HS SERIES SECURITY OVERVIEW
HS SERIES SECURITY OVERVIEW (CONT.)
Encryption algorithms are complex mathematical equations that use a number,
called a key, to encrypt data before transmission. This is done so that
unauthorized persons who may intercept the transmission cannot access the
data. In order to decrypt the transmission, the decoder must use the same key
that was used to encrypt it. The decoder will perform the same calculations as
the encoder and, if the key is the same, the data will be recovered.
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 HS Series uses the CipherLinx™ algorithm, which is based on Skipjack, a
cipher designed by the U.S. National Security Agency (NSA). At the time of this
writing, there are no known cryptographic attacks on the full Skipjack algorithm.
Skipjack uses 80-bit keys to encipher 64-bit data blocks. The CipherLinx™
algorithm uses Skipjack in a provably secure authenticated encryption mode
both to protect the secrecy of the data and ensure that it is not modified by an
adversary. 8 bits of data are combined with a 40-bit counter and 80 bits of
integrity protection before being encrypted to produce each 128-bit packet.
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.
Preamble
128-Bit Encrypted Data
RX Noise Logic
Balancing Filter Filter
Integrity Check
80 bits
Data
8 bits
Counter
40 bits
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.
Figure 6: HS Series Data Structure
There are several methods an attacker may use to try to gain access to the data
or the secured area. Because a key is used to interpret an encrypted message,
trying to find the key is one way to attack the protected message. The attacker
would either try using random numbers or go through all possible numbers
sequentially to try to get the key and access the data. Because of this, it is
sometimes believed that a larger key size will determine the strength of the
encryption. This is not entirely true. Although it is a factor in the equation, there
are many other factors that need to be included to maintain secure encryption.
Another factor in system security is the control of the encoder. If attackers gain
control of the encoder, typically they would be able to access the system. The
HS offers the option of adding a Personal Identification Number (PIN) to the
encoder that must be entered before the encoder will activate. Furthermore,
since each encoder has its own key and the Control Permissions are stored in
the decoder, all the attackers would be able to do is duplicate the device that
they have already taken. They will not be able to grant themselves greater
authority, create a new controller, or replicate another encoder.
One factor is the way that the underlying cipher (in the case of the CipherLinx™
algorithm, Skipjack) is used to encrypt the data. This is referred to as the cipher’s
“mode of operation.” If a highly secure cipher is used in an insecure mode, the
resulting encryption will be insecure. For example, some encryption modes allow
an adversary to combine parts of legitimate encrypted messages together to
create a new (and possibly malicious) encrypted message. This is known as a
“cut-and-paste” attack. The mode of operation used by the CipherLinx™
algorithm is proven to prevent this type of attack.
Before the encoder sends a packet, it will calculate the Hamming Weight (the
number of ‘1’s in the string) of the packet to determine the duty cycle. If the duty
cycle is greater than 50% (more ‘1’s than ‘0’s), the encoder will logically invert all
of the bits. This ensures that every packet will always contain 50% or less ‘1’s.
Since the FCC allows transmitter output power to be averaged over 100mS, this
allows a legal improvement in link range and performance for many devices
using an ASK / OOK transmitter. A 50% duty cycle is generally the best
compromise between data volume and output power.
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.
Some other manufacturers may use a Pulse Width Modulation (PWM) scheme
or Manchester Encoding scheme to maintain a 50% duty cycle. Both of these
methods work, but are inefficient and do not make use of the full link budget. The
HS Series uses true serial data while maintaining a 50% duty cycle. Application
Note AN-00310 covers these issues in detail.
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