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 Oberschelp de Meneses Anna Oberschelp de Meneses

Anna Sophia Oberschelp de Meneses is an associate in the Data Privacy and Cybersecurity Practice Group.  Anna is a qualified Portuguese lawyer, but is both a native Portuguese and German speaker.  Anna advises companies on European data protection law and helps clients coordinate…

Anna Sophia Oberschelp de Meneses is an associate in the Data Privacy and Cybersecurity Practice Group.  Anna is a qualified Portuguese lawyer, but is both a native Portuguese and German speaker.  Anna advises companies on European data protection law and helps clients coordinate international data protection law projects.  She has obtained a certificate for “corporate data protection officer” by the German Association for Data Protection and Data Security (“Gesellschaft für Datenschutz und Datensicherheit e.V.”). She is also Certified Information Privacy Professional Europe (CIPPE/EU) by the International Association of Privacy Professionals (IAPP).  Anna also advises companies in the field of EU consumer law and has been closely tracking the developments in this area.  Her extensive language skills allow her to monitor developments and help clients tackle EU Data Privacy, Cybersecurity and Consumer Law issues in various EU and ROW jurisdictions.