MCS3122-ISLASHST datasheet(16/26 Pages) MICROCHIP

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MCS3122

DS40001762A-page 16

 2014 Microchip Technology Inc.

FIGURE 6-2:

SYNCHRONIZATION WINDOW

6.3

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 and 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 the manufacturer would want to

protect.

To protect the security of other receivers with the same

manufacturer’s code, the manufacturer should use the

random seed for secure learn. It is a second secret that

is unique for each transmitter. If a manufacturer’s key is

compromised, clone transmitters can be created, but

without the unique seed, they have to 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 make

brute force attacks that guess the hopping code take

years. To make the receiver less susceptible to this

attack, it should test all bits in the decrypted code for

the correct value, not just the low counter bits and

function code.

The main benefit of hopping codes is to prevent the

retransmission of captured code words. This works

very well for code words which the receiver decodes.

Its weakness is that, 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. The

receiver should increment the counter on questionable

code word receptions. The transmitter should use

separate buttons for lock and unlock functions. A

different method would be to require two different

buttons in sequence to gain access.

There are more ways to make KEELOQ systems more

secure, but they all have trade-offs. The user should

find a balance between security, design effort and

usability, particularly in failure modes. For example, if a

button sticks or kids play with it, the counter should not

advance into the Blocked Code window, rendering the

transmitter useless or requiring retraining.

Blocked

Window

(8M Codes)

Double Operation

(Resynchronization

Window)

(8M Codes)

Stored

Synchronization

Counter Value

Single Operation

Window

(16 Codes)

Entire window rotates

to eliminate use of

previously used codes

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