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.
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 255 bytes of data. A response APDU is sent by the card to the reader – it contains a mandatory 2-byte status word and from 0 to 256 bytes of data.
|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. "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|
|Command data||Nc||Nc bytes of data|
|Le||0, 1, 2 or 3||Encodes the maximum number (Ne) of response bytes expected|
|Response data||Nr (at most Ne)||Response data|
|2||Command processing status, e.g. 90 00 (hexadecimal) indicates success|
- "ISO/IEC 7816-4:2005 Identification cards — Integrated circuit cards — Part 4: Organization, security and commands for interchange". Iso.org. 2008-10-03. Retrieved 2012-01-27.