APDU

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.

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

Abbreviations and Notation

For the purposes of this part of the ISO/IEC 7816, the following abbreviations apply :

APDU	Application protocol data unit
ATR	Answer to reset
BER	Basic encoding rules of ASN.1 (see annex D)
CLA	Class byte
DIR	Directory
DF	Dedicated file
EF	Elementary file
FCI	File control information
FCP	File control parameter
FMD	File management data
INS	Instruction byte
MF	Instruction byte
P1-P2	Parameter bytes
PTS	Protocol type selection
RFU	Reserved for future use
SM	Secure messaging
SW1-SW2	Status bytes
TLV	Tag length value
TPDU	Transmission protocol data unit

For the purposes of this part of ISO/IEC 7816, the following notation applies :

< TR>

'0'-'9' and 'A'-'F'	The sixteen hexadecimal digits
(B1)	Value of byte B1
B1||B2	Concatenation of bytes B1 (the most significant byte) and B2 (the least significant byte)
#	Number