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.
A command APDU is sent by the reader to the card – it contains a mandatory 4-byte header (CLA, INS, P1, P2)[2] 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).
0 bytes denotes Nc=0 1 byte with a value from 1 to 255 denotes Nc with the same length 3 bytes, the first of which must be 0, denotes Nc in the range 1 to 65 535 (all three bytes may not be zero) 0 bytes denotes Ne=0 1 byte in the range 1 to 255 denotes that value of Ne, or 0 denotes Ne=256 2 bytes (if extended Lc was present in the command) in the range 1 to 65 535 denotes Ne of that value, or two zero bytes denotes 65 536 3 bytes (if Lc was not present in the command), the first of which must be 0, denote Ne in the same way as two-byte Le