Smart card application protocol data unit


In the context of smart cards, an application protocol data unit 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.

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 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.