Zero-Knowledge Proof (ZKP)

A Zero-Knowledge Proof (ZKP) is a cryptographic technique that enables one party (the prover) to demonstrate to another party (the verifier) that a specific statement is true without revealing any information beyond the truth of the statement itself. In the context of digital identity and eIDAS 2.0, ZKPs represent a powerful tool for privacy preservation.

For example, a wallet holder could prove they are over 18 years old without revealing their actual date of birth, or prove they hold a valid professional licence without disclosing the licence number or issuing authority. The eIDAS 2.0 regulation emphasises user privacy and data minimisation, and the Architecture Reference Framework acknowledges the importance of privacy-enhancing technologies.

While the initial ARF specifications focus on selective disclosure mechanisms within SD-JWT and mdoc formats, which provide a practical level of privacy by allowing holders to reveal only chosen attributes, zero-knowledge proofs offer a more advanced layer of privacy by enabling predicate proofs (proving a value meets a condition without revealing the value) and unlinkability (preventing verifiers from correlating transactions). The implementation of ZKPs in the EUDIW context is an area of active research and standardisation. Some credential formats, such as those based on BBS+ signatures or Camenisch-Lysyanskaya signatures, natively support ZKP-based selective disclosure.

Whether and how ZKPs will be incorporated into the EUDIW remains a topic of ongoing discussion. For organisations interested in privacy-first identity verification, understanding ZKPs provides insight into the future direction of the European digital identity ecosystem and the technical possibilities for going beyond basic selective disclosure.