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Daily Dose of Curious
HOW IT WORKS
The range is a consequence, not a safety feature bolted on afterwards. Inductive coupling falls away sharply with distance, so a card a few centimetres too far away does not get a weak connection — it gets no power, and nothing happens at all.
The reader powers the card. Its coil throws out a magnetic field, the card's coil takes energy from it, and the card answers by changing how much of that energy it absorbs.
What actually happens
The reader makes a field
A coil in the terminal drives an alternating magnetic field at 13.56 megahertz, and it is doing this constantly, whether a card is there or not.
The card wakes up
The card is a coil of wire around a chip. Held in the field, a voltage is induced in that coil — enough to run the chip for the moment it is needed.
They talk by loading
The card does not transmit. It varies the load across its own coil, which the reader feels as a change in its field. That is how the card's answer gets back.
The chip does sums
It never sends a fixed number. It computes a one-time code from a key that has never left the chip and cannot be read out of it.
The clever bit
Nothing is broadcast by the card at all. It talks by absorbing slightly more and slightly less of the reader's own field — the electrical equivalent of answering somebody by tugging on the rope they are holding.
The words
- Inductive coupling
- Passing energy between two coils through a magnetic field, with no connection between them.
- Load modulation
- Sending a message by changing how much energy you draw, rather than by transmitting.
Where this comes from
- ISO/IEC 14443 — proximity cards: the 13.56 MHz carrier and load modulation
- EMV contactless transactions use a per-transaction cryptogram rather than a static card number
This one is not an opinion. It is how the thing works, and these are what that rests on.