Inside Wireless: A New Era Of Secure RF. Linx Technologies Newsletter

Hello Valued Customer,

Linx Technologies is pleased to announce CipherLinx(TM), a new era of security for Remote Keyless Entry (RKE) and remote control devices. This highly encrypted remote control technology offers a level of security and features believed to be unmatched by any existing solution. CipherLinx(TM) may well redefine the wireless security marketplace.

CipherLinx(TM) technology is based on a core algorithm called Skipjack, developed by the United States National Security Agency (NSA). This algorithm is widely considered to be one of the most secure encryption algorithms available. At this time, there are no known cryptographic attacks on the full Skipjack algorithm.

Skipjack uses 80-bit keys to encipher 64-bit data blocks. The CipherLinx(TM) algorithm uses Skipjack in a proven secure, authenticated encryption mode both to protect the secrecy of the data and ensure that it is not modified by an adversary. Furthermore, CipherLinx(TM) is designed to operate on 128 bits, not just the 64 bits of the base Skipjack algorithm. 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.

One of the key features of CipherLinx(TM) technology is that it will never send or accept the same packet twice. This alone makes it more secure than "rolling code" devices which can repeat packets multiple times. Furthermore, CipherLinx(TM) changes codes with every packet sent, not just every button press. Even if the device transmits continuously at the fastest rate possible, it will still take 890 years to run out of valid packets.

In addition to offering state-of-the-art security, CipherLinx(TM)-based parts offer numerous features superior to competitive solutions. These include a large number of data lines, multiple baud rates, individual "button level" user permissions, an optional encoder PIN, encoder identity output, a control line for external transmitters, and easy “programming-free” setup. Products employing CipherLinx(TM) components are easily configured by the manufacturer or end user and require no production programming.

The first CipherLinx(TM)-based parts are the Linx HS Series. This series consists of an encoder that will encode the status of up to 8 buttons or contacts into a secure transmission. The matching Linx HS Series decoder will decode the transmission and, following authentication, replicate the button or contact states to control external circuitry or devices. These are implemented in tiny 20-pin SSOP packages with very low power consumption, making them ideal for battery-powered applications.

CipherLinx(TM) has been evaluated by Independent Security Evaluators (ISE), a well-known security evaluation company, who concluded an extensive report by noting the CipherLinx(TM) protocols are "well-designed" and "an excellent choice". ISE's full evaluation report and additional information may be obtained by contacting Linx Technologies at 800-736-6677 or visiting www.cipherlinx.com.

The HS Series' compelling combination of security, features, and ease of use make it the logical choice for all high-security RKE applications. The HS is currently in sample production and is priced at $2.54 in production volume quantities of 10,000.

Click here to visit CipherLinx.com. >>
Click here for more information about the HS Series. >>
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Creating a secure remote control system with the HS Series encoder and decoder is very simple. The application circuits shown here use LR Series RF modules. First, we will look at the transmitter / encoder side.

Click here for a detailed PDF diagram.

In this example, the data lines (D0 - D7) are connected to buttons, and when any button is pressed, the SEND line is pulled high, which causes the encoder to transmit. Diodes are used to prevent the voltage on one data line from affecting another.

The KEY_IN line is attached to a port, which allows the key to be transferred from the decoder during setup. To ensure security, this would normally be a wire, contact, or short-range IR link, although any connection capable of transferring asynchronous serial data may be utilized.

None of the inputs have internal pull-up or pull-down resistors, so 100kΩ pull-down resistors are used on the data, SEND, and CREATE_PIN lines. These resistors are used to pull the lines to ground when the buttons are not being pressed, which ensures that the pins are always in a known state and not floating. Without these resistors, the state of the lines cannot be guaranteed and encoder operation may not be predictable.

A LED is attached to the MODE_IND line to provide visual feedback to the user that an operation is taking place. This line will source a maximum of 25mA, so the limiting resistor may not be needed, depending on the LED chosen and the brightness desired. A LED can also be connected to the TX_CNTL line to provide visual indication that the encoder is sending data.

Outgoing encrypted data will be sent via the DATA_OUT line at the baud rate determined by the state of the SEL_BAUD line. In the circuit above, the baud has been set for 4,800bps by pulling it to ground. The DATA_OUT line is connected directly to the DATA line of the LR Series transmitter.

The TX_CNTL line is connected to the PDN line of the transmitter so that the encoder can put the module into a low-power state when not in use. Since this line can source 25mA and the module only consumes 6mA, the encoder can be used to power the transmitter rather than using Vcc. This means that the transmitter will not pull any current when not in use rather than the 5 nanoamps (nA) it will consume when powered down.

In the example above, the data lines are pulled high by simple pushbutton switches, but many other methods may be employed. Contacts, reed switches, or microcontrollers are just some examples of other ways to pull the data lines high.

Next, we will look at the receiver / decoder side.

Click here for a detailed PDF diagram.

In this circuit, the baud rate has been set for 4,800bps by pulling the SEL_BAUD line to ground.

SEND_COPY, CREATE_KEY, and LEARN are all connected to buttons that will pull the line high when pressed. Since the lines do not have internal resistors, 100kΩ resistors are used to pull the lines to ground when not in use.

COPY_IN is connected to a port that allows the transfer of the User Data from another decoder. This port can be a simple wire, an infrared receiver, or any other device that allows the transmission of asynchronous serial data.

The KEY_OUT line is connected to a port that allows the transfer of the key to an encoder or another decoder. This port can be a simple wire, an infrared diode, or any other device that allows the transmission of asynchronous serial data.

The KEY_OUT line can also be connected to a microprocessor or PC to record the transmitter identity. Linx Application Note AN-00156 has sample C code that will read the transmitter ID and display the ID number on an LCD screen.

A LED indicator is attached to the MODE_IND line to provide visual feedback that an operation is taking place. This line will source a maximum of 25mA, so the limiting resistor may not be needed, depending on the LED chosen and the brightness desired.

The DATA_IN line is connected directly to the DATA output of the receiver.

Data Lines D0 through D7 can be connected directly to the external circuitry that is to be activated remotely. In this example, D5 is connected directly to a piezoelectric buzzer, which will cause the buzzer to sound when the D5 line on the encoder goes high. Line D6 will activate a relay through a transistor buffer when it goes high. A buffer like this may be needed if the decoder cannot source enough current or voltage to energize the relay coil. The decoder will turn on the transistor, which will provide the appropriate drive levels to the relay.

As can be seen by these examples, creating a highly secure remote control system with the HS Series is very straightforward. The flexibility of the encoder, combined with the associative options of the matching decoder, opens a new world of possibilities for creative product designers. For more information on the HS Series and the LR Series, please see our website, www.linxtechnologies.com.

Click here for more information about the HS Series. >>
Click here for more information about the LR Series. >>

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