The mode of operation used by the CipherLinx? algorithm is proven to
prevent this type of attack.
Another critical factor is how often the message changes. To prevent code
grabbing, most high-security systems send different data with each
transmission. Some remote control applications will encrypt the message
once per activation and repeat the same message over again until it is
deactivated. This gives an attacker the opportunity to copy the message
and retransmit it to maintain the state of the protected device and “hold
the door open”, or worse yet, have the option to come back later and
gain access. The HS Series goes a step further and sends different data
with EACH PACKET, so the data will change continuously during each
transmission. This means that at 28,800bps, there will be a completely new
128-bit message sent every 25.5mS.
Another factor is how often the message will be repeated and the intervals
between repeats. Some applications use a counter to change the
appearance of the message. This is good, but at some point, the counter
will roll over and the message will be repeated. For example, if attackers
were to copy an encrypted message and save it, they could potentially gain
access to the protected device at a later time. Depending on the size of
the counter, this vulnerability could occur frequently. The HS Series uses a
40-bit decrementing counter to keep this from ever happening. If the SEND
line was held high continuously at the high baud rate (28,800bps), it would
take 889 years before the counter would reach zero, at which point the key
would be erased and the encoder would have to get a new key. The math
used is: [(240 * 25.5ms) / (1000mS*60s*60m*24h*365d)] = 889 years. This
large counter prevents a packet from ever being sent twice and prevents
the encoder from ever losing sync with the decoder.
The key is generated with the decoder by the user through multiple button
presses. This is ensures that the key is random and chosen from all 280
possible keys. Since all of the keys are created by the user and are internal
to the part, there is no list of numbers anywhere that could be accessed to
compromise the system.
Encryption of the transmitted data is only one factor in the security of a
system. With most systems, once an encoder is authorized to access a
decoder, it can activate all of the decoder data lines. With the HS Series,
each encoder can be set to only activate certain lines. This means that the
same hardware can be set up with multiple levels of control, all at the press
of a button.
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