How to handle S-block received by reader?

Question

I use MFRC522 as the reader, while Sony C4 is executing Google HCE sample. The reader is sending frame 1:

02 00 A4 04 00 07 F0 01 02 03 04 05 06 crc1 crc2

Then the reader receives frame 2:

f2 08 50 dd

I think it may be an S-block or WTX.

However the log of the HCE app shows:

received apdu 00 A4 04 00 07 F0 01 02 03 04 05 06

sending account number number 123456

Even if the reader sends frame 1 again and again, I always receive the same frame 2, why?

sorry, I had a typo.... apdu1 is 02 00 A4 04 00 07 F0 01 02 03 04 05 06 crc1 crc2 and the log shows 00 A4 04 00 07 F0 01 02 03 04 05 06. — jorgewang168, Dec 08 '15 at 11:06

score 1 · Accepted Answer · answered Dec 08 '15 at 12:57

1

f2 08 50 dd is an S-block indicating a waiting time extension (WTX):

PCB = f2:
- Bit 8..7 = '11': S-block
- Bit 6..5 = '11': S(WTX)
INF = 08:
- WTXM = 8
EDC = 50 dd

If the reader receives a S(WTX) request from the card, it needs to acknowledge this request by responding with a S(WTX) frame. Thus, you could, for instance, send the same S(WTX) frame back to the card.

As HCE processing is usually rather slow, it's not uncommon to exchange multiple S(WTX) frames before you actually receive any response data for the APDU.

answered Dec 08 '15 at 12:57

Michael Roland

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thanks, it helps a lot. – jorgewang168 Dec 09 '15 at 00:29
When the reader sends the same S(WTX) frame back to the card, the card only responses 02 90 00 crc1 crc2. How can I get the account number 123456? – jorgewang168 Dec 09 '15 at 02:53
@jorgewang168 `02 90 00 CRC` looks like a perfectly valid response to the select command that you sent. So I suggest that you accept this answer and then create a new question asking for that new problem (and including the relevant parts of your Android HCE code). – Michael Roland Dec 09 '15 at 07:15
Where can we get more input on this? – Langusten Gustel Jan 27 '16 at 10:33
@LangustenGustel E.g. in ISO standard that specifies that protocol (ISO/IEC 14443-4). – Michael Roland Jan 27 '16 at 15:32
I am getting nuts finding out how long to wait after the WTX is received. Any input on that? – Langusten Gustel Jan 27 '16 at 17:27
@LangustenGustel You mean how to calculate the time value (that the reader needs to wait before generating a timeout) from the acknowledged WTXM value? – Michael Roland Jan 27 '16 at 18:20

How to handle S-block received by reader?

1 Answers1