Verifier App アプリ
The Digital Green Certificates Verifier App is responsible for scanning and verifying DGCs using public keys from national backend servers. Offline verification is supported, if the latest public keys are present in the app's key store. Consequently, once up-to-date keys have been downloaded, the verification works without active internet connection.
Details on the Digital Green Certificate itself can be found at the corresponding web page by the EU Commission. Quoting from there:
A Digital Green Certificate is a digital proof that a person has been vaccinated against COVID-19, has recovered from COVID-19 or has a test result. The Digital Green Certificate is therefore a name used to collectively refer to 3 types of certificates: vaccination, recovery, test.
