Smart card application protocol data unit

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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[edit]

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 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. "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 APDU
Response data Nr (at most Ne) Response data
SW1-SW2
(Response trailer)
2 Command processing status, e.g. 90 00 (hexadecimal) indicates success

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