A recent post in radioscanner (my nickname is "ansanto" there), and some interesting discussions with a friend of mine, gave me the opportunity to study that signal and deepen the theme that is dealt with by MIL 188-220. In particular, the signal in question (courtesy by KarapuZ) concerns a transmission with link encryption provided by external COMSEC devices, in accordance with what is reported in the Appendix D of the cited standard, ie "transmission frame structure in either external and embedded COMSEC modes".
In short, in external COMSEC transmission frame the clear-text (not encrypted) part of the transmission frame is the "COMSEC Preamble", depicted below, which consists of the subfields:
a. COMSEC Bit Synchronization
b. COMSEC Frame Synchronization
c. Message Indicator (MI)
The COMSEC Frame Synchronization subfield is used to provide a framing signal indicating the start of the encoded MI to the receiving station. This subfield is 465 bits long, consisting of 31 Phi-encoded bits. The Phi patterns are a method of redundantly encoding data bits. The logical data bits 1 and 0 are encoded as shown in figure.
The Message Indicator subfield (MI) contains the COMSEC-provided MI, a stream of random bits that are redundantly encoded using Phi patterns.
Alternatively, in the embedded COMSEC transmission frame the clear-text part, depicted below, is composed of the following components:
a. Phasing
b. Frame synchronization (Standard Frame Sync or Robust Frame Sync)
c. Robust Frame Format (RCP only)
d. Message Indicator (MI)
Back to the signal, it was heard by KarapuZ in the 33 MHz band and it is a GFSK modulation at a rate of 16000 Baud that can be easily processed using SA (Fig. 1).
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| Fig. 1 |
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| Fig. 2 - COMSEC Preamble |
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| Fig. 3 |
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| Fig. 4 |
KG-84 is similar to the emebedded COMSEC transmission frame, although the 64-bit pattern of KG-84 Standard Frame sync
1111101111001110101100001011100011011010010001001100101010000001
is different from the patterns reported in Appendix D for Standard and Robust Frame sync. Moreover, the Message Indicator is not encoded using the Phi 15 bits, although it is redundantly encoded. That's probably ok since KG-84 is based on SAVILLE algorithm and 188-220 is VINSON based. Note as in some KG-84 "detectors" the Standard Frame sync is termed as "SYNC" and the MI data as "Initialization vectors". |
| Fig. 5 - NATO KG-84 encryption and 188-220 embedded COMSEC |
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| Fig. 6 - Turkish-Mil FSK and 188-220 embedded COMSEC |
(to be continued)
[1] http://www.cryptomuseum.com/radio/sincgars/