#APDU Interpreter

#Principles

In the PC/SC architecture, the SCardTransmit function implements the dialog between an application and a smart card, the coupler being only a “passive” gateway.

The coupler transmits frames in both directions, without any specific processing.

The dialog MUST follow the ISO 7816-4 standard for APDU format, and this constraint is enforced by Windows’ PC/SC subsystem:

• [CLA] [INS] [P1] [P2] <Lc DataIn> <[Le]> in application-to-card (uplink) direction
• <DataOut> [SW1] [SW2] in card-to-application (downlink) direction.

In order to be able to work with smart cards that do not respect this constraint (for instance, the 1st generation of Desfire cards which uses a custom APDU format) and with cards that are not smart cards at all (e.g. wired-logic cards), the SpringCore couplers feature an APDU Interpreter that translates ISO 7816-4 APDUs into commands that are significant for the very card under operation.

#CLA byte of the APDU Interpreter

The APDU Interpreter processes all commands going through SCardTransmit with the CLA field set to FF.