View source for Smart card application protocol data unit

{{Short description|Communication unit between a reader and a smart card}}
In the context of [[smart card]]s, an '''application protocol data unit''' ('''APDU''') is the communication unit between a [[Card reader#Smart card readers|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''.<ref name="iso">{{Cite ISO standard
|csnumber=77180
|title=ISO/IEC 7816-4:2020 — Identification cards — Integrated circuit cards
}}</ref>
==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 &ndash; it contains a mandatory 4-[[byte]] header (CLA, INS, P1, P2)<ref name="celer">{{cite journal
|last=Celer
|first=Victor
|date=2021-12-25
|title=Using the SIMcard as a Security Module (HSM)
|url=https://www.celersms.com/android-SIM-HSM.htm
|journal=CelerSMS
|volume=1
|issue=2
|pages=13–17
|issn=2745-2336
|oclc=1295467772
}}</ref> and from 0 to 65 535 bytes of data. A response APDU is sent by the card to the reader &ndash; it contains from 0 to 65 536 bytes of data, and 2 mandatory status bytes (SW1, SW2).
{| class="wikitable"
! colspan="3" | 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
|-
| L<sub>c</sub>
| 0, 1 or 3
| Encodes the number (N<sub>c</sub>) of bytes of command data to follow
0 bytes denotes N<sub>c</sub>=0<br />
1 byte with a value from 1 to 255 denotes N<sub>c</sub> with the same length<br />
3 bytes, the first of which must be 0, denotes N<sub>c</sub> in the range 1 to 65 535 (all three bytes may not be zero)
|-
| Command data
| N<sub>c</sub>
| N<sub>c</sub> bytes of data
|-
| L<sub>e</sub>
| 0, 1, 2 or 3
| Encodes the maximum number (N<sub>e</sub>) of response bytes expected
0 bytes denotes N<sub>e</sub>=0<br />
1 byte in the range 1 to 255 denotes that value of N<sub>e</sub>, or 0 denotes N<sub>e</sub>=256<br />
2 bytes (if extended L<sub>c</sub> was present in the command) in the range 1 to 65 535 denotes N<sub>e</sub> of that value, or two zero bytes denotes 65 536<br />
3 bytes (if L<sub>c</sub> was not present in the command), the first of which must be 0, denote N<sub>e</sub> in the same way as two-byte L<sub>e</sub>
|-
! colspan="3" | Response APDU
|-
| Response data
| N<sub>r</sub> (at most N<sub>e</sub>)
| Response data
|-
| SW1-SW2<br />(Response trailer)
| 2
| Command processing status, e.g., 90 00 ([[hexadecimal]]) indicates success<ref name="celer"/>
|}
==References==
{{reflist}}
==See also==
* [[Protocol data unit]]
==External links==
* [https://web.archive.org/web/20090624235219/http://cheef.ru/docs/iso/ Smartcard ISOs, contents ]
* [https://web.archive.org/web/20090630004017/http://cheef.ru/docs/HowTo/APDU.info Selected list of smartcard APDU commands]
* [https://web.archive.org/web/20090623030155/http://cheef.ru/docs/HowTo/SW1SW2.info Selected list of SW1 SW2 Status bytes]
* [https://web.archive.org/web/20090330082954/http://www.wrankl.de/SCTables/SCTables.html More information about APDU commands and APDU responses]
[[Category:Smart cards]]
[[Category:ISO standards]]
[[Category:IEC standards]]