GDPR definitely applies. Your scenario seems to raise two questions:
- What is the legal basis for processing?
- Are the security measures appropriate?
Every processing activity of personal data requires a legal basis. Most well-known is consent, but there are six categories in Art 6(1) GDPR including legitimate interests and necessity for performing a contract with the data subject. Conditions on consent are laid out in Art 7 GDPR.
In the context of certifications, the legal basis would likely be necessity for performing a contract with the data subject: You were contracted to provide this validation service to the data subjects. You should however make it clear which information will be displayed on the validation page (other than a valid/not valid) response. For example, I am concerned that showing the email address would not be strictly necessary for providing the validation service. You should also consider whether this validation service is a core component of your certification service, or whether there should be an opt-in or opt-out here.
The GDPR requires that you implement appropriate security measures, “taking into account the state of the art, the costs of implementation and the nature, scope, context and purposes of processing as well as the risks of varying likelihood and severity for the rights and freedoms” of the data subjects (see Art 32 GDPR).
This gives you a lot of leeway to determine what measures are appropriate, but also criteria that you must consider in your determination. A number of security measures are explicitly suggested and must also be considered:
- pseudonymization and encryption
- ensuring confidentiality, integrity, availability, and resilience of your systems
- ability to restore the service (e.g. backups)
- processes for regularly testing and evaluating your measures
Aside from generic IT security measures like backups and providing the service over a HTTPS connection, I would be concerned about “insecure direct object reference” style attacks on the database. If I know that certification no 1234 exists, I could try to get the information for other numbers like no 1233 and so on. This would leak personal data.
Basic defenses could include requiring additional information such as the data subject's name to be provided, or using anti-bot measures such as captchas and rate limits.
However, the real solution is to avoid sequential IDs, and to generate sufficiently large tokens with a cryptographically secure random number generator. Instead of a “certification number”, it would be better to view this as a “validation code”.