While there are ways to set access bits in such a way that the keys are blocked from reading or changing each other, that is not my primary concern. The Mifare spec dictates that access bits be written in a specific way with both normal and matching inverted bits… and if you write an invalid bit pattern or there is a bit flip error during transmission, the entire sector will be bricked… no data will be readable and no keys either. This is my primary concern when playing around with writing data to sectors, especially since we are dealing with magic chips with sometimes wonky signal detection, small antennas, and one-handed proxmark3 operation.
NFC-Access-Control-for-Mifare-S50.pdf (631.1 KB)
That said, it would still be a concern if you were dealing with a mifare card that had secured one or more sectors using access bit settings which permanently set the entire block to read-only with no key access (meaning you can’t change the keys or access bits).