Pseudonymisation
The process of replacing directly identifying data with pseudonyms, so the data can no longer be attributed to a specific individual without additional information.
Full Definition
Pseudonymisation is a data protection technique in which directly identifying information is replaced with a pseudonym (a key or token) and the linking information is kept separately and securely. Unlike anonymisation, pseudonymisation is reversible — the data can be re-identified using the separately stored key. GDPR explicitly recognises pseudonymisation as a way to reduce privacy risks and incentivises its use, but pseudonymised data is still considered personal data and remains subject to GDPR. Common pseudonymisation approaches include tokenisation, encryption, and hashing. It is a key tool for privacy-preserving analytics.
How each law treats it
'Pseudonymisation' means processing personal data in such a manner that it can no longer be attributed to a specific data subject without additional information. Pseudonymised data remains personal data.
DPDP treats pseudonymisation as a reasonable security safeguard under Section 8. It is not a scope-out from the Act.
In practice
- Keep the pseudonymisation key separate from the data, with strict access controls and key rotation.
- Pseudonymise at the earliest point in the pipeline — ideally at collection or immediately after.
- Where multiple pseudonymised datasets are joinable through a shared key, treat the combined set as personal data regardless of individual field pseudonymisation.
Common pitfalls
- Calling pseudonymised data 'anonymous'. It is still personal data; the GDPR scope applies in full.
- Keeping the pseudonymisation key in the same database as the data. The separation is the entire point of the technique.
- Hashing without salt. SHA-256 of an email is reproducible by anyone who knows the plaintext.
Frequently asked questions
Is pseudonymised data in GDPR scope?+
Yes. Recital 28 recognises pseudonymisation as a risk-reducing measure but does not take the data out of scope. Only properly anonymised data falls outside the GDPR.
What's the difference between pseudonymisation and tokenisation?+
Tokenisation is a form of pseudonymisation — a non-reversible token replaces the original value. The token can still be used as a key to retrieve the original where that mapping is stored securely.
Does pseudonymisation reduce DPIA risk?+
Yes. It's one of the standard mitigation measures in a DPIA. The residual risk score after pseudonymisation is usually meaningfully lower.
Related terms
Anonymisation
The irreversible process of altering data so that individuals can no longer be identified, directly or indirectly.
Data Minimisation
The principle that only personal data that is adequate, relevant, and limited to what is necessary should be collected and processed.
Personal Data
Any information that relates to an identified or identifiable natural person.
Relevant regulations
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