AN2284 datasheet(22/40 Pages) STMICROELECTRONICS

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7 ISO 7816-3 - Electronic Signal and Transmission Protocol

AN2284

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7.2.1

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.

The 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 T=0 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 i.e. 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.

The 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

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