A contactless reader powers the card. It creates the radio field, sends commands into it, and the card answers by modulating that same field. That is why a reader cannot observe someone else's transaction: the moment it energises, there are two fields and neither exchange completes.
| You want to | Role | What that needs |
|---|---|---|
| Talk to a card | Reader / initiator | Any contactless reader. The easy case. |
| Look like a card to a terminal | Target / emulator | Hardware that can be a target, and a terminal willing to talk to it. |
| Watch a real transaction | Passive sniffer | A device that listens without energising. This is the one a reader cannot do. |
| Change bytes in flight | Man in the middle | Two interfaces at once — a target facing the terminal, a reader facing the card. |
The last row is worth dwelling on, because it explains the shape of the hardware in the published research. A wedge sitting between a card and a terminal is not one device being clever; it is two radios and a link, presenting as a card on one side and as a terminal on the other. That is also why the attacks built on it are described in terms of what each side believes — the two halves genuinely are talking to different things.