UICCSupport

Introduction

The SIM card is connected to the modem or baseband processor, but not to the application processor. The Android system runs on the application processor, thus has limited or no access to the SIM card. Communication with the SIM card is provided by the baseband processor over AT commands (e.g. AT+CPIN, AT+CRSM) defined by 3GPP 27.007 specification or through an proprietary IPC interface.
As the (more and more common) IPC interface is not standardized we define the requested extensions in the AT command language that needs to be mapped to proprietary function calls if AT commands are no longer supported.

Details

In order to access card applications on the SIM card like any other Secure Element using the SmartcardAPI the baseband processor must provide some additional AT commands to enable transparent APDU exchange. For security reasons the APDU exchange must be limited to logical channels between the Android world and the card applications. The basic channel of the UICC must be reserved to the baseband processor.

The 3GPP TS 27.007 Technical Specification describes all required AT commands.

• AT+CSIM (Generic SIM access)

+CSIM=, +CSIM: ,```

The AT+CSIM command transmits the command APDU to the SIM card, and returns the response APDU to the caller. This command allows exchange of any APDU, and should be handled with great care. The baseband must limit the range of command APDUs to the essential needs. The blocks or filters: * SELECT by NAME commands * CHANNEL MANAGEMENT commands * CLA bytes that contain logical channel numbers (CLA = CLA & 0xFC) The channel management and applet selection functionality should be implemented by the following AT commands.

• AT+CCHO (Open Logical Channel)

+CCHO= ```

The AT+CCHO commands opens a logical channel to the card application dfname, and returns a sessionid. The sessionid is to be used when sending commands with the AT+CGLA command.
dfname matches the applet AID that needs to be selected or can be null if no applet needs to be selected on a new logical channel
sessionid is from 1 to 4294967295 in case of success. The value 0 is reserved.

In case of an error the baseband returns an error code.

+CME ERROR: ```

The upper layers need to distinguish between certain error cases. For this reason the baseband shall return one of the following values for err.
memory full in case no logical channel was available
not found in case the card application was not found
unknown in case of any other error

• AT+CCHC (Close Logical Channel)

+CCHC=```

This command asks the baseband processor to close a communication session with the UICC.

• AT+CGLA (Generic UICC Logical Channel Access)

+CGLA=,, +CGLA: ,```

The AT+CGLA command transmits the command APDU to the selected card application, and returns the response APDU to the caller.

The uicc.patch adds support for SIM card access to the telephony framework. The additional telephony framework methods shall be available for the SmartCard API exclusively thus no malicious application can use the telephony extensions directly.

Once the baseband processor supports the required AT commands, and the UICC patch has been applied, the SmartCard API supports access to the SIM card with mentioned limitations. The RIL implementation on current Android phones do not support the required AT commands. For this reason the UICC patch includes an emulator extension (emulator.patch), which adds support for the required AT commands to the emulator's RIL implementation (reference-ril).
Refer to RIL_Extensions.tgz for more information.

Error handling

Multiple error conditions can arise where the system needs to be able to handle them. For this case the SmartCard API defines in addition to the proposed changes a getLastError() function to determine the error condition.

getLastError	definition
0	no error
1	general error (communication error, no further diagnosis)
2	no resources (no logical channel available on SE)
3	not found (applet selection failed)
4	close failed (MANAGE CHANNEL CLOSE error)
5	sessionid mismatch (invalid session id provided)

Mapping to the corresponding AT commands:

getLastError	CSIM	CCHO	CGLA	CCHC	interpretation
0	+		+	+	no error occurred while processing the command
1	+		+	+	an undefined error occurred, e.g. communication error
2		+			card does not support logical channels or no logical channel available
3		+			applet selection failed with 6A 82 (file/application not found)
4				+	an error occurred while closing the logical channel
5				+	session ID mismatch or tampered

Transmit functions like +CSIM or +CGLA always return SW1SW2 if available or empty data when no SW1SW2 was received and indicate 1 in getLastError