart government. Smart governance may also have an important role in smart city initiatives, which require complex interactions between governments, citizens and other stakeholders. Based on the literature review, this paper coins a definition of ‘smart city governance’ and contributes to developing a framework for building new, smart governance models addressing the challenges of the digital society, collaborative governance, information sharing, citizen engagement, transparency and openness.

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Smart governance in the context of smart cities: A literature review

Cloud computing research: a review of research themes, frameworks, methods and future research directions

Resource scheduling for infrastructure as a service (IaaS) in cloud computing

Outsourcing data to external providers has gained momentum with the advent of cloud computing. Encryption allows data confidentiality to be preserved when outsourcing data to untrusted external providers that may be compromised by attackers. However, encryption has to be applied in a way that still allows the external provider to evaluate queries received from the client. Even though confidential database-as-a-service (DaaS) is still an active field of research, various techniques already address this problem, which we call confidentiality preserving indexing approaches (CPIs). CPIs make individual tradeoffs between the functionality provided, i.e., the types of queries that can be evaluated, the level of protection achieved, and performance.

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Confidential database-as-a-service approaches: taxonomy and survey

The distributed hash table (DHT) formed by BitTorrent has become very popular as a basis for various kinds of services. Services based on this DHT often assume certain characteristics of the DHT. For instance, for realizing a decentralized bootstrapping service a minimum number of peers running on a certain port are required. However, key characteristics change over time. Our measurements show that e.g. the number of concurrent users grew from 5 to over 7 millions of users during the last months. For making reliable assumptions it is thus essential to monitor the P2P network. This demo presents BitMON, a Java-based out-of-the-box platform for monitoring the BitTorrent DHT. This tool does not only crawl the network, but also automatically analyzes the collected data and visualizes the results. BitMON monitors the DHT's size in peers as well as the peers' IP addresses, port numbers, countries of origin and session length. Also, the long-term evolution of these indicators can be graphically displayed. Furthermore, BitMON is designed as a framework and can easily be extended or adapted to monitor other P2P networks.

BitMON: A Tool for Automated Monitoring of the BitTorrent DHT

Distributed Hash Tables (DHTs) prove valuable for distributed architectures by providing distributed information lookup, routing, and data storage while promising good scalability and robustness at the same time. From this point of view, a DHT could be seen as a basic service that can be used to build distributed applications. Whereas today no widely deployed and publicly accessible basic DHT service exists and thus DHT- based applications have to deploy their very own DHT networks, DHTs consisting of millions of peers are formed by file sharing clients. Although the interfaces of typical DHTs used for file sharing are too narrow, a basic DHT service could probably be created by bundling a suitable client implementation with file sharing software. In this paper, we evaluate whether a basic DHT service could suit the needs of DHT- based applications in terms of stability, number of participating peers, the peers'' session lengths, geographical distribution and peer connectivity when deployed similar to DHTs driven by file sharing. As these metrics mostly depend on end user behavior rather than on the DHT protocol, we report on measurement results gathered from monitoring of the BitTorrent Mainline client''s DHT over six months. We analyze which metrics would fit a basic DHT service and which could prove problematic. Furthermore, we discuss resulting technical requirements for an appropriate DHT protocol.

Towards a Basic DHT Service: Analyzing Network Characteristics of a Widely Deployed DHT

For cloud-based outsourcing of confidential data, various techniques based on cryptography or data-fragmentation have been proposed, each with its own tradeoff between confidentiality, performance, and the set of supported queries. However, it is complex and error-prone to select appropriate techniques to individual scenarios manually. In this paper, we present a policy-based approach consisting of a domain specific language and a policy-transformator to automatically generate scenario-specific software adapters called mediators that set up data outsourcing and govern data access. Mediators combine state-of-the-art confidentiality techniques to ensure a user-specified level of confidentiality while still offering efficient data access. Thus, our approach simplifies data outsourcing by decoupling policy decisions from their technical implementation and realizes appropriate tradeoffs between confidentiality and efficiency.

Data Outsourcing Simplified: Generating Data Connectors from Confidentiality and Access Policies

The benefits of distributing a network simulation depend on characteristics of the simulated network. Performance improvements reported in the literature are comparatively low for peer-to-peer overlay networks in particular, as the logical topology of these networks can necessitate frequent synchronization between the processors executing the simulation. In this paper, we show that a speedup of up to a factor of 6.0 using 16 nodes connected using InfiniBand and close to linear reductions in memory usage are possible for simulations of Kademlia-based networks. Our distributed simulator implementation enables simulations of one of the largest peer-to-peer networks at full scale of about 10 million peers. Based on the two fundamental goals of minimizing communication between processors and minimizing synchronization overheads, we propose two strategies for assigning simulated nodes to processors. We analyze the effects of the two strategies and show that each strategy supports one of the goals, while being detrimental to the other. We propose efficiency metrics that expose how much of the potential for parallel execution is exploited by a simulator. Through detailed performance measurements and by applying the new metrics to our simulator implementation, we quantify remaining efficiency potentials.

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Konrad Jünemann

Papers

Confidential database-as-a-service approaches: taxonomy and survey

BitMON: A Tool for Automated Monitoring of the BitTorrent DHT

Towards a Basic DHT Service: Analyzing Network Characteristics of a Widely Deployed DHT

Data Outsourcing Simplified: Generating Data Connectors from Confidentiality and Access Policies

Parallelism potentials in distributed simulations of Kademlia-based peer-to-peer networks