Security of the XStream and XCite radios (encryption)

Is the wireless communication from Digi radios secure?

Many scada or financial applications are sensitive to the potential for intercepted data or unauthorized control of a scada system through a wireless link.

We will outline the factors that affect the security for the Digi radio modems (XStream and XCite):

1. Digi radio modems transmit using FSK modulation while hopping among 25 unique frequencies in a pseudo-random sequence. This proprietary frequency hopping spread spectrum (FHSS) technique is not intercepted by other commercial radios on the market. In addition to the FHSS, Digi uses a proprietary modulation technique that is not published. With no documentation or information about it being publicly available it would require sophisticated, specialized equipment to be able to record and analyze the wireless transmission.
    
2. While the proprietary frequency hopping modulation and the number of networks and addresses can make it very difficult for an outsider to eavesdrop on communication, it would be possible for another Digi radio to listen in on communications provided it was configured to the proper channel and radio address. For additional security, Digi can program a unique Vendor Identification number (VID) into the firmware of the radio for a fee. Unique VIDs are given to one vendor, and one vendor only. Only units with identical vendor identification numbers will communicate.
   
   Every transmission from a Digi radio modem contains a 16-bit VID that is settable only at the factory. Digi radio modems can only interact with other modems that have a matching VID. If a Digi customer chooses to purchase a VID, they receive a special ordering part number and each radio modem is configured to work with their systems only prior to shipping from the factory. No other individual or company can purchase modems that might interact with the modems containing a special VID.
    
3. Digi XCite and XStream radios do not encrypt the data that is transmitted over-the-air. For maximum security we recommend the use of the Digi XTend modules which support 256-bit AES (Advanced Encryption Standard) encoding to protect over-the-air data.
   
   But how secure is AES? There are 1.1 x 10⁷⁷ possible 256-bit keys. In comparison, DES keys are 56 bits long, which means there are approximately 7.2 x 10¹⁶ possible DES keys. Thus, there are on the order of 10⁶¹ times more AES 256-bit keys than DES 56-bit keys.
   
   It is important to note that Digi's implementation of AES is designed to provide for confidentiality of the messages. Any additional security needs such as "message authentication" or "data integrity" should to be implemented outside the radio. Techniques such as rolling codes, time stamps etc. can be used to guard against spoofing valid messages by simply capturing and replaying an encrypted message.