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SMART CARDS - How Do They Work?
=================================
SMART cards are already in use in the VideoCrypt and D2-MAC
Eurocrypt scrambling systems. They are also widely used for pay
telephone applications. At this stage the SMART card is throwaway
technology. Despite being portrayed as being the ultimate in security,
there are ways to hack the SMART card based systems though the
cards are relatively secure.
A SMART card has a microprocessor on the card. It may also have
additional memory. This makes it somewhat different from memory
only cards as used for prepaid services like telephone calls or rail
travel.
There are basically two classes of SMART cards; contactless and
contact. The contact type SMART card requires direct electrical
connection to the decoder. This is the type of SMART card used in
the VideoCrypt system. It is the cheapest format.
The non-contact type SMART card has not been used in decoder
applications yet. It uses oscillators running at different frequencies to
supply power to the card. A filter and rectifier arrangement circuit on
the card picks up the signal generated by the descrambler and
produces a DC voltage. Data can be transferred in a similar fashion.
This type of card may be used in the future but the cost is prohibitive.
The connector specifications for the contact SMART card have
been established as an ISO standard. The ISO standard specifies
eight connections of which only six are actively used. Many of the
SMART cards used for payphones and banking applications follow
the pattern that appears in the diagram. The typical payphone card is
an EPROM memory card rather than a smart card. Videocrypt's card
does not look like it follows the pattern but it does have the same six
connections on the connector array.
Due to the difficulties involved in reverse engineering a card, it is
very difficult to extract the data from the card without destroying the
card. The fact that the structure of the card is known does not imply
that the actual program in the card is known.
It should be stated that extracting the program from a smart card is
not impossible. It will happen and according to one company, the first
pirate. clones will appear with in the next few months.
SMART CARD STRUCTURE
====================
The structure of the card is basically simple. It consists of a
microprocessor and memory. This description fits the microcontrollers
used to control receivers and video recorders. The type of memory
used can vary. It generally involves Read Only Memory, Erasable
Programmable Read Only Memory, Electrically Erasable programm-
able Read Only Memory and Random Access Memory.
The information stored in the ROM is fixed and cannot be altered
without changing the design of the SMART card. The information in
the EPROM generally has to be erased with ultra violet light. This
would imply that once the card has been programmed, the
information cannot be erased in the card. The EEPROM is more
usable in SMART cards for one specific reason - it can be
reprogrammed in the card. The manner in which the VideoCrypt
cards can be turned on and off seems to indicated that the
VideoCrypt SMART card uses EEPROM memory rather than
EPROM.
The microprocessor used on the France Telecom Visiopasse card
is based on the 6805 microcontroller. This microcontroller is an eight
bit type and has on chip ROM, EEPROM and RAM. The VideoCrypt
card may be similar in structure.
The specification for the Visiopasse PC2 card
CPU : 6805
RAM : 128 Bytes
ROM : 6144 Bytes
EPROM: 8192 Bytes
The specification for the Thomson STl834.
CPU : 8 Bit
RAM : 76 Bytes
EPROM: 4095 Bytes
Vcc: +5 Volts Vpp: +21 Volts
______________________________________
/ \
| |
| |
| C1 C5 |
| C2 C6 |
| C3 C7 |
| C4 C8 |
| |
| |
| |
\________________________________________/
These contacts have the following purpose:
C1 VCC Supply voltage (+5 V, max. 200 mA)
C2 RST Reset signal
C3 CLK Clock signal
C4 - reserved
C5 GND Ground
C6 VPP Programming voltage
C7 I/O Data input/output
C8 - reserved
The actual card type used by Sky is believed to be a Motorola type
though this has not been established. It would seem logical as the
main crypto processor is a Motorola microcontroller.
A Smart Card Memory Map
=======================
-------------------- MASKED ROM
| |
| | This section contains the Card,s
| | housekeeper programs and the
| | Primary Decryption algroithm. This
| | algorithm is used on all services.
| |
| |
| |
|------------------| EEPROM
| |
| | This section contains the service
| | data, the authorisation, the card
| | identity, the subsciber identity, the
| | billing data and period data
| |
| |
|------------------| RAM
| |
| | This section is used by the
-------------------- decryption algorithm and other
programs as a tempory storage
area.
The Smart Card's Chip Structure
===============================
---- EEPROM R/W Voltage
|
|
----------- --------------
| | | |----- Reset
| |....| |
| EEPROM | | Micro- |----------- Data Port
| |....| Processor |
| | | |----- Clock
----------- | |
| |----- 5v Supply
--------------
. . . .
. . . .
------------ -----------
| | | |
| | | |
| | | Masked |
| RAM | | ROM |
| | | |
| | | |
| | | |
------------ -----------
The Smart Card's Microcontroller is fabricated on
one chip. This chip contains EEPROM. If any attempt
is made to scan the chip with an electron microscope,
the EEPROM will be wiped. Since the design is all on a
single chip the data flow between the memory areas
cannot be directly examined.
From this information, a memory map of a VideoCrypt SMART card
can be guessed. The ROM area of the memory is the area where the
main operational program and decryption algorithms are stored. The
EEPROM area contains the service data, the pay per view data, the
card identity data and the billing period data. The RAM is used for
temporary storage of data.
The ROM area of the card is not accessible. It cannot be read out
by pumping the card. For this reason it holds the most critical
information. The programs held in ROM are used for all services or
channels that use the card. The algorithms will be identical but the
keys will be totally different. This section will also hold the routine for
decryption the data in the EEPROM area of the memory.
The EEPROM contains the enabling data for each channel that the
card user has paid for. The data entry for each channel would consist
of a channel identifier, a billing period, a regional identifier, key data
and authorisation data for the channel.
The design of a SMART card is complex and prototyping can take
a few months. The ROM in the card has to be mask programmed.
This essentially means that the programs to be stored in the ROM are
designed as part of the chip. The procedure is straightforward.
The programs to be included in the ROM are developed on a
SMART card emulator. This is a microprocessor development system
that is configured to imitate a SMART card. It is hooked to a personal
computer. The program developer will write the programs on the
computer, test them, and if they run successfully, load them into the
SMART card emulator. The SMART card emulator will then be
plugged into a decoder to ensure that the programs work.
The programs will then be supplied on floppy disc to the chip
manufacturer. The manufacturer will program an EPROM with the
programs and send it to the card issuer for verification. Once the
EPROM is checked, the manufacturer will then produce the chips in
sample quantity. These chips will also be tested for correct operation.
The chips can then be mass produced.
The chips are glued to a printed circuit board substrate with epoxy
resin. The connection pins on the chip are wired to the connections
on the substrate. The substrate connections are then wired to the
connector array. The actual plastic card is injection moulded with an
indent for the chip. The chip is then glued into the indent. The card is
then tested to ensure that it is operational.
The SMART card at this stage will only have the bare minimum of
data. There will be no service data in the EEPROM. This data is
programmed into the card by the card issuer. In VideoCrypt's case,
the programming and card assembly would be carried out at the
Gemplus factory in Scotland.
SMART CARD OPERATION
=====================
The SMART card is essentially a partial computer on a card. It is a
partial computer because it requires other circuitry and inputs to
operate. The first requirement is supply voltage. This is generally a 5
Volt DC supply.
The second requirement is a clock signal. This is a stable
frequency square wave of 5 volts amplitude. This frequency is derived
from a crystal in the descrambler. The frequency used in the
VideoCrypt card is 3.5 MHz.
The third requirement is a reset line. This is used to initiate the l@
programs and routines in the card when the card is inserted into the
socket.
The fourth requirement is the EEPROM voltage. The EEPROM
programming voltage is high, typically over twenty volts. This voltage
is only on for a few milliseconds every three seconds. The chip would
generate too much heat if it was continually fed with high voltage.
The fifth requirement is the data port. The data flows to and from
the card on one line. It is serial data. This port would be connected to
the RAM in the card. The serial data would be clocked into the RAM.
The microprocessor on the chip would then read the data in a parallel
format. The data flows at 9600 Baud or at 9600 bits per second. The
serial data line makes the card more secure. I
When the card is inserted into the descrambler, the reset pin is
activated. This zeroes the RAM and causes the microprocessor to
select the boot-up program. This program will verify that the card is
valid for the period and not on the blacklist. The card will then read
the data from the descrambler. This data, along with service data from
the EEPROM, will be used in the decryption algorithm stored in the
ROM. The product of the decryption algorithm will then be passed
back to the descrambler.
In the VideoCrypt system, the information flowing to and from the
card is not useful on its own. The data is not the actual key used to
descramble the picture. This data is passed via the 8052 Housekee-
per microcontroller to a secure microprocessor, the ZC404044 or
ZC404047, where it is then used in a further algorithm to generate
the seed for cutpoint generator. The secure microprocessor is actually
a Mask ROM version of the 6805 microcontroller.
Pay Per View is extremely easy to implement with a SMART card.
The card user will purchase a number of credits or tokens each billing
period. A typical number would be 99 tokens. The SMART card would
be programmed so that the token counter would read 99 tokens.
When ever the user wanted to watch a PPV film or event, a message
would be shown on screen stating the number of tokens that the
event is valued at. To watch the programme, the user would press the
authorise or pay button on the front of the descrambler. The
descrambler would then decrease the token register by the correct
amount.
Each service could have a token register. The actual operation of
the counting mechanism would be more complex. It would be too
easy to intercept the taken count value and substitute a continual 99
tokens. This type of hack is commonly used in computer games and
is known as an "Infinite Lives POKE". At present the actual PPV
algorithm for VideoCrypt lies mainly in the 8052. There is the
possibility that the Infinite Lives Poke would work The issue of the O9
smart cards may have taken this into consideration.
SMART CARD SECURITY AND ADDRESSING
===================================
The VIdeoCrypt cards are valid once they leave the subscription
centre. This means that they can be used in any VideoCrypt
descrambler. They can be deactivated over the air by Sky.
When this occurs, a section of the EEPROM in the card is
overwritten so that when the card is inserted into the descrambler, it
will not work. In order to reactivate the card, the program providers
Sky send out a message to the descrambler to reprogram the
overwritten section of EEPROM.
This weakness was actually used to hack the VideoCrypt system
though it was quickly discovered and countered. The details of the
hack are given later.
Other systems using SMART card use over the air enabling. In this
case, the cards cannot be used immediately. They have to be inserted
into the descrambler and the subscription centre has to be informed.
The subscription centre will then activate the card. This method of
addressing is more time consuming and thus less economical. it is
basically a trade off between medium security and very high security.
Reverse engineering a SMART card is not an economically viable
hack. The chip on the SMART card is covered in epoxy resin. Trying
to remove this resin can sometimes destroy the chip. If the memory
and the microprocessor are on different chips, it would be barely
possible to attack the connecting wires and monitor the data flow. if
the memory and the microprocessor are on the same chip it is
impossible. Using an electron microscope to read the memory may
partially work on EPROM type SMART cards. Smart cards using
EEPROM memory are reasonably secure against this hack. The
scanning electron beam erases the EEPROM.
It is possible that a method for reprogramming the EEPROM on a
SMART card could be developed. Using such a method, the
EEPROM contents of a valid card could be copied and loaded on to
an old SMART card. There is virtually no problem in obtaining old Sky
SMART cards. This hack has one fatal flaw. The ROM data on
SMART cards is changed from billing period to billing period.
Therefore the algorithms and the EEPROM data decrypting algorithm
are different. One potential weakness on a multi service card would
be a cloning of a fully authorised card using a SMART card with the
minimum authorisation. This hack relies on the development of an
EEPROM reading and writing method.
It would appear that the SMART card is the most difficult aspect of
the system to hack. It is certainly not economically viable to hack it.
The use of a SMART card does not confer immunity to hacking upon
a system. There are usually fatal flaws in the descrambler that the
hacker can exploit. The VideoCrypt system has become a rather
lucrative target after the Sky / BSB merger. There are now three
movie channels, two foreign language channels, The Adult Channel
and one sports channel using the system. Other users will adopt the
system with in the next year or so.