6 July 2018

Link-11 SLEW, KG-40A encryption (tentative)

TADIL-A/Link-11 SLEW half-duplex transmissions spotted on 8223.5/usb (July, 3). Demodulated stream has a 252-bit period that likely is due to KG-40A, a crypto device which is just used to provide cryptographic security for Link 11 communications. If so, the structure of the COMSEC preamble should consist of 7 x 252-bit encoded Message Indicator followed by 96-bit sequence of "10" (phasing or time to load the key).

Link-11 SLEW 252-bit period

The KG-40A interfaces between a computer and a data terminal, and operates in accordance with control signals from those two units. The KG-40A can be wired two different ways for use in Naval Tactical Data Systems (NTDS) and Airborne Tactical Data System (ATDS) systems. The KG-40AR is a "form, fit and function" replacement for the now obsolete KG-40A cryptographic equipment.

typical Link-11 system

Link-11 functions as a computer-to-computer communications link to exchange information between shipboard, airborne and land based tactical data systems. SLEW is the more recent Link-11 development but unfortunately it is not interoperable with CLEW. Consequently, a Link-11 net can only be operated on one of them. If less capable stations need to be in the net then only CLEW waveform may be used.
Link-11 SLEW waveform is discussed here:

Link-11 SLEW waveform period

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