Enigma procedure (1937)

Introduction

The great novelty of this 1937 procedure is the camouflage of the network identifier. In the modified 1930 manual (1931?), the network is identified by a three-digit number (Kennzeichnung) present at the end of the header.

From this point on, it is represented by a trigram (called Kenngruppe) included in the message even inside a group whose position is part of the daily key. To increase its camouflage, each daily key was associated with four trigrams (Kenngruppen). The cipher clerk chose one of them and, what is more, could switch its letters. In the following example, the selected Kenngruppe, DGV is thus transformed into GVD.

Example of decryption

Here is an example of a key table. Compared to previous versions, the daily key also contains four kenngruppe to identify the network. The location of the group containing the chosen kengruppe is also specified (Kenngruppen Einsatzstelle Gruppe).

Geheim   - Heer -   Maschinenschlüssel für Monat Mai 1938 - Allgemeine    - N°354
Tag  Walzenlage      Ring.     Grund.     Steckerverbind.  K.E.G. Kenngruppen
 31  III  I    II     1 16 12    2 15  9  HZ RV BG AD EY LP   2   zpe vtm hxz vms
 30  I    III  II     9 22 11   23 19 14  CP EZ IQ FT WX HJ   3   vjc sjw vtk ziw
 29  III  I    II    25  5  3    4 25  8  BZ KS IV EP DR JU   3   lqo var xmx tsw
 28  II   III  I     21 19 19   26  8  8  IM CO LR HJ YZ DE   2   tra gtm ohl xsw
 27  I    III  II    24  3  7   26  7 16  JS FZ TW VX QR AM   1   xql wwy wbe tux
 26  I    III  II     9 25 10   24 16 10  LR EZ GV BM WX UY   2   qgj amu frd wbw
 25  I    III  II    26 18 20   14  6 21  GX HT OZ AM RW BQ   3   ysl dgv rkq mfg
 24  III  II   I     23 11  8    8  9  5  AN BT GK JU VX OS   1   poh wca mdk wzb
...

At midnight, we configure the Enigma to the daily key (May 25): Walzenlage: I-III-II, Ringstellung: ZRT, Grundstellung: NFU, Steckerverbindungen: GX, HT, OZ, AM, TW, BQ.

Suppose we receive this message:

2505 – 0530 - 133
XPLNY HQSUC WIGVD LKVUJ TOOGQ HWDHC BWYFW MLDCW NCDUO LROLU
HYOAE LTUHN CKOQO FNLNI ICNMI HCFAE JQWWT WUAOU BHCMW BSJYL
TXNYP RWEJY TGUTZ PNSHW JNWEM UTWCW ORJ

In the header, there is the date (25 may = 2505), the hour 05:30 (0530) and the number of characters (133). We can verify that the date of issue corresponds to today's date.

Note: if the date corresponded to the day before, it would have been smart to keep an Enigma configured to the key of the day before.

The cipher clerk who receives the message (the recipient) first identifies the group of letters that contains the Kenngruppe (Buchstabenkenngruppe). Its position is indicated in the daily key. In the example provided, it is in the third position: it is the WIGVD group. The first two letters, WI (Füllbuchstaben), were chosen at random by the sender. The next three letters correspond to one of the four Kenngruppe of the daily key or a permutation of one of them. The daily key provided these four groups: YSL DGV RKQ MFG. The GVD group corresponds indeed to the permuted DGV group, meaning the message has been encrypted with the key in the possession of the recipient.

Then the recipient deletes the group containing the Kenngruppe. The beginning of the message therefore becomes XPLNY HQSUC LKVUJ… Then he extracts the first six letters XPLNYH which correspond to the message key encrypted twice (Verschlüsselter Spruchschlüssel).

The recipient deciphers this text at the basic position (Grundstellung) 14-06-21 (NFU) indicated by the daily key. The recipient obtains the string SHASHA. The message key is therefore SHA (19-08-01).

The recipient removes the twice-encrypted message key from the message. The rest of the message (QSUC LKVUJ…) corresponds to the cryptogram. The recipient now only has to decipher it with the key SHA (we let the reader decipher the message).