Issue
97, August 1998
Designing
for Smart Cards
- Part 2: Practical Implementation
SYSTEM
TYPES
There
are two types of smart-card systems—open and closed.
In an open system, the card may be used for transactions
between many operators. In closed systems, the cards
are used with one operator (usually the issuer).
Examples
of an open system include phone cards and electronic
purses. In those cases, the card establishes a unique
identity and eligible services for use with a phone
system or for making debits at various point-of-sale
terminals.
The
simplest case—a closed smart-card system—involves
one type of card and one type of reader (e.g., a card
used with one mass transit carrier). And, there is a
known identification procedure between them, which can
be implemented in the answer-to-reset (ATR) string.
The
ATR is the first message in a smart-card session with
a reader and is sent by the card immediately after powerup
(provided that a clock is supplied along with the power).
The ATR message contains data that defines relevant
transmission protocols and other information.
ISO
defines the ATR to have a maximum length of 33 bytes,
although this length is seldom reached. As you see in
Figure 2, the ATR consists of:
- an
initial character, TS
- a
format character, T0
- several
interface characters
- a
maximum of 15 historical characters
- a
check character, TCK
|
Figure 2—The
ATR string, which has a maximum of 33 bytes,
is broken down into five main sections.
|
TS
defines the conventions to code data bytes in all subsequent
transmissions. There are two conventions—direct
($3B) and inverse ($3F). All terminals can read both,
and most card manufacturers are capable of both.
The
format character consists of two parts. One part indicates
the subsequent interface characters, and the other indicates
the number of historical bytes to follow (1–15).
Information
transmitted via the interface characters includes the
transmission protocol (e.g., T=0 or T=1), the maximum
current the reader should supply to the card, and any
other transmission parameters pertinent to communicating
with the card.
The
historical characters may contain any information, such
as the manufacturer of the card, software version on
the chip, or traceability data. The number of historical
bytes is indicated in the least significant half of
the interface character.
The
last byte of the ATR string is the check character.
This byte is defined by ISO 7816-4 to be "a value
such that the exclusive ORing of all bytes from T0 to
TCK included is null."
