HCS473-SLROM datasheet(6/68 Pages) MICROCHIP

6 / 68 page

HCS473

DS40035C-page 4

Preliminary

 2002 Microchip Technology Inc.

• Transponder Reader (Reader, for short) - A

device that authenticates a transponder using bi-

directional communication.

• Transport code - An access code, ‘password’

known only by the manufacturer, allowing write

access to certain secure device memory areas

(Section 3.2.3.2).

1.2

Encoder Overview

The HCS473 code hopping transcoder is designed

specifically for passive entry systems; particularly vehi-

cle access. The transcoder portion of a passive entry

system is integrated into a fob, carried by the user and

operated to gain access to a vehicle or restricted area.

The HCS473 is meant to be a cost-effective yet secure

solution to such systems, requiring very few external

components (Figure 2-1).

1.2.1

LOW-END SYSTEM SECURITY

RISKS

Most low-end keyless entry transmitters are given a

fixed identification code that is transmitted every time a

button is pushed. The number of unique identification

codes in a low-end system is usually a relatively small

number. These shortcomings provide an opportunity

for a sophisticated thief to create a device that ‘grabs’

a transmission and retransmits it later, or a device that

quickly ‘scans’ all possible identification codes until the

correct one is found.

1.2.2

HCS473 SECURITY

The HCS473, on the other hand, employs the K

EELOQ

code hopping technology coupled with a transmission

length of 69 bits to virtually eliminate the use of code

‘grabbing’ or code ‘scanning’. The high security level of

the HCS473 is based on the patented K

EELOQ technol-

ogy. A block cipher based on a block length of 32 bits

and a key length of 64 bits is used. The algorithm

obscures the information in such a way that even if the

transmission’s pre-encrypted information differs by

only one bit from that of the previous transmission, sta-

tistically greater than 50 percent of the transmission’s

encrypted result will change.

1.2.3

HCS473 HOPPING CODE

The 16-bit synchronization counter is the basis behind

the transmitted code word changing for each transmis-

sion; it increments each time a button is pressed.

Once the device detects a button press, it reads the

button inputs and updates the synchronization counter.

The synchronization counter and crypto key are input

to the encryption algorithm and the output is 32 bits of

encrypted information. This encrypted data will change

with every button press, its value appearing externally

to ‘randomly hop around’, 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 serial

number to form the code word transmitted to the

receiver. The code word format is explained in greater

detail in Section 3.1.2.

1.3

Identify Friend or Foe (IFF)

Overview

Validation of a transponder first involves an authenti-

cating device sending a random challenge to the

device. The transponder then replies with a calculated

response that is a function of the received challenge

and its stored crypto key. The authenticating device,

transponder reader, performs the same calculation and

compares it to the transponder’s response. If they

match, the transponder is identified as valid and the

transponder reader can take appropriate action.

The HCS473’s IFF response is generated using one of

two possible crypto keys. The authenticating device

precedes the challenge with a three bit field dictating

which key to use in calculating the response.

The bi-directional communication path required for IFF

is typically inductive for short range (<10cm) transpon-

der applications with an inductive challenge and induc-

tive response. Longer range (~1.5m) passive entry

applications still transmit using the LF inductive path

but the response is transmitted RF.

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