# Special Issue on Security and Privacy in Blockchains and the IoT
###### Abstract  
The increasing digitalization in all areas of life is leading step-by-step to a data-driven society. From an information technology perspective, this process is particularly promoted by the Internet of Things (IoT). Nowadays, a variety of sensors can be embedded in virtually any everyday object, enabling users to continuously quantify a wide range of aspects of life. For instance, a smartwatch can use GPS technologies to determine the current location of its user, an accelerometer and gyroscope to recognize the user's activity, and a microphone to capture and interpret voice messages and spoken instructions of its user. Even special use sensors are installed in those IoT devices, such as a heart rate sensor or sensors for recording insulin levels, which can be used to capture and monitor health data. Additionally, such IoT devices have the ability to communicate with each other and exchange the data they gather. In this way, large amounts of data can be collected. Comprehensive processing and analysis of these data (e.g., in a powerful cloud backend) makes it possible to draw conclusions about the context in which an IoT device is used and generate knowledge about the data subjects. This gained knowledge represents the foundation of any smart service, not only in the private sector but also in the public and industrial sectors, such as in the smart home, eHealth, and Industry 4.0 domains. This renders data as one of the most valuable assets in the information age. Therefore, it is important to manage data securely. Blockchain technologies are often applied to this end, as they ensure the immutability and tamper-resistance of data when they have to be exchanged between multiple parties that do not entirely trust each other. In additions to these information security measures, such highly sensitive data also pose great challenges with respect to data privacy. Applicable data protection laws, such as the EU's General Data Protection Regulation (GDPR), therefore demand the development and application of technical mechanisms that ensure the protection of any natural person when processing their data. In order to ensure that such protective measures are effective, however, they must be tailored to the IoT and blockchain technologies. In this regard, it must be investigated, e.g., how lightweight and privacy-preserving authentication in the IoT is possible; which trust-building approaches regarding the genuineness and validity of IoT data can be applied; and how blockchain systems can efficiently manage big data. These and related research questions regarding security and privacy in blockchains and the IoT are addressed by six research articles and two literature reviews in this Special Issue. In the following, these eight papers are briefly outlined. 202220220220220220220220220220220220220222022202  
group communications against malicious things. [PERSON] et al. [2] present a different approach to increasing trust in IoT applications. They look at the Social Internet of Things (SIoT), in which smart devices autonomously establish social connections with each other. In this way, whenever necessary, things can become service requesters or service providers on their own, without the need for any human intervention. It is obvious that--similar to real-world services--trust in the service provider is required, e.g., whether it is able to provide the advertised services. While there are approaches that can adequately model this kind of trust in SIoT, these state-of-the-art approaches completely ignore the fact that a service provider has to trust a service requester as well. This work therefore focuses on modeling bidirectional trust in SIoT. This kind of modeling introduces additional complexity due to the fact that trust is context-dependent and can vary depending on the given situation.