ATQA in Mifare and RFU configurations

Question

I received some cards that are supposed to be Mifare Classic cards. When I perform the level 1 of anticollision (REQA) the ATQA is 04 00. According to ISO/IEC 14443-3, the first byte of the ATQA is RFU and the second part defines the UID size and anticollision bits. The value 00000 for the anticollision is defined as RFU.

But according to NXP MIFARE Type Identification Procedure (AN10833) byte 1 of the ATQA can be 0x04. Anyways I can not find MIFARE Classic hex value for the ATQA compatible in the previously mentioned document. Also the document MF1S50YYX_V1 states that the hex value of the ATQA of a Mifare Classic card should be 00 xx.

I am pretty sure that I am reading the ATQA in the correct endianess (I am able to perform a full anticollision procedure) so I can't figure what's going on with the ATQA. Any hint would be very appreciated.

Answer 1

I'm not sure I understand your problem. 04 00 looks like a valid ATQA for MIFARE Classic under the assumption that the octets are ordered in transmission byte order (lower byte first).

The coding of the ATQA according to ISO/IEC 14443-3 is:

+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
|  16 |  15 |  14 |  13 |  12 |  11 |  10 |   9 |   8 |   7 |   6 |   5 |   4 |   3 |   2 |   1 |
|          RFU          |     PROPR. CODING     |  UID SIZE | RFU |   BIT FRAME ANTICOLLISION   |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----

Since bits 8..1 are the LSB (first transmitted octet) and bits 16..9 are the MSB (second transmitted octet), your ATQA would map to:

+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
|  16 |  15 |  14 |  13 |  12 |  11 |  10 |   9 |   8 |   7 |   6 |   5 |   4 |   3 |   2 |   1 |
|          RFU          |     PROPR. CODING     |  UID SIZE | RFU |   BIT FRAME ANTICOLLISION   |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
|                      0x00                     |                      0x04                     |
|  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  1  |  0  |  0  |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----

So your MIFARE Classic card could be either Classic 1K or Mini (or Plus) with a 4-byte (N)UID. Note that you should not rely on the ATQA to detect UID length and chip type though (this should be done through selection and evaluation of the SAK value).