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.

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Due to their interesting features, blockchains have become popular in recent years. They are full-stack systems where security is a critical factor for their success. The main focus of this work is to systematize knowledge about security and privacy issues of blockchains. To this end, we propose a security reference architecture based on models that demonstrate the stacked hierarchy of various threats (similar to the ISO/OSI hierarchy) as well as threat-risk assessment using ISO/IEC 15408. In contrast to the previous surveys [39], [8], [139], [20], we focus on the categorization of security incidents based on their origins and using the proposed architecture we present existing prevention and mitigation techniques. The scope of our work mainly covers aspects related to decentralized nature of blockchains, while we mention common operational security issues and countermeasures only tangentially.Index Terms-blockchain • distributed ledgers • reference architecture • threat-risk assessment arXiv:1904.06898v1 [cs.CR]

To fully benefit from a multiprocessor system, the tasks of a program are to be carefully assigned and scheduled on the processors of the system such that the overall execution time is minimal. The associated task scheduling problem with communication delays, P jprec; c ij jC max , is a well known NP-hard problem. We propose a novel mixed integer linear programming (MILP) solution to this scheduling problem, despite the fact that scheduling problems are often difficult to handle by MILP solvers. The proposed MILP solution uses problem specific knowledge to eliminate the need to linearise the bi-linear equations arising out of communication delays. Further, the size of the proposed formulation in terms of variables is independent of the number of processors. We analyse and discuss the influence of the different MILP components in respect to characteristics of the task graph such as structure and communication to computation ratio. The proposed MILP formulation is experimentally compared with previous MILP formulations used to solve this scheduling problem. The proposed formulation displays a drastic improvement in performance, which allows to solve larger problems optimally. We also observe strengths and weaknesses of the formulation related to the input characteristics. Index Terms-Parallel task scheduling with communication delays, multiprocessor system, mixed integer linear programming, scheduling optimisation Ç

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