HCS360_01 Datasheet, PDF (2/28 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder

HCS360
The length of the transmission eliminates the threat of
code scanning and the code hopping mechanism
makes each transmission unique, thus rendering code
capture and resend (code grabbing) schemes useless.
The encryption key, serial number, and configuration
data are stored in EEPROM which is not accessible via
any external connection. This makes the HCS360 a
very secure unit. The HCS360 provides an easy to use
serial interface for programming the necessary security
keys, system parameters, and configuration data.
The encryption keys and code combinations are pro-
grammable but read-protected. The keys can only be
verified after an automatic erase and programming
operation. This protects against attempts to gain
access to keys and manipulate synchronization values.
The HCS360 operates over a wide voltage range of
2.0V to 6.6V and has four button inputs in an 8-pin
configuration. This allows the system designer the
freedom to utilize up to 15 functions. The only
components required for device operation are the but-
tons and RF circuitry, allowing a very low system cost.
1.0 SYSTEM OVERVIEW
1.1 Key Terms
• Manufacturer’s code – a 64-bit word, unique to
each manufacturer, used to produce a unique
encryption key in each transmitter (encoder).
• Encryption Key – a unique 64-bit key generated
and programmed into the encoder during the
manufacturing process. The encryption key
controls the encryption algorithm and is stored in
EEPROM on the encoder device.
• Learn – The HCS product family facilitates sev-
eral learning strategies to be implemented on the
decoder. The following are examples of what can
be done.
Normal Learning
The receiver uses the same information that is
transmitted during normal operation to derive the
transmitter’s secret key, decrypt the discrimination
value and the synchronization counter.
Secure Learn*
The transmitter is activated through a special but-
ton combination to transmit a stored 48-bit value
(random seed) that can be used for key genera-
tion or be part of the key. Transmission of the ran-
dom seed can be disabled after learning is
completed.
The HCS360 is a code hopping encoder device that is
designed specifically for keyless entry systems,
primarily for vehicles and home garage door openers.
It is meant to be a cost-effective, yet secure solution to
such systems. The encoder portion of a keyless entry
system is meant to be held by the user and operated to
gain access to a vehicle or restricted area. The
HCS360 requires very few external components
(Figure 2-1).
DS40152D-page 2
Most keyless entry systems transmit the same code
from a transmitter every time a button is pushed. The
relative number of code combinations for a low end
system is also a relatively small number. These
shortcomings provide the means for a sophisticated
thief to create a device that ‘grabs’ a transmission and
retransmits it later or a device that scans all possible
combinations until the correct one is found.
The HCS360 employs the KEELOQ code hopping tech-
nology and an encryption algorithm to achieve a high
level of security. Code hopping is a method by which
the code transmitted from the transmitter to the
receiver is different every time a button is pushed. This
method, coupled with a transmission length of 67 bits,
virtually eliminates the use of code ‘grabbing’ or code
‘scanning’.
As indicated in the block diagram on page one, the
HCS360 has a small EEPROM array which must be
loaded with several parameters before use. The most
important of these values are:
• A 28/32-bit serial number which is meant to be
unique for every encoder
• An encryption key that is generated at the time of
production
• A 16-bit synchronization value
The serial number for each transmitter is programmed
by the manufacturer at the time of production. The
generation of the encryption key is done using a key
generation algorithm (Figure 1-1). Typically, inputs to
the key generation algorithm are the serial number of
the transmitter or seed value, and a 64-bit manufac-
turer’s code. The manufacturer’s code is chosen by the
system manufacturer and must be carefully controlled.
The manufacturer’s code is a pivotal part of the overall
system security.
The 16-bit synchronization value is the basis for the
transmitted code changing for each transmission, and
is updated each time a button is pressed. Because of
the complexity of the code hopping encryption algo-
rithm, a change in one bit of the synchronization value
will result in a large change in the actual transmitted
code. There is a relationship (Figure 1-2) between the
key values in EEPROM and how they are used in the
encoder. Once the encoder detects that a button has
been pressed, the encoder reads the button and
updates the synchronization counter. The synchroniza-
tion value is then combined with the encryption key in
the encryption algorithm and the output is 32 bits of
encrypted information. This data will change with every
button press, hence, it is referred to as the hopping
portion of the code word. The 32-bit hopping code is
combined with the button information and the serial
number to form the code word transmitted to the
receiver. The code word format is explained in detail
in Section 4.2.
 2001 Microchip Technology Inc.