HCS473-SLROM datasheet(43/68 Pages) MICROCHIP

43 / 68 page

 2002 Microchip Technology Inc.

Preliminary

DS40035C-page 41

HCS473

5.5

Transponder Design

You must initially decide if a ferrite core or an air core

antenna will be used. There are advantages and disad-

vantages to using each. One advantage of using a fer-

rite core is that the coil can have a larger inductance for

a given volume. Volume will usually be the primary con-

straint as it will need to fit into a:

• key fob

• credit card

• other small package.

First step: choose the transponder coil external dimen-

sions because packaging places large constraints on

antenna design.

Second step: properties of the core, coil windings, as

well as the equivalent load placed across the coil must

be determined. Calculations from the first two steps will

fix the initial coil specification. The initial coil specifica-

tion includes:

• Minimum number of wire turns on the coil

• Wire diameter

• Wire resistance

• Coil inductance

• Required resonating capacitor.

Build the initial coil and take appropriate measure-

ments to determine the coil quality factor. The data

gathered to this point may then be used to calculate an

Optimum Coil Specification.

It is not this data sheet’s purpose to present in-depth

details regarding LC antennae and their tuning. Please

refer to “Low Frequency Magnetic Transmitter Design

Application Note”, AN232, for appropriate LF antenna

design details.

5.6

Security Considerations

The strength of this security is based on keeping a

secret inside the transmitter that can be verified by

encrypted transmissions to a trained receiver. The

transmitter's secret is the manufacturer's key, not the

encryption algorithm. If that key is compromised, then

a smart transceiver can:

• capture any serial number

• create a valid code word

• trick all receivers trained with that serial number.

The key cannot be read from the EEPROM without

costly die probing, but it can be calculated by brute

force decryption attacks on transmitted code words.

The cost for these attacks should exceed what you

would want to protect.

To protect the security of other receivers with the same

manufacturer's code, you need to use the random seed

for secure learn. It is a second secret that is unique for

each transmitter. It’s transmission on a special button

press combination can be disabled if the receiver has

another way to find it, or is limited to the first 127 trans-

missions for the receiver to learn it. This way it is very

unlikely to ever be captured. Now if a manufacturer's

key is compromised, new transmitters can be created.

But without the unique seed, they must be relearned by

the receiver. In the same way, if the transmissions are

decrypted by brute force on a computer, the random

seed hides the manufacturer's key and prevents more

than one transmitter from being compromised.

The length of the code word at these baud rates makes

brute force attacks that guess the hopping code require

years to perform. To make the receiver less susceptible

to this attack, make sure that you test all the bits in the

decrypted code for the correct value. Do not just test

low counter bits for sync and the bit for the button input

of interest.

The main benefit of hopping codes is to prevent the

retransmission of captured code words. This works

very well for code words that the receiver decodes. Its

weakness is if a code is captured when the receiver

misses it, the code may trick the receiver once if it is

used before the next valid transmission. To make the

receiver more secure it could increment the counter on

questionable code word receptions. To make the trans-

mitter more secure it could use separate buttons for

lock and unlock functions. Another way would be to

require two different buttons in sequence to gain

access.

There are other ways to make K

EELOQ systems more

secure, but these are all trade-offs. You need to find a

balance between:

• Security

• Design effort

• Usability (particularly in failure modes).

For example, if a button sticks or someone plays with

it, the counter should not end up in the blocked code

window, rendering the transmitter useless or requiring

the receiver to relearn the transmitter.

Note:

The exact number of turns may be

tweaked such that a standard value reso-

nant capacitor may be used.

Note:

Microchip also has a confidential Applica-

tion Note on Magnetic Sensors (AN832C).

Contact Microchip for a Non-Disclosure

Agreement in order to obtain this applica-

tion note.

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