30 March 2020

few comments on the secured 50-75Bd/850 FSK transmissions

Just to spend a bit of time during this ugly period, I thought I'd take a look at the naval BRASS systems (1) that use the 50Bd/850 and 75Bd/850 (STANAG-4481F) FSK waveforms as well as the  crypto devices that are used in those transmissions. 
A necessary introduction, to not forget, concerns the use of the terms KG-84 and KW-46: well, it does not necessarily mean that those devices are physically deployed ashore or aboard. Rather than to the equipments, these names must be understood as referring to the used "algorithms", since - unless few exceptions - many of those devices are now obsolete and no longer used. Actually, the algorithms are emulated by interoperable and more compact devices such as, for example, the KIV-7M Programmable Multi-Channel Encryptor that can be used for communicating with the older KG-84/KIV-7 family of devices. So, I talk about

- KG-84: when is detected the presence of the 64-bit frame sync
followed by the 128-bit message indicator;

- KW-46: when is is detected the presence of the Fibonacci's bits generated by the polinomyal
x^31 + x^3 +1 (KW-46T uses that M-sequences to synch the KW-46R receive devices).

Using KiwiSDRs located in various parts of the world, and with some lists of logs from UDXF friends, I spent some days browsing the HF spectrum, recording and analyzing as many 50-75Bd/850 FSK transmissions as possible (so far I have come up to 51 different FSK channels), and logging the results into a spreadsheet. By the way, I added the "false 75Bd" frequencies (see this post) to the group of the 50Bd FSK.
Beyond that the "working" list of frequencies is certainly not complete, a fact appears to emerge quite clearly: all the 50Bd/850 FSK transmissions use KW-46 encryption while all the 75Bd/850 FSK use KG-84/KIV-7 encryption!

Fig. 1
(French Navy 50Bd/850 FSK is a separate discussion: they use a 21-bit period stream consisting of frames which are delimited by two LFSR markers M1 M2 generated by the polynomials x^6+x^5+1 and x^7+x^6+1 and a logical "1" value bit; these transmissions were not considered here).

Given that KW-46 is used to secure the fleet broadcasts and KG-84 is used to secure Point-To-Point  (PTP) circuits and multi-station nets [1], it follows that 50Bd/850 FSK is used for broadcast.
For what concerns the 75Bd/850 FSK transmissions, they consist of a continuous  flow of short/long messages; thus, since by their nature  PTP and MRL (2) transmissions are sporadic  and short-lived, 75Bd/850 FSK is used for a "some type of broadcast" for those unspecified multi-station nets. In this regards, it's to notice that some stations may operate simultaneously with the two waveforms, as for example NPN Lualualei (Hawaii, US) which has been heard on 9075 KHz (75Bd) and 9112 KHz (50Bd), thus serving  two different scopes at the same time (Fig. 2).

Fig. 2
It's also to notice that different baud rates and encryption devices were used for GENSER (General Service) traffic and for CRITICOMM (operational intelligence) traffic, but that information is from old non-classified documents from 50 years ago.
I've not clear in mind what "multi-station net" stand for neither if they still refer to naval communications (it could also be that 75Bd just repeate the 50Bd broadcast). The FSK hunt continues...

(1) BRASS (Broadcast and Ship to Shore) is an approach used by Navies, particularly in NATO countries, to communicate between Ships and Shore using HF Radio. The core of a BRASS service is a continuous HF broadcast going out to multiple ships and on several frequencies (typically four or five) to allow ships to select a frequency that works.

(2) The BRASS service makes use of three types of point to point link:
- Ship to Shore. A special link that supports message flow from a ship to shore and request to re-send missing or corrupt messages. Frequency Assignment Broadcast (FAB) is used to allow ships to share a pool of HF frequencies for ship to shore communication.
- Ship to Ship: to support direct communications between ships.
- Maritime Rear Link (MRL).  A link between selected ships (usually "Command Afloat") and shore, supporting message flow in both directions.

No comments:

Post a comment