What mechanism is embedded within, for instance, tram cards that you can use to check in and out of your train ride, that prevents you from copying the signal that is coming from that card ? I could maybe understand this for cards that have an account linked to them (e.g. bank cards or other personalised cards), but what about cards you just [top up](https://en.wikipedia.org/wiki/Stored-value_card?wprov=sfti1) (e.g. [train cards in the Netherlands](https://en.wikipedia.org/wiki/OV-chipkaart?wprov=sfti1)). Why couldn’t you just copy every signal that is coming out of that card and clone it to a different one making a receiving device think you have a topped up card?
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First off, the cards don’t transmit a signal in the way your question implied. They interact with a signal being transmitted from the card reader. So, in order to “clone” this, you’d need a copy of the chip on the card, AND a copy of the signal being sent from the reader, AND the challenge/response of that signal, and…
Basically, it could be done, but it pretty much requires direct interaction with both devices. That’s why physical card skimmers on things like gas pumps are a thing. They provide that direct interaction so they can record all the parts of the transaction and *hopefully* reproduce them elsewhere (they are not 100% effective, thankfully)
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