The Mifare Classic Card Recovery Tool is a solid utility for recovering data from damaged Mifare Classic RFID cards. While it has some limitations, its effectiveness, user-friendly interface, and robust security features make it a valuable asset for those working with these cards.

The card uses a challenge-response protocol. The reader (PDC) requests authentication for a specific block. The card replies with a random number (nonce). The reader and card then exchange encrypted data to verify they both possess the same key.

Once a sector is authenticated, the protocol allows for "nested authentication," where the reader can authenticate to a different sector without resetting the communication stream. The critical flaw is that during a nested authentication transaction, the card generates a new random number ($n_T$) that is encrypted using the keystream of the already authenticated session. If the attacker knows the key of Sector A, they can authenticate to Sector A and then request authentication to Sector B. The response from the card leaks information about the random number generated for Sector B, encrypted under the known keystream.

MIFARE Classic Tool (MCT) is the most popular open-source application for interacting with these tags using an Android device's internal NFC controller.