On November 4, 2019, the Spanish Supervisory Authority (“AEPD”), in collaboration with the European Data Protection Supervisor, published guidance on the use of hashing techniques for pseudonymization and anonymization purposes. In particular, the guidance analyses what factors increase the probability of re-identifying hashed messages.

The AEPD explains that the probability of re-identification increases if more information is available on the hash values used (e.g., that they were created on the basis of Spanish phone numbers of a certain operator). The guidance provides examples of how controllers can make the re-identification of hashed messages more difficult. These examples include encrypting the message (prior to hashing), encrypting the hash value, or adding “salt” or “noise” (i.e., a random number) to the original message.

According to the AEPD, the use of hashing techniques for pseudonymization and anonymization purposes requires companies to analyze the risk of re-identification, taking into account the hashing technique used. The risk analysis must assess the hashing process and all the other related elements, such as the information that the controller retains about the hash value after the hashing (e.g., that the hash values consist of Spanish phone numbers). The analysis should lead to an objective evaluation of the probability of re-identification of the hashed message over time.

The guidance also lists a number of “basic” considerations when using hashing for anonymization or pseudonymization purposes, such as ensuring secure access to the hashing process and periodically auditing the management processes of the hashing system.

Finally, according to the guidance, in order for a hashing technique to be considered an anonymization technique, the risk analysis must—in addition to the above considerations—assess two factors:

  • whether information which permits the re-identification of the hashed message has been deleted; and
  • whether the applied hashing technique will remain sufficiently robust over time.

Note that earlier this year the AEPD also released guidance on applying K-anonymization to data sets.

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Photo of Kristof Van Quathem Kristof Van Quathem

Kristof Van Quathem advises clients on information technology matters and policy, with a focus on data protection, cybercrime and various EU data-related initiatives, such as the Data Act, the AI Act and EHDS.

Kristof has been specializing in this area for over twenty…

Kristof Van Quathem advises clients on information technology matters and policy, with a focus on data protection, cybercrime and various EU data-related initiatives, such as the Data Act, the AI Act and EHDS.

Kristof has been specializing in this area for over twenty years and developed particular experience in the life science and information technology sectors. He counsels clients on government affairs strategies concerning EU lawmaking and their compliance with applicable regulatory frameworks, and has represented clients in non-contentious and contentious matters before data protection authorities, national courts and the Court of the Justice of the EU.

Kristof is admitted to practice in Belgium.

Photo of Anna Sophia Oberschelp de Meneses Anna Sophia Oberschelp de Meneses

I advise companies across the EU on technology laws, with a focus on data protection, cybersecurity, and current consumer protection laws. I help businesses navigate complex regulations like the GDPR, AI Act, Digital Services Act, Unfair Commercial Practices Directive, and the upcoming Digital…

I advise companies across the EU on technology laws, with a focus on data protection, cybersecurity, and current consumer protection laws. I help businesses navigate complex regulations like the GDPR, AI Act, Digital Services Act, Unfair Commercial Practices Directive, and the upcoming Digital Fairness Act, turning legal requirements into practical, business-friendly solutions.

In data protection, I support tailored GDPR compliance, international data transfers, and privacy-conscious marketing. On cybersecurity, I guide clients through risk assessments, incident response, and evolving laws such as NIS2 and the Cyber Resilience Act. Regarding consumer protection, I advise on existing laws to help businesses revise their terms and conditions for compliance and review online interfaces to ensure all mandatory consumer information is clearly provided, tackling issues like dark patterns and unfair contract clauses.

Fluent in multiple languages and experienced across borders, I’m passionate about helping clients embed compliance into their operations and thrive in the fast-changing digital landscape.