## 2 Background
### Personal Data Protection--Legal Provisions  
The right to privacy has been recognized as a fundamental human right by the United Nations Declaration of Human Rights [29], the International Covenant on Civil and Political Rights [30], the Charter of Fundamental Rights in the European Union [31], and other international treaties. Privacy is strongly related to personal data protection; as stated in the Charter of Fundamental Rights, personal data \"_must be processed fairly for specified purposes and on the basis of the consent of the person concerned or some other legitimate basis laid down by law_\" (see Article 8(2) of the Charter). In Europe, as stated above, the main legal instrument for personal data protection is the General Data Protection Regulation (GDPR). The GDPR also applies to the processing of personal data by organizations not established in the European Union in the case of when they process the data of individuals in the Union, and the processing activities are related to either \"_the offering of goods or services_\" or \"_the monitoring of their behavior as far as their behavior takes place within the European Union_\" (see Art. 3, par. 2 of the GDPR). As explicitly stated in [32], the intentionally global reach of the GDPR, with the threat of the huge fines it sets if fundamental rights are not properly protected, has led companies around the world to amend their privacy practices, as well as countries around the world to update their privacy laws. According to the GDPR (see Art. 4(1)), the term _personal data_ refers to _any information relating to an identified or identifiable natural person_, that is a person who can be identified (referred to as a _data subject_); as explicitly stated in the same provision (i.e., Art. 4(1)) in the GDPR, an identifiable natural person _is one who can be identified, directly or indirectly, in particular by reference to an identifier such as a name, an identification number, location data, an online identifier or to one or more factors specific to the physical, physiological, genetic, mental, economic, cultural or social identity of that natural person_. It should be pointed out that this definition goes beyond the well-known definition of personally identifiable information (PII), which is generally considered information that can be used to distinguish or trace an individual's identity; all PII is considered personal data, but the reverse does not hold (see, e.g., [33]). _Personal data processing_ means _any operation that is performed on personal data, including the collection, recording, structuring, storage, adaptation or alteration, retrieval, use, disclosure by transmission, dissemination, combination and erasure_ (see Art. 4(2) in the GDPR). The entity that, alone or jointly with others, determines the purposes and means of the processing of personal data, is the so-called _data controller_, whereas the entity that processes personal data on behalf of the controller is the _data processor_ (see Art. 4(7) and 4(8), respectively, in the GDPR). If personal data are modified in such a way that the person(s) is (are) no longer identifiable, then the data should be considered anonymous, i.e., nonpersonal. In such a case, as stated in Recital 26 in the GDPR, the GDPR's provisions do not apply since we do not have personal data (although the process of anonymization, by itself, constitutes personal data processing). However, to determine whether a natural person is identifiable from a dataset that is considered anonymous, \"_account should be taken of all the means reasonably likely to be used to identify the natural person directly or indirectly_\". (This is described in Recital 26 of the GDPR.) In other words, performing truly effective anonymization is typically a difficult task; therefore, the GDPR actually \"confirms\" the research on known anonymization fallacies (see, e.g., [34]).