AN2646 datasheet(18/43 Pages) STMICROELECTRONICS

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ISO 7816-4 – smartcard commands

AN2646 - Application note

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ISO 7816-4 – smartcard commands

The previous chapter discussed the answer-to-reset (ATR) mechanism, which establishes a

basic communication channel between the smartcard and the reader. This channel is a half-

duplex physical channel. This chapter investigates the use of more complex protocols on top

of this physical channel.

A link-level communication protocol resides directly on top of the physical channel, providing

error-free communication between the reader and the smartcard. Once this link-level

protocol is established, application-level protocols can be defined. ISO 7816-4 defines two

such application-level protocols:

●

File system API providing a set of functions to manipulate files (for example read, write,

select, etc.).

●

Security service API allowing the smartcard and the reader to mutually authenticate

themselves and also to encrypt data to be exchanged between the card and the reader.

ISO 7816-4 defines a protocol message structure to support the application protocol APIs.

This message structure consists of application protocol data units (APDUs) which are

exchanged between the reader application and the smartcard application by the link-level

protocol. This chapter will provide an overview of the file access and security APIs.

5.1

T0 protocol

The T0 protocol is a byte-oriented protocol where a character is transmitted across the

channel between the reader and the card. In addition, error handling is performed on each

byte by looking at the parity bit. If the actual parity bit does not correspond to the parity of

the transmitted data, then an error must have occurred. In the T0 protocol, the receiving side

signals that it requires the byte to be retransmitted in the case of detecting a parity error.

This is done by holding the I/O line low (normally the I/O line is set high preceding the

transfer of a byte). When the transmitting side detects this, it resends the byte that was not

received correctly.

The reader and the smartcard exchange data structures known as transmission protocol

data units (TPDUs). It consists of two distinct structures:

●

a command that is sent from the reader to the card

●

a response that is sent from the card to the reader

The command header includes the following five fields each of one byte in length:

●

CLA: class designation of the command set to establish a collection of instructions

●

INS: specifies a specific instruction from within the set of instructions

●

P1: used to specify the addressing used by the [CLA, INS] instruction

●

P2: also used to specify the addressing used by the [CLA, INS] instruction

●

P3: specifies the number of data bytes transferred to or from the card as part of the

[CLA, INS] instruction execution.

Each value of CLA defines an application-specific set of instructions. Table 8. below lists

values for some sets of instructions.

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