Simple Startup
Firstly, it's the Mifare Classic Tool that I used to write hex data back to Mifare 1k card with my NFC phone - Moto X 2013. And after several steps of studies and development, I find a way to read the money data from the card and generate legal hex data then write back it to the card. And it works perfectly, which made me so happy. Seemed like I was a real hacker, can easily change the money on the card with a simple app. After that, I want to share this happiness with all guys who are interested in RFID. After about 3 weeks of development, the first version of MTools came out. Here're the things that you need to know to use it better.
  1. 1.
    Get the keys from all sectors
  2. 2.
    Compare to find the sector with money data
  3. 3.
    Add card, sector, and keys to MTools
  4. 4.
    Record data from after every consumption in sniffer
  5. 5.
    Compare and analyze the rules of dynamic bytes
  6. 6.
    Finish the expression
  7. 7.
    Charge Mifare 1k card with one click

Get the keys from all sectors

Some cards using default keys like FFFFFFFFFFFF 000000000000 A1B2C3D4E56F etc, which you can try to dump keys with Mifare Classic Tools on the NFC android phone. If the card is not fullly encrypted, it can be cracked with an RFID device to burst crack. Like ACR122U or PN532.
If the card is fully encrypted, only PM3 can crack the keys.

Compare to find the sector with money data

With keyA or keyB of all sectors of the card, data need to be gathered. And compare them after every consumption, so you can find the sector that data change, which means the money data may in that sector.

Add card, sector, and keys to MTools

The 4th block of every sector contains
keyA(6 bytes) + Access Control(4) + keyB(6)

Record data from in sniffer

With sector numbers and valid keys added, it can be easy to compare data with different money amounts.
Mark the money bytes(contain money data) and check bytes(mostly the changing bytes)

Compare and analyze the rules of dynamic bytes

Pay attention to HEX or DEC, Reverse or not and Rate on Money bytes.
Compare the Summation or XOR values with up and down check bytes.

Finish the expression

MTools use mXparser to calculate expression in rules. Please follow the tips in MTools to add expression.

Charge Mifare 1k card with one click

Before charging a card, an accurate simulation is necessary, just by a long click on the $ button after tag card.

One more thing

MTools support work with an extra device like ACR122U, PN532, PN532 BLE, which means you can use the features on a phone without NFC hardware.