The Kenngruppen System (Navy)


By Michael Hörenberg

How to use Kenngruppenbuch (K.Buch) and Doppelbuchstabentauschtafel ... or how did the German Navy got the message key (Enigma - Nachrichtenschlüssel)

Image "Enigma Kenngruppenbuch (codebook)"
KBook
The code system is explained in the beginning of the Kenngruppenbuch. The short version is on page IX. On almost all our U-534-messages the first part of the work is done already by the operators. But when a message is received, we normally only have the two first groups, and the two last as control.

(Explicit graphical representation removed)

Image "Enigma Schlüsselzettel"
P1030690
An example:
We take the first two groups from the Enigma message P1030690 (indicator groups - Kenngruppen).

FNHC
GVET

Image "Tauschtafelplan"
Tauschtafel - Plan
The "Doppelbuchstaben - tauschtafel" (bigram substitution table) for the day is "Quelle" Tafel A. We got this information from the Tauschtafelplan (table selection chart).

1. May 1945

Image "Enigma Doppelbuchstabentauschtafel (substitution table)"
"Quelle" Tafel A
We take the letters in pairs horizontally and look them up in Tafel A.
FN = KY
HC = DM
GV = UU
ET = ZZ
The four "new" pairs are then arranged vertically

(K) D  U  Z    Schlüsselkenngruppe
Y  M  U (Z)   Verfahrenkenngruppe

Image "Gruppenliste"
Gruppenliste
In 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 DUZ. With that we go into part 2, Gruppenliste of the Kenngruppenbuch and find that DUZ = 12 9 .

Image "Zuteilungsliste"
Z-Liste
In the Zuteilungsliste (Z-Liste) für Kenngruppen Part B, the last page in the Kenngruppenbuch, we find that 1 – 20 is Potsdam. If we want to doublecheck, we find 12 9 in the first part of the book, Spaltenliste, and can verify that Spalte (column) 12, line (row) 9 is actually DUZ. We can check the Verfahrenkenngruppe in the same way, but that is not so important because we know that it is Enigma M.

Now we should have used the keysheet for Potsdam (1. May 1945). But keysheets were printed on water-soluble paper, so they have not survived. There is a little help on P1030690. The operator wrote the Grundstellung (basic key) on the top of the message sheet IBFK. With this basic key and the ENIGMA machine set up to the daily key, we can decode the message key.

Daily key:

Reflector: B
Greek: C
Wheels: 438
Rings: VCCH*
Plugs: CH EJ NV OU TY LG SZ PK DI QB

With Grundstellung IBFK: Verfahrenkenngruppe YMUZ --> ODFF the correct message key is recovered! (See also the Schlüsselzettel P1030690)

*Note:
drariG naD .rM recovered the original "Ring" setting for the U 534 messages (1. May 1945). The messages deciphers now at the wheel position/rings we found, or at the original position. Some of the "Schlüsselzettel" have their message keys written above the "Funkgruppen" column. The message keys for these messages all work with the ring setting VCCH: e.g. P1030682: HXYL