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AN2646 - Application note

ISO 7816-4 – smartcard commands

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5.2.1

The ISO 7816-4 APDU

There are two types of messages used to support the ISO 7816-4 application protocols: the

command APDU (sent from the reader to the card) and the response APDU (sent from the

card to the reader).

Figure 7.

Command APDU structure

The command APDU consists of a header and a body (this can be seen in the diagram

above). The header includes CLA, INS, P1 and P2 fields. As in the T0 protocol, CLA and

INS specify an application class and instruction. P1 and P2 are used to qualify specific

instructions and are given specific definitions by each [CLA, INS] instruction. The body of

the APDU can vary in size and is used to transmit data to the card's APDU processor as

part of a command or to convey a response from the card to the reader. The Lc field

specifies the number of bytes to be transmitted to the card as part of the instruction, that is,

the length of the data field. The data field contains information that must be sent to the card

to allow its APDU processor to execute the command specified in the APDU. The Le field

specifies the number of bytes that will be returned to the reader in the response APDU.

The body of the APDU can take on four different forms:

●

Case 1: No data is transferred to or from the card, so the APDU only contains the

header.

●

Case 2: No data is transferred to the card, but data is returned from the card. The body

of the APDU only contains a non-null Le field.

●

Case 3: Data is transferred to the card, but none is returned from it. The body of the

APDU includes the Lc and data fields.

●

Case 4: Data is transferred to the card and is also returned from the card as a result of

the command. The body of the APDU includes the Lc, data and Le fields.

Figure 8.

Response APDU structure

The response APDU has a much simpler structure than that of the command APDU. It

consists of a body and a trailer. The body is either null or it includes a data field - depending

on the specific command. The length of the data field is determined by the Le field in the

corresponding command APDU. The trailer consists of up to two fields of status information

called SW1 and SW2. These fields return a status code in which one byte is used to specify

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