Germany has decided to implement the proof of vacination for Corona based on 5 Blockchains. It's supposed to be anonymus and safe and the verification could hapen with a QR-Code.
Use X509 (SSL) certificates to verify that someone is vacinated, by including additional information and making the Countries CA's.
We reuse the existing and proven code for issuing and checking SSL certificates like OpenSSL and expand the issued certificate with a field for with information with which it's possible to uniquely identify a person, like birthplace and date, while the CN field can be the name itself.
Every Country which is part of the system becomes a CA, or if you have a group of states like the EU they become the CA. Then create the chain of trust for example : EU -> Germany -> Bavaria -> Vacination Center -> Anton (Person). The certificate Anton gets issued can't sign other. Anyone who knows the root certificate can then verify that Anton has recieved his jab. The personal information is safe, only Anton has the certificate. No one, except for the other party, even knows that Anton showed his proof.
The only problem would be how to safe and show the certificat. QR-Codes wouldn't work, the cert. is to big, however one could use NFC. The people with a device where you can easily access the NFC Antena, like Android, could use their phone. Everyone else could use a boring old NFC Tag with sufficient Storage, 8KB should be enough, which I could find for 1,10€. (That's kind of expensive and maybe a deal breaker.) It would be easy to use (just tap your card on the reader), safe (only allowed people can issue cards and the underlying infrastructure is tested) and protects personal data (only you carry your information).
If the EU wants to honour the cert issued in the USA, they could add them as a root CA. If it turns out a someone has issued wrong certificates for whatever reason: revoke the cert which is highest in the chain, which was compromised.
Could this scheme work or did I overlook something, (other than countries not agreeing on one method, because what they are doing is best)?