I use a key card to enter a building, but it only works reliably when I keep it separate from the other cards in my wallet. If I hold the whole wallet against the reader, the door usually does not recognize it. Would placing an RFID-blocking card between the access card and my other cards prevent the reader from picking up the others, so I could keep everything together and always present the same side of the wallet?
3 Answers
RFID blockers work by attenuating the radio-frequency field with conductive material, such as aluminum or copper. They are not magic, and a thin blocking card in one position may not stop signals coming around its edges. Also, foil can be awkward and tear, so a separate badge holder or keeping the access card outside the wallet is usually the more reliable solution.
Probably not if the blocker is only between the cards. The reader's field can still interact with cards beside or behind it, and several cards responding at once can confuse the reader. To shield something effectively, the conductive material generally needs to surround the card or shield the unwanted cards on all relevant sides. A simple test is to place several layers of foil around the other cards and see whether the access card works by itself.
The wallet may already be failing because multiple cards are answering the reader at the same time, not necessarily because the cards are being read as one. Some access cards also use stronger security than ordinary contactless payment or identification tags, so the issue is likely card collision or poor coupling rather than someone being able to copy the card just by standing nearby.
A practical workaround is to keep the access card in a badge holder or outer pocket and attach it to a retractable clip. That avoids having to experiment with shielding and makes it easier to present only the intended card.