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I have a Nokia 301 connected to APDU snoop device. I have 2 SIM cards one older 2G (SIM) card and a newer 3G (USIM).

My understanding is that the CLA (of the APDU) would be A0 in the SIM case and 00/80 in the USIM case. I would like to know how the device knows which CLA to use.

The traces I have taken start off with ATR then: the SIM immediately issues a A0 A4 00 00 (SELECT) of the 7F 20 (DF_GSM)
whereas the USIM immediately issues a 00 A4 00 0C (SELECT) of the 3F 00 (MF)

This leads me to believe the ATR in some way enables the device to distinguish SIM/USIM? My understanding was that the ATR holds more interface information like voltages, frequencies and protocols etc.

My question is, how does the phone know from the word go which CLA to use without querying EF_DIR or trying a CLA then failing?

QuickPrototype
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    Is it dual-SIM phone? First slot: supports SIM/USIM. Second slot: SIM only. The correct one is: "phone trying a CLA then failing". If using CLA=0x00 failed, then use CLA=0xA0. EF DIR is read only after a successful SELECT with CLA=0x00. – David Aug 07 '15 at 02:45
  • No, single SIM phone. I would have assumed that a CLA would be tried, but as I mentioned in the post this phone just seems to know without failing on CLA. – QuickPrototype Aug 11 '15 at 13:41

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