## 3 Related Work
### Data Security  
In modern data administration, four protection goals in particular have to be addressed, namely confidentiality, integrity, availability, and authenticity [81]. Next, we discuss research approaches designed to fulfill these protection goals. We look at the protection goals separately. In the practical application, however, there are correlations with other protection goals. For instance, effective authenticity is a cornerstone of access control and thus a prerequisite for all the other protection goals. The protection goals are also contradictory to some extent, e.g., the highest level of confidentiality can be achieved if no one has access to the data. Yet, this conflicts with availability [82]. Such mutual effects have to be taken into account when adopting solutions to ensure the protection goals. In our discussion, we refer to the definitions of these four protection goals given in the ISO/IEC 27000-series [83], which is the internationally recognized standard for information security management systems. Confidentiality means that information is not disclosed to unauthorized third parties. Integrity ensures that the information made available is complete and uncorrupted. Availability means that authorized parties have access to the information at all times. Finally, authenticity ensures that both the origin of the information and the identity of the parties interacting with the information are verified. Confidentiality.To protect against the disclosure of sensitive information, cryptography approaches are typically used. That is, data are available in encrypted form and can only be decrypted (and thus read) with the appropriate key. Both symmetric encryption--in which the same key is used for encryption and decryption--and asymmetric encryption--in which a key pair with different keys for encryption and decryption is used--can be applied to this end. While symmetric encryption approaches generally require less encryption time, asymmetric encryption approaches facilitate key distribution, since the private key always remains with the key owner, while the corresponding public key is shared with anybody without compromising confidentiality [84]. Combinations of these two techniques are also found, particularly in IoT environments, in order to reconcile simple key management with reduced hardware requirements [85]. To reduce the overall decryption effort required for each data access, homomorphic encryption can be used. Here, encrypted data are also unreadable without a corresponding key, but certain predefined operators can still be applied to them, such as aggregation functions, for statistical surveys [86] or search queries [87]. That is, the data can be preprocessed and even analyzed without being fully exposed [88]. An access control policy can be used to specify who has access to which data and for what purpose [89]. However, since the IoT is a dynamic environment, this must also be reflected in an access control system [90]. Thus, policy rules also have to consider the current context in which data are accessed (e.g., a spatiotemporal context in which the access takes place or a role-based context of the accessor) [91]. As a result, such a policy becomes highly complex, which is why access control systems in practice have to solve problems regarding conflicting policy rules [92] and scalable access management [93]. Integrity.Currently, data integrity is often ensured by the use of blockchain technologies. A blockchain can be regarded as an immutable and tamper-resistant data store. By organizing the data in blocks that are inseparably linked to each other via cryptographic hashing, it can be ensured that neither individual data items within a block nor entire blocks can be manipulated. As this chain of blocks is managed in a distributed manner, i.e., multiple parties manage an equivalent copy of the chain, manipulations can be easily detected and reversed [94].