International audienceWe study the problem of making forecasts about the future availability of bicycles in stations of a bike-sharing system (BSS). This is relevant in order to make recommendations guaranteeing that the probability that a user will be able to make a journey is sufficiently high. To do this we use probabilistic predictions obtained from a queuing theoretical time-inhomogeneous model of a BSS. The model is parametrized and successfully validated using historical data from the Vélib ' BSS of the City of Paris. We develop a critique of the standard root-mean-square-error (RMSE), commonly adopted in the bike-sharing research as an index of the prediction accuracy, because it does not account for the stochasticity inherent in the real system. Instead we introduce a new metric based on scoring rules. We evaluate the average score of our model against classical predictors used in the literature. We show that these are outperformed by our model for prediction horizons of up to a few hours. We also discuss that, in general, measuring the current number of available bikes is only relevant for prediction horizons of up to few hours

We present an overview of hybrid Casper the Friendly Finality Gadget (FFG): a Proof-of-Stake checkpointing protocol overlaid onto Ethereum's Proof-of-Work blockchain. We describe its core functionalities and reward scheme, and explore its properties. Our findings indicate that Casper's implemented incentives mechanism ensures liveness, while providing safety guarantees that improve over standard Proof-of-Work protocols. Based on a minimal-impact implementation of the protocol as a smart contract on the blockchain, we discuss additional issues related to parametrisation, funding, throughput and network overhead and detect potential limitations.

Hypothesis testing is an important part of statistical model checking (SMC). It is typically used to verify statements of the form p > p 0 or p < p 0 , where p is an unknown probability intrinsic to the system model and p 0 is a given threshold value. Many techniques for this have been introduced in the SMC literature. We give a comprehensive overview and comparison of these techniques, starting by introducing a framework in which they can all be described. We distinguish between three classes of techniques, differing in what type of output correctness guarantees they give when the true p is very close to the threshold p 0 . For each technique, we show how to parametrise it in terms of quantities that are meaningful to the user. Having parametrised them consistently, we graphically compare the boundaries of their decision thresholds, and numerically compare the correctness, power and efficiency of the tests. A companion website allows users to get more insight in the properties of the tests by interactively manipulating the parameters.

Blockchains are distributed systems, in which security is a critical factor for their success. However, despite their increasing popularity and adoption, there is a lack of standardized models that study blockchain-related security threats. To fill this gap, the main focus of our work is to systematize and extend the knowledge about the security and privacy aspects of blockchains and contribute to the standardization of this domain.We propose the security reference architecture (SRA) for blockchains, which adopts a stacked model (similar to the ISO/OSI) describing the nature and hierarchy of various security and privacy aspects. The SRA contains four layers: (1) the network layer, (2) the consensus layer, (3) the replicated state machine layer, and (4) the application layer. At each of these layers, we identify known security threats, their origin, and countermeasures, while we also analyze several cross-layer dependencies. Next, to enable better reasoning about security aspects of blockchains by the practitioners, we propose a blockchain-specific version of the threat-risk assessment standard ISO/IEC 15408 by embedding the stacked model into this standard. Finally, we provide designers of blockchain platforms and applications with a design methodology following the model of SRA and its hierarchy.

The rapid evolution of the blockchain community has brought together stakeholders from fundamentally different backgrounds: cryptographers, protocol designers, software developers, startup entrepreneurs, corporate executives and investors, academics of various disciplines, and end users. The result is a diverse ecosystem, presently exemplified by the development of a wide range of different blockchain protocols. This raises questions for policy and decision makers: How do different protocols compare? What are their trade-offs? Existing efforts to survey the area reveal a fragmented terminology, and the lack of a unified framework to make the different desirable properties of blockchain protocols explicit.In this paper, we work towards bridging this gap. We evaluate protocols within a five-dimensional design space with the following axes. Optimality: does the protocol achieve its main goals? Stability: are the incentives of its participating agents well-aligned? Efficiency: is its output maximal relative to its use of resources? Robustness: can it cope when its operational assumptions are invalid or perturbed? Persistence: can it recover from catastrophic events? Based on the relevant literature, we organize the properties of existing protocols in subcategories of increasing granularity. The result is a dynamic scheme -termed the PREStO framework. Its scope is to aid the communication between stakeholders of different backgrounds, including managers and investors, and to identify research challenges and opportunities for blockchain protocols in a systematic way. We illustrate this via use cases and make a first step to understand the blockchain ecosystem through a more comprehensive lens.