# Special Issue on Security and Privacy in Blockchains and the IoT
###### Abstract
Based on their bidirectional trust model, the authors discuss a trust-based service delegation method in SIoT, which considers not only the level of trust between a service requester and a service provider but also the utility of the offered service. Two of the research articles are dedicated to blockchain technologies. While blockchain systems enable secure data management in terms of immutability and tamper resistance, they typically lack comprehensive query capabilities. [PERSON] et al. [3] therefore review the current state of query processing in blockchain systems and the future challenges in this research area. For this purpose, they initially investigate in which application domains blockchain technologies are used as part of big data management systems. Based on this, they determine which types of data and which data models are primarily used in this context. They then study the query capabilities of today's blockchain systems and discuss to what extent they meet the requirements of the use cases from the application domains. Furthermore, they give an outlook on how the internal data structures as well as the block structures of a blockchain system have to be adapted in order to efficiently support complex queries, such as history queries over time series data. [PERSON] et al. [4] address another inherent problem in blockchain systems. As the blockchain uses a distributed ledger as its underlying infrastructure, i.e., a replicated, shared, and synchronized data store whose instances are managed by multiple parties, all involved parties have to agree on what data should be added to the blockchain. To synchronize changes, consensus methods such as proof of work (PoW) are used. A major disadvantage of PoW, however, is that it is very computation-intensive and therefore favors parties that have access to powerful computing capacities. In order to provide more fairness in the case of heterogeneous parties, e.g., in IoT environments, the authors interpose edge devices that monitor each computing node participating in PoW. This monitoring is based on a digital twin approach that simulates the normally expected behavior of each computing node. As a result, misbehavior by dishonest participants can be detected, e.g., computing nodes that use extra computing power to outperform their competitors. By means of a proof-of-concept implementation, the authors demonstrate not only the feasibility but also the efficiency of their approach. The remaining two research articles focus on how blockchain technologies can be applied in the IoT to ensure privacy aspects, namely, access control and privacy-aware data sharing. While IoT applications typically rely on a central data backend that is responsible for the management of the collected data, such an approach poses a risk from a security perspective. Since a single entity operates this backend and thus has full control over the data, tampering is easily possible. Blockchain-based solutions, which manage the data in a distributed manner and are jointly operated by multiple parties, overcome this problem. However, they cause major privacy concerns, as an access policy for confidential data must be reliably applied to all data nodes involved. [PERSON] et al. [5] therefore introduce a two-pronged approach by which access to sensitive IoT data can only take place with the consent of the data subject. This approach uses a combination of a secure hash function and a key derivation function to encrypt the data. The data in the blockchain can only be decrypted if the data subject has given their consent. With their approach, the authors not only improve reliability but also reduce the computation time compared to state-of-the-art approaches. [PERSON] et al. [6] also present an approach with which confidential IoT data can be trustworthily shared among multiple parties using distributed ledger technology. However, their approach relies on IOTA, a distributed-ledger-based communication protocol specifically tailored to the requirements of the IoT.