Smart card application protocol data unit

"APDU" redirects here. For other uses, see Apdu.

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.

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) 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

See also

• Protocol data unit

External links

• Smartcard ISOs, contents
• Selected list of smartcard APDU commands
• Selected list of SW1 SW2 Status bytes
• More information about APDU commands and APDU responses