HCS361 datasheet(2/24 Pages) MICROCHIP

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HCS361

DS40146C-page 2

Preliminary

The encryption key, serial number, and conﬁguration

data are stored in EEPROM which is not accessible via

any external connection. This makes the HCS361 a

very secure unit. The HCS361 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 HCS361 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 several

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 HCS361 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

HCS361 requires very few external components

(Figure 2-1).

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 sys-

tem

also

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 HCS361 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

HCS361 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.

Any type of controller may be used as a receiver, but it

is typically a microcontroller with compatible ﬁrmware

that allows the receiver to operate in conjunction with a

transmitter,

based

the

HCS361. Section 7.0

provides more detail on integrating the HCS361 into a

total system.

*Secure Learning patents pending.

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