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) 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. "write data"
P1-P2	2	Instruction parameters for the command, e.g. offset into file at which to write the data
L_c	0, 1 or 3	Encodes the number (N_c) of bytes of command data to follow 0 bytes denotes N_c=0 1 byte with a value from 1 to 255 denotes N_c with the same value 3 bytes, the first of which must be 0, denotes N_c in the range 1 to 65 535 (all three bytes may not be zero)
Command data	N_c	N_c bytes of data
L_e	0, 1, 2 or 3	Encodes the maximum number (N_e) of response bytes expected 0 bytes denotes N_e=0 1 byte in the range 1 to 255 denotes that value of N_e, or 0 denotes N_e=256 2 bytes (if extended L_c was present in the command) in the range 1 to 65 535 denotes N_e of that value, or two zero bytes denotes 65 536 3 bytes (if L_c was not present in the command), the first of which must be 0, denote N_e in the same way as two-byte L_e
Response APDU
Response data	N_r (at most N_e)	Response data
SW1-SW2 (Response trailer)	2	Command processing status, e.g. 90 00 (hexadecimal) indicates success

References

^ "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.

External links

[1] "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.

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