What materials block the NFC scanning of RFID cards?

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I'm given to understand that the scanning of wallets and purses is now even easier because of NFC smartphones(eg, Samsung)

Card protective sleeves, wallets and cases are on the market to block the scanners/smartphones from stealing card data and ultimately $$$$.

I never believe advertisers out of principle, and so I want to know from a Physics perspective what can block the scanning of this private data.

Thanks in advance for sharing your knowledge.

The RFID chip has no power source. It relies on the energy of the signal received to echo a reply. It's supposed to have a very short range, but experts say range is all about the quality/sensitivity of the receiving antenna trying to scan the card.

Supposedly those card protectors are good enough. Acting like a Faraday cage to block/reflect the incoming signal. It'd be interesting to know what it takes to overcome that level of protection.

FWIW, a person involved in national security issues told me flat out that the US currency can be scanned and identified for tracking purposes. So, while cash is somewhat anonymous, if someone in government wants you tracked, they could use the money in your wallet using some very common equipment already in place.

Interesting.

My concern is with identity thieves and other criminals seeking credit and debit card data, etc...

I'm not worried about the currency issue because thieves can't use the data from fiat currencies for anything, it's the use of the data from the cards that scares me.

Line your wallet with aluminum foil. I've tested this with HID scanners and found it to be effective in blocking 125kHz RFID fields from RFID cards inside the wallet.

A piece of foil slightly longer than a dollar bill and slightly larger than twice the height of a dollar bill, then folded in half lengthwise with your cash inside will effectively block any RF, but not X-Rays.

Some people will "cook" their passports in a microwave oven for about 5 seconds to burn out its RFID chip - I DO NOT RECOMMEND THIS, as I am not certain of the legalities involved ... all considerations for fire hazards aside ...

thank you