Pseudonymised data

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Article 4(5) of the UK GDPR defines pseudonymisation as:

“…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, provided that such additional information is kept separately and is subject to technical and organisational measures to ensure that the personal data are not attributed to an identified or identifiable natural person.”

The ICO’s guidance adds:

“At a basic level, pseudonymisation starts with a single input (the original data) and ends with two outputs (the pseudonymised dataset and the additional information). Together, these can reconstruct the original data. However, in relation to the individuals concerned, each output has meaning only in combination with the other.”

So putting in words JC understands:

• If you anonymise data — switch out all references to someone identifiable IRL with unique identifiers, randomised numbers and so on so that there is no possibility of anyone — whether inside your organisation of outside it — reverse engineering who the individuals are, then you are not processing personal data.
• If you pseudonymise data — which is to anonymise it as per above but separately hold some kind of key which decodes it into something identifiable, then it still counts as personal data. If you are a third party given pseudonymised data, even though in your hands it is anonymous, seeing as someone else could reverse engineer it, it is still, in your hands, pseudonymised. This seems a bit harsh, but the risk is you (and the person holding the “key”) suffer some data breach or something like that.

See also

• Personal data
• ICO guidance