It looks like technically bogus to me. A technically sound proof which doesn't require online access is like this. The authority encrypt some private informat

It looks like technically bogus to me. A technically sound proof which doesn't require online access is like this. The authority encrypt some private information(name and birth date for example) with the private key, and encode it to QR code and give it to the customer. On entering the pub, the customer show the document(passport, driver's license etc) which prove his private information. Staff then decode the QR code and decrypt it with authority's public key. Check the decrypted text.