*updated *

I noticed that in their STANAG-4285 **600bps/Short** fleet broadcast the German Navy use a 7-bit framing consisting of [5-bit data] + [one parity bit (even parity)] + [one pahsing bit], such framed stream is differential encoded and then forwarded to the S4285 modem.

Fig. 1 - 7-bit framing adopted in S4285 600bps/Short broadcasts |

Fig. 2 - the differential decoded stream is parity bit checkd |

Such peculiar framing, a "new one" for me, is used in several frequencies (see Table I) and most likely is thinked for their "domestic" fleet broadcast (as the French Navy does with their characteristic 21-bit framing); indeed in other S4285 frequencies the German-Ny run the NATO "standard" fleet broadcast consisting of the S4285 **600bps/Long** sub-mode and KW-46 encryption (figure 3).

Fig. 3 - KW-46 sync sequence in S4285 600bps/Long broadcasts |

The 5-bit user data are most likely encrypted, the quality of the cryptography can be evaluated with a statistical method or by calculating the Shannon Entropy of a stream. The statistical test determines the randomness of the bit stream, the number of single bits in the stream is counted, then the double bits, then the triple bits and so on to the end. The result is a graph: If the information is not systematic, the adjacent columns should be half the size of the previous ones. The tes for these bit streams show good encryption quality. The measure of the Shannon Entropy can be used, in a broad sense, to detect whether data is likely to be structured or unstructured. 8 is the maximum, representing highly unstructured, 'random' data. Properly encrypted or compressed data should have an entropy of over 7.5

Fig. 4 - measurements about the presence of encryption in the 5-bit user data sub-frames |

I also noticed that each 773 frames (5411-bit length blocks) a sequence of 588 bit length is inserted into the stream for a total length of 5999 bits (figures 5,6): that sequences are not repetitive, do not have a defined period, do not have a parity bit check, and do not seem generated by a polynomial (at least in my attempts).

Fig. 5 |

Fig. 6 |

- (588,4638) if the check is applied to [5-bit data]+[parity bit]

- (588,3865) if the check is applied to [5-bit data] only

**this page**.

**18th June update**

Repeating the tests of my friend, I took a small amount of bits from an unsystematic stream (fig. 7a) and reshaped them to a 10-bit period stream (fig. 7b), then I edited the last column by inverting some bits and turned them into a phasing bits column (fig. 1c). As a result of the differential decoding, I received a uniform parity check, except for the first combination of bits (fig. 7d).

Fig. 7 |

*(1) As usual, code verification is carried out by comparing each line of code in turn with all the 588 rows of the check sub-matrix: the vertical correspondences of the "1s" positions in the code line and in the row #n of the check sub matrix are counted. If the matches are even then the correspondent position #n in the EDAC bits will be "1", otherwhise (ie matches are a odd number) will be "0". The values 1/0 of the EDAC bit will be the opposite in case of odd parity.*

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