A personal data breach, as defined in Article 4(12) of the GDPR, is a breach of security leading to the accidental or unlawful destruction, loss, alteration, unauthorized disclosure of, or access to, personal data. This definition encompasses breaches of confidentiality, integrity, and availability. In this scenario, the loss of the decryption key renders the customer data permanently inaccessible. This constitutes an "accidental... loss of access to... personal data," which is an availability breach. The cause of the incident (e.g., human error vs. hacking) is irrelevant to whether it qualifies as a breach.

A. The cause of a breach is not a determining factor; accidental incidents qualify as breaches just as malicious ones do under the GDPR definition.
B. A personal data breach is not limited to the exposure of data (a confidentiality breach); it also includes the loss of availability or integrity.
D. This option incorrectly identifies the nature of the breach. The primary issue is the loss of availability, not confidentiality (the data remains encrypted) or integrity (the data is not altered).

1. General Data Protection Regulation (GDPR) - Regulation (EU) 2016/679:

Article 4(12): Defines 'personal data breach' as "a breach of security leading to the accidental or unlawful destruction, loss, alteration, unauthorised disclosure of, or access to, personal data transmitted, stored or otherwise processed". The term "loss" directly covers the loss of availability described in the scenario.

2. European Data Protection Board (EDPB) - Guidelines 01/2021 on Examples regarding Data Breach Notification, Version 2.0 (14 January 2022):

Section 3.3, "Availability breach," page 11: "An availability breach is the result of an accidental or unauthorised loss of access to, or destruction of, personal data. Example: A ransomware attack where data is encrypted and the controller has no back-up to restore the data from." The loss of a decryption key is functionally identical to this example, resulting in a permanent loss of access.

3. Article 29 Working Party (Predecessor to the EDPB) - Guidelines on Personal data breach notification under Regulation 2016/679 (WP250rev.01), page 7:

This document explicitly categorizes breaches based on information security principles: "‘Availability breach’ – where there is an accidental or unauthorised loss of access to, or destruction of, personal data." This directly supports that the inaccessibility of data constitutes a breach.