Why do you trust the reader though? It could display one thing and send another. Although I guess this also happens with payment card terminals. Who's to say the €3 displayed is not charged as €300...
If the ID is on your phone, you can make it so that the transaction details have to be digitally signed by the person authorizing them in order to be valid. Then, if 3€ shows up on your phone, that's what you're authorizing, not 300€.
Sure, given an advanced enough device anything is possible. But I think here we are still discussing a "card" form factor for ID? (Being an "unperson" simply because you don't have a smartphone or have a rooted one would be "interesting").
Most places with digital IDs use either a phone card reader or the phone’s own NFC terminal to read a contactless smart card. The cryptographic key comes from the smart card, with the phone as a payment terminal.
Nothing advanced is required. And sure, your phone can be hacked, but there’s only so much fearmongering to go around.
