[quesomaster9000]

The door guard needs to validate your face matches your ID. In you example you provide the door guard with proof that you are in real-time communication with a real ID, not that it's yours.

A challenge response would be the door guard provides you with a picture of yourself, and then you run their verification code with the picture and your ID.

However, you would also want to take into account multi-venue barring lists, which would require you to provide that you're not on a list of people (that you don't have access to).

In reality, this gets complicated very fast, and I would much prefer to just show the door guard my ID rather than involve computers in any way shape or form, let alone ZK systems.

[latchkey]

The app shows your face on it and says it is 21+.

[quesomaster9000]

That isn't a challenge/response flow, that's the app on your device asserting something in isolation.

[latchkey]

The challenge is from the security guard who won't let you into the club without proof that you're 21+.