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The Application Protocol Data Unit (APDU) is the communication unit between a smartcard reader and a smartcard. The structure of an APDU is defined by the ISO/IEC 7816 standards.

APDU message Command-response pair

Field Length Description Code
Class byte 1 Class of instruction CLA
Instruction byte 1 Instruction code INS
Parameter bytes 2 Instruction parameters P1-P2
Lc field 0, 1 or 3 The Lc field fixes number Lc -
Command data field Lc String of Lc bytes -
Le field 0, 1, 2 or 3 The Le field fixes number Le -
Response data field Lr String of Lr bytes -
Status bytes 2 Command processing status SW1-SW2

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 5-byte header and from 0 to up 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 up to 256 bytes of data.

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Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary




  1. (computing) application protocol data unit


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