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Smart Cards Lab COMPGA12 University College London

7 APDUs

Smart Card commands are called APDUs (Application Protocol Data Unit).

They are strings of bytes, written in order of their transmission.

The card is only answering to commands (master/slave principle, half-

duplex) and never initiates any communication.

Quiz We call an C-APDU a:

APDU used in contactless smart

cards

string of bytes sent to the smart

card.

Quiz We call an R-APDU a:

string of bytes received from the

smart card

APDU that updates registers of

the reader.

7.1 Industrial standards

Quiz Cross-industry standardized APDU commands are described in

ISO 7816-1 ISO 7816-4.

Structure of an APDU

There are several possible cases, depending on the instruction.

Header - required Body - optional

CLA INS P1 P2 Lc Data Le

Explanations:

CLA is the byte that identiﬁes the type of command, the application,

and also indicates to what extent the command is complaint to ISO specs.

INS is a byte that encodes the instruction.

Quiz The optional ﬁeld Lc is for:

size of the input data length of the instruction.

Quiz The optional ﬁeld Le is for:

length of the instruction expected output size.

The size is always in bytes. Each ﬁeld Lc or Le can be encoded on 1 or

3 bytes, but most of the time it is one byte and a number between 0 and

127. Any larger number requires encoding on 3 bytes.

If the command is successful, the answer should be:

Nicolas T. Courtois 2009-10

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Classic Electronics Smart 4F Especificaciones Pagina 9