HCS365_11 Datasheet, PDF (20/40 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder Crypt keys are read protected

HCS365
FIGURE 7-3:
SYNCHRONIZATION WINDOW
Entire Window
rotates to eliminate
use of previously
used codes
Blocked
Window
(32K Codes)
Double Operation
(resynchronization)
Window
(32K Codes)
Stored
Synchronization
Counter Value
Single Operation
Window
(16 Codes)
7.4 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, cre-
ate 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 trans-
mitted 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. Its transmission on a special button
press combination can be disabled if the receiver has
another way to find it, or 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, 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 ran-
dom 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 take
years. 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
DS41109E-page 20
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 more ways to make KEELOQ systems more
secure, but they all have trade-offs. You need to 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 end up
in the blocked code window rendering the transmitter
useless or requiring retraining.
© 2011 Microchip Technology Inc.