just a note. take a picture of the QR code, but it in gallery. someone scans it, checks your id. finished. no need for an app. Of course, You have to get the paper version. But it’s completely trivial.
The QR code is just a picture. It needs to be verified against another picture (passport, etc). So, I’m not sure what we’re still talking about? Is there any place you can’t just get a print out?
EDIT: so far I’ve only showed my WHO yellow vaccination certificate.