Before revision:  We describe the underlying principles, ethical considerations, and a 9-step usecase scenario .
After revision:  For the test certificate Holder, Issuer (e.g. doctor, pharmacy) and Verifier (e.g. employer), it is 'just another app' which takes only minutes to use. Conclusions: The app and distributed server architecture offer a prototype proof of concept that is readily scalable, applicable generically, and in effect 'waiting in the wings' for the biological issues, plus key ethical issues raised in the discussion section, to be resolved .