The paper MIFARE Classic: exposing the static encrypted nonce variant was quite an interesting paper. A Chinese variant (FM11RF08S) of MIFARE Classic cards was found to be backdoored by the manufacturer. Special auth commands leak (static) encrypted nonces, which can then be used to recover sector keys and dump the card. Quarkslab’s write-up adds that a second backdoor key works on all older Fudan FM11RF08 cards and even on old NXP (MF1ICS5003, MF1ICS5004) and Infineon (SLE66R35) cards, and that these chips turn up in hotels across the US, Europe and India.
By 2024, we all know MIFARE Classic is badly broken
Move on to DESFire or some other newer, safer chips.