CIS-1200 SDPSK 1200Bd ("Makhovik", T-230-1A)

updated (23 July 2024)

This transmission, along with a probably spurius emission 600 Hz above, was recorded on 13002.5 KHz (cf) thanks to the remote KiwiSDR located in Azumino-city Nagano, Japan [1].

Fig. 1 - main signal and its spurius

The signal that I assume is the "actual" one and that I analyzed is characterized by a SDPSK (Simmetrical Differential PSK) modulation at a speed of 1200 Baud. Indeed SDPSK is equivalent to π/2 DBPSK or PSK2 with phase rotation: ie, as shown by the transitions in absolute mode, SDPSK assumes that the phase is rotated by +π/2 for bit “0” and by -π/2 for bit “1” thus there is not a 180° turn (transitions do not pass through 0). The information transmitted is encoded in the transition and not in the state. The signal can be demodulated using the differential mode (diff=1).

Fig. 2 - SDPSK modulation

The transmission consists of some segments that differ by the presence or absence of an initial preamble (signals A and B in Figure 3) which consists of a repeated 511-bit length pseudo-random sequence generated by the polynomial x^9+x^5+1 (1) as for the ITU Recommendation O.153 [2] (188-110B "39-tone parallel mode" too uses that sequences).

Fig. 3

 

Fig. 4 - 511 bits length sequence

The presence of such sequences is one of the features of the so-called Makhovik (aka the "flywheel"), a well known Soviet-Mil crypto system. Although someone classifies Makhovik as vocoder, it can can be used for time-multiplexed encryption of both voice and data up to 9600 bps. It's official name is "T-230 bundle ciphering device for teleprinter and  data connections" and was designed to operate in UHF but very often is found in LF and in HF.
After the removal of the initial preamble, the following data block consists of a "common" sequence:

110101100100011110101100100011

followed by 240-bit Initialization Vectors that are sent in 8x30-bit groups, each group repeted three times (Figure 5): these 30-bit groups are another peculiar feature of  the Makhovik system.

010000111011001110010100001110
011101100101000011001010000111
110010100001110010000111011001
001110110010100000111011001010
001110110010100011001010000111
111111111111111000011101100101
001110110010100111011001010000
101100101000011011101100101000

Fig. 5

Segments sent w/out the initial preamble (type B in Figure 3) show exactly the same structure: note as the Initialization Vectors slightly differ (Figure 6): this feature should be further studied (it is probably somehow related to the presence/absence of the initial preamble) but it is necessary to obtain several more recordings.

010000111011001110010100001110
011101100101000011001010000111
110010100001110010000111011001
001110110010100000111011001010
001110110010100011001010000111
111111111111111000011101100101
001110110010100111011001010000
101100101000011011101100101000

010000111011001110010100001110
011101100101000011001010000111
011001010000111100001110110010
111011001010000110010100001110
010100001110110110110010100001
100101000011101001010000111011
111011001010000111111111111111
011101100101000100001110110010

Fig. 6

It's worth noting that in some previous Makhovik recordings I saw differential encoded data & BPSK, while this ones consist of  plain encoded data & SDPSK [3].

update (23 July 2024)
I willingly add a comment sent me by my friend cryptomaster.
The common sequence in Figs 5,6

110101100100011110101100100011

shall be right shifted to appear as

111101011001000111101011001000

which in turn is the repetition of the 15 bits length M-sequence generated by the polynomial x^4+x+1 (Figure 7).

111101011001000

Fig. 7 - the repetition of the 15 bits M-sequence generated by the polynomial x^4+x+1

https://disk.yandex.com/d/Vg5XruORhd8_5A

(1) the use of the polynomial x^9+x^5+1 is quite common in CIS waveforms,see http://i56578-swl.blogspot.com/p/polynomials.html

[1] http://jf0fumkiwi.ddns.net:8073/
[2] https://www.itu.int/rec/T-REC-O.153/en
[3] https://i56578-swl.blogspot.com/search/label/Makhovik