RHV Part 4 - Message decoding procedure
By Michael Hörenberg
To decode a RHV message, we can use the whole procedure inverse. After decoding the radio groups with the correct bigram substitution tables, the box will be filled vertically with the book groups. Read out is now horizontally. See also Part 3 of the RHV procedure tutorial.
Our RHV message:
MCBP JQTB FPDZ MNRE LPDU OZHI BUSB ROLH KQFQ RHPA HTCL KWCL YVDJ YDBT KGTA DHCQ EMDE CIAQ QCKH VHGP WDIS DIZC RDSQ HCLA YNPD INKL ZWYM KFPJ HLZG MCBP JQTB
Let´s have a closer look to the "Kenngruppen" (indicator groups) recovery procedure:
"Quelle" Tafel AWe take the letters in pairs horizontally and look them up in Tafel A. This table is determined by the "Tauschtafelplan für Kenngruppen" (daily bigram table selection chart for indicator groups).
MC = BJ
BP = JM
JQ = KK
TB = GX
The four "new" pairs are then arranged vertically
(B) J K G Schlüsselkenngruppe
J M K (X) Verfahrenkenngruppe
K-Book GruppenlisteIn the Kenngruppenbuch we can see that the first letter in the Schlüsselkenngruppe and the last letter in the Verfahrenkenngruppe shall be skipped. So we have Schlüsselkenngruppe JKG and Verfahrenkenngruppe JMK. With that we go into part 2 of the K-Book, Gruppenliste of the Kenngruppenbuch (list of groups) and find that JKG = 217 6 and JMK = 320 13.
Indicator group allocation listIn the Zuteilungsliste für Kenngruppen (indicator group allocation list) Part B, the last page in the Kenngruppenbuch, we find that 201-250 for the "Schlüsselkenngruppe" is "Reservehandverfahren" RHV. If we want to doublecheck, we find 217 6 in the first part of the book, Spaltenliste, and can verify that Spalte (column) 217, line (row) 6 is actually JKG.
With this information we can look up the "Tauschtafelfolge" in the RHV Tauschtafelweiser (table sequence selection chart), we find the "Zahlenreihe" (number sequence) in the RHV "Zahlenreihentafel" (number sequence selection chart) and we can create the "Kastenwürfel" (box). As seen in Part 3. After decoding the radio groups with the correct bigram substitution tables, the box will be filled vertically with the book groups. Read out is now horizontally. Vice versa to Part 3 !
Table of contentInhaltsverzeichnis Kenngruppenbuch (Table of content: German naval K-Book for Enigma and RHV)
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