The definition of pseudonymization can be found in Art. 4(5) GDPR. The following discusses this definition and provides a technical interpretation that visualized in Figure 5:

Figure 5: Pseudonymization as a manner of processing according to Art. 4(5) GDPR.

The upper part of the figure corresponds to the partial sentence “processing of personal data in such a manner that the personal data can no longer be attributed to a specific data subject without the use of additional information” of Art. 4(5) GDPR.

The lower part corresponds to the partial sentence “provided that such additional information is kept separately”. It further visualized the partial sentence “[provided that such additional information] is subject to technical and organizational measures to ensure that the personal data are not attributed to an identified or identifiable natural person”.

Note that in the lower part of the figure that is concerned with additional information, no processing (beyond storage) is mentioned or implied in Art. 4(5). The additional information is solely kept for the possibility of exiting (through re-identification) or entering (through data pseudonymization) the realm of pseudonymization.

Between the upper and lower part of the figure respectively, a black bar represents the separation. Separation of the pseudonymized data from the additional information is a key concept of pseudonymization. It is this separation that guarantees “that the personal data can no longer be attributed to a specific data subject without the use of additional information”.

The figure also illustrates the technical and organizational measures to which the additional information is subject as a double box around the additional information. Since these measures “ensure that the personal data are not attributed to an identified or identifiable natural person”, they also enforce the mentioned separation. They are discussed in more detail in section 4.6 below.