## 3 Related Work
### Data Security
Here, a verifiable delay function (VDF) is applied to the data, which is slow to compute but easy to verify [118]. Therefore, if a data owner poses challenges to instances of the cloud provider that are supposed to hold the data in question that relate to this function, the provider can only answer them if the data are actually at rest here. If the response takes too long, this is proof that there is no replica on the instance and the cloud provider needs to compute the VFD on the fly. In addition to unreliable cloud providers, a central server always represents a bottleneck and thus an inherent weak point with regard to the availability of data in traditional client-server structures. If such a server is flooded with requests, e.g., due to a distributed denial of service attack (DDoS), and thus becomes unavailable, all data managed by it are also no longer available to the clients. IoT environments in particular are severely vulnerable to such attacks [119]. In order to ensure data availability, it is therefore necessary to replace such centralized structures with highly distributed models that store the data in multiple replicas on different nodes to be resilient in the event of a single node failure [120]. Consequently, the use of blockchain technologies is also suitable for ensuring data availability as the blockchain is based on the distributed ledger technology [121]. This refers to technologies that enable data to be stored and shared over distributed computer networks. In simplified terms, a distributed ledger is a data storage system that manages data on multiple computer nodes with equal rights [122]. Due to the decentralized nature, no central authority has control and interpretational sovereignty over the data. Moreover, the collective of nodes can decide which data should be available and thus keep false or harmful data out of the data store [123]. As blockchain-based database systems typically require each node to manage the entire blockchain, this incurs excessive storage costs. Therefore, there are approaches in which only a few server nodes need to store the entire blockchain, while clients can still verify the authenticity of the data by means of authenticated data structures (ADS) [124]. Since this constrains the data distribution, which can endanger availability if the number of expected malicious or compromised nodes is remarkably high, other approaches rely on data partitioning. In sharding, the complete data stock of a blockchain is divided into several parts and distributed to the available nodes according to well-defined rules [125]. **Authenticity. ** In order to identify users, i.e., to verify their authenticity, passwords are commonly used. These can either be real words or PIN codes or lock patterns that have to be entered for authentication [126]. The IoT also offers other authentication options based on biometrics features such as voice, fingerprints, or facial expressions [127]. All of these methods have in common, however, that they can be easily exploited by shoulder surfing during input [128] or replay attacks [129]. To reduce the number of authentications required and thus mitigate some of these threats, there are OAuth-based approaches for the IoT. Here, an authentication service issues a token that authorizes the use of devices or services for a certain period of time [130]. However, this only shifts the problem of illegally acquired authentication data to the OAuth service. To address this, the ownership model relies on using a technical device for authentication that has a unique hardwired fingerprint [131], e.g., by means of physical unclonable functions (PUF) [132]. Yet, the loss of such a device inevitably enables another person to gain possession of the authentication data. In the IoT, this issue is further exacerbated as devices are not linked to specific users but are used by several people. Moreover, IoT devices have limited input capabilities, which means that users cannot enter their credentials like on a traditional computer [133].