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I've heard about RFID-shielding wallets and even individual card sleeves, but my wallet has multiple RFID cards in it (credit card, bus card, building entry card), and if I tap my whole wallet on a POS reader to try to pay for a transaction, it fails.

Why does it fail? Is the reader only reading 1 card with the strongest signal, but it's the wrong one (for example the bus card)? Or is it reading a mixed signal which it can't interpret?

Does having 2 RFID cards in a wallet cause them to shield each other from any possible attack? (even though I understand there aren't many attacks to be concerned about at the moment)

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Failure of reading two cards in the field is a result of what happens in anticollision loop. Basically, readers can decide how they handle that situation.

For some applications it makes sense to perform only collision detection (where it is important for the operation context that user consciously selects the card he wants to use in the operation, like when it's payment transaction) and require one card only in the field.

ISO14443 standard allows to resolve such situation and reader can select the card it wishes to talk to and perform the operation, even with all of them.

Answer to your other questions is then that two cards side by side do not counter any attacks. It's simply the usage that defines the necessity to present one.

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