REMOTE CONTROL OVERVIEW
HS SERIES OVERVIEW
Wireless remote control is growing in popularity and finding its way into more
unique applications. Remote Keyless Entry (RKE) systems for unlocking cars or
opening garage doors quickly come to mind, but how about a trash container that
signals the maintenance office when it needs to be emptied? The idea behind
remote control is simple: a button press or contact closure on one end causes
some action to be taken at the other. Implementation of the wireless RF stage
has traditionally been complicated, but with the advent of simpler discrete
solutions and modular products, such as those from Linx, implementation has
become significantly easier.
The HS Series encoder encodes the status of up to eight buttons or contacts into
highly secure encrypted serial data stream intended for wireless transmission via
an RF or infrared link. The series uses CipherLinx™ technology, which is based
on the Skipjack algorithm developed by the United States National Security
Agency (NSA). The CipherLinx™ protocol in the HS Series has been
independently evaluated by Independent Security Evaluators (ISE). A full
evaluation white paper is available at www.linxtechnologies.com/cipherlinx.
The encoder combines eight bits representing the state of the eight data lines
with counter bits and integrity bits to form a 128-bit message. To prevent
unauthorized access this message is encrypted with CipherLinx™ in a mode of
operation that provides data integrity as well as secrecy. CipherLinx™ never
sends or accepts the same data twice, never loses sync, and changes codes on
every packet, not just every button press.
Encoder and decoder ICs are
generally employed to maintain the
security and uniqueness of a wireless
RF or IR link. These devices encode
the status of inputs, usually button or
contact closures, into a data stream
suitable for wireless transmission.
Upon successful recovery and
validation, the decoder’s outputs are
set to replicate the states of the
encoder’s inputs. These outputs can
then be used to control the circuitry
required by the application.
VCC
Tx
LR
ENC
HS
Series
Decoding of the received data signal is accomplished by the HS Series decoder.
When the decoder receives a valid command from an encoder, it will activate its
logic-level outputs, which can be used to activate external circuitry. The encoder
will send data continuously as long as the SEND line is held high. Each time the
algorithm is executed, the counter is decremented, causing the code to be
changed with the transmission of each packet. This, combined with the large
counter value and the timing associated with the protocol, ensures that the same
transmission is never sent twice.
Series
DEC
Rx
LR
GND
HS
Series
Series
Figure 5: Remote Control Block Diagram
An 80-bit key used to encrypt the data is created in the decoder by the user. The
decoder is placed into Create Key Mode, and a line is toggled 10 times, usually
by a button. This is required to gather entropy to ensure that the key is random
and chosen from all 280 possible keys. A high-speed timer is triggered by each
rise and fall of voltage, recording the time that the line is high and low. The 80-
bit key is generated by combining the low-order bits of the twenty timer values.
To create an association, the key, a 40-bit counter, and a decoder-generated ID
are sent to the encoder via a wire, contacts, IR, or other secure serial connection.
Prior to the arrival of the Linx HS Series, encoders and decoders typically fell into
one of two categories. First were older generation, low-security devices that
transmitted a fixed address code, usually set manually with a DIP switch. These
products were easy to use, but had significant security vulnerabilities. Since they
sent the same code in every transmission, they were subject to code grabbing.
This is where an attacker records the transmission from an authorized
transmitter and then replays the transmission to gain access to the system.
Since the same code was transmitted every time, the decoder had no way to
validate the transmission.
The HS Series allows the end user or manufacturer to create associations
between the encoder and decoder. If the encoder and decoder have been
associated through a successful key exchange, then the decoder will respond to
the encoder’s commands based on its permissions. If an encoder has not been
associated with a decoder, its commands will not be recognized.
These concerns resulted in the development of a second type of encoder and
decoder that focused on security and utilized a changing code to guard against
code grabbing. Typically, the contents of each transmission changes based on
complex mathematical algorithms to prevent someone from reusing a
transmission. These devices gained rapid popularity due to their security and the
elimination of manual switches; however, they imposed some limitations of their
own. Such devices typically offer a limited number of inputs, the transmitter and
receiver can become desynchronized, and creating relationships and
associations among groups of transmitters and receivers is difficult.
The user or manufacturer may also set “button level” permissions. Permission
settings control how the decoder will respond to the reception of a valid
command, either allowing the activation of an individual data line or not. The
decoder is programmed with the permission settings during set-up, and those
permissions are retained in the decoder’s non-volatile memory.
The HS decoder has the ability to identify and output a decoder-assigned identi-
fication number for a specific encoder. An encoder’s key, a 40-bit counter, and
permissions are stored in one of fifteen memory locations within the decoder.
The decoder is able to output an 8-bit binary number that corresponds to the
memory location of the encoder’s information. This provides the ability to identify
the specific encoder from which a signal originated. This identification can be
used in various ways, including systems that record access attempts or in
applications where the originating user needs to be known.
The HS Series offers the best of all worlds. The HS Series uses an advanced
high security encryption algorithm called CipherLinx™ that will never become
desynchronized or send the same packet twice. It is easily configured without
production programming and allows for “button level” permissions and unique
encoder and decoder relationships. Eight inputs are available, allowing a large
number of buttons or contacts to be connected.
To learn more about different encoder and decoder methodologies please refer
to Application Note AN-00310.
Page 6
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