It's an easy-to-state largely foolproof test to see if data really is anonymized.

The thing that you're worried about with poorly-anonymized datasets is that if you have another non-anonymized dataset you can combine them to deduce the original information. "Your data set must not be able to be combined with any others that would allow them to infer the original data" is hard. How could you possibly test them all?

Well it turns out that there is one such non-anonymized dataset with the property that if you can't connect your anonymized data with it at all then you can be pretty sure that you couldn't connect them with any others -- the original data!

Let's say you're doing a study of fingerprint patterns. You anonymize a collection of fingerprints from a non-anonymized source by stripping everything but the fingerprint images. Because fingerprints are unique it seems like it'd be impossible to meet the GDPR criteria; even if the only thing that was left was the fingerprints, when compared against the source dataset they will be identified. a) is this interpretation accurate? b) if so, it seems that there's large swaths of data that can never be in compliance. What are the implications for medical research, for instance?

I think you nailed it that some data can’t really be anonymized. How could you anonymize emails, names, social security numbers, DNA samples?

You don’t have to use anonymized data all the time, it’s just that the requirements for handling and passing around such data is lower.

I don't understand the point though; if someone has the source data, what good is the anonymized data to them? What value is added by requiring more stringent safeguards on data that can't be anonymized this way?

If someone makes inferences on the de-identified data, or joins it against another dataset. The source dataset lets those inferences or joins be tied back to the original identifying data.

The main point is that de-identified data can still be "personal" so it's regulated. If you share or make public psuedonymous data, that data is still covered by GDPR so you have to inform the individuals, have a legal basis (such as consent), let them opt out (if applicable), etc. Even if it's been pseudonymized, I would want to know if/when my data is sold to a marketing firm or whatever.

> The source dataset lets those inferences or joins be tied back to the original identifying data.

But if the attacker lacks the source dataset, they can't do this, and if they possess the source dataset, they'd use it for their analysis rather than using the anonymised dataset.

The point is that if the attacker can connect your user record in the source data with user # 188da24a7789d in the "anonymized" data, they can use that de-identify all information derived or built on the "anonymized" data.

Oh, there is Netflix account for user # 188da24a7789d and the IRS released tax summaries for user # 188da24a7789d? That's interesting, since I know that user # 188da24a7789d is really MaxBarraclough.

If a dataset removes all information except for, say, a user's fingerprints, meaning the only information stored in the anonymous dataset is an image of a fingerprint. The nature of fingerprints prevents them from meeting this requirement, as stated, which effectively eliminates any research that can be done with the data. Given that the only way the dataset could be linked to the original user is if an attacker already had access to the source data, how is this regulation benefiting anyone?

Privacy and security are not the same. Security is to protect against malicious actors. Privacy is to protect data from everyone that’s not the person PII itself.