Smart card application protocol data unit

In the context of smart cards, an application protocol data unit (APDU) is the communication unit between a smart card reader and a smart card. The structure of the APDU is defined by ISO/IEC 7816-4 Organization, security and commands for interchange.^[1]

APDU message command-response pair

There are two categories of APDUs: command APDUs and response APDUs. A command APDU is sent by the reader to the card – it contains a mandatory 4-byte header (CLA, INS, P1, P2)^[2] and from 0 to 65 535 bytes of data. A response APDU is sent by the card to the reader – it contains from 0 to 65 536 bytes of data, and 2 mandatory status bytes (SW1, SW2).

Command APDU
Field name	Length (bytes)	Description
CLA	1	Instruction class - indicates the type of command, e.g., interindustry or proprietary
INS	1	Instruction code - indicates the specific command, e.g., "select", "write data"
P1-P2	2	Instruction parameters for the command, e.g., offset into file at which to write the data
Lc	0, 1 or 3	Encodes the number (Nc) of bytes of command data to follow 0 bytes denotes Nc=0 1 byte with a value from 1 to 255 denotes Nc with the same length 3 bytes, the first of which must be 0, denotes Nc in the range 1 to 65 535 (all three bytes may not be zero)
Command data	Nc	Nc bytes of data
Le	0, 1, 2 or 3	Encodes the maximum number (Ne) of response bytes expected 0 bytes denotes Ne=0 1 byte in the range 1 to 255 denotes that value of Ne, or 0 denotes Ne=256 2 bytes (if extended Lc was present in the command) in the range 1 to 65 535 denotes Ne of that value, or two zero bytes denotes 65 536 3 bytes (if Lc was not present in the command), the first of which must be 0, denote Ne in the same way as two-byte Le
Response APDU
Response data	Nr (at most Ne)	Response data
SW1-SW2 (Response trailer)	2	Command processing status, e.g., 90 00 (hexadecimal) indicates success[2]