MFRC522 RFID reader authentication keys

I purchased an MFRC522 RFID reader and have it working pretty well, but I have a question about the authentication keys.

I'm using the MIFARE Classic 1K card (I'm pretty sure). It claims to have two authentication keys in the 4th block of each sector. From the MFRC522.h file...

  • MIFARE Classic 1K (MF1S503x):
  • Has 16 sectors * 4 blocks/sector * 16 bytes/block = 1024 bytes.
  • The blocks are numbered 0-63.
  • Block 3 in each sector is the Sector Trailer. See
  • sections 8.6 and 8.7:
  • Bytes 0-5: Key A
  • Bytes 6-8: Access Bits
  • Bytes 9: User data
  • Bytes 10-15: Key B (or user data)

This would suggest that I have a key in both the first 6 bytes and also in the last 6 bytes. But when I print out the contents of the block it looks like all 0's in the first 6 bytes while the last 6 bytes appears to have the expected factory key FFFFFF.

It appears to work just fine if I use Key A or Key B. Can someone tell me what the difference is between the two and why it looks like I have 0's where I expect to see Key A?

Thanks very much for any tips!