Enigma procedure (1938)

Introduction

On September 15th, 1938, an important change took place: it was the abandonment of a single Grundstellung for all members of a network. Since then, each cipher clerk chose a Grundstellung for each message that was mentioned in the header of the message.

This new procedure was part of an update to the Enigma manual of 1937. The appendix to the book “Enigma and its Achilles Heel”, written by Hugh Skillen (1992) provides Enigma's 1937 encryption notice. In this, one can see manual additions which make it conform to the procedure of September 1938.

Decryption Example

In the key table, the Grundstellung column disappears.

Geheim - Heer - Maschinenschlüssel für Monat November 1938     - N°146
Tag  Walzenlage      Ring.     Steckerverbind.  K.E.G. Kenngruppen
 31  I    III  II    18 11 17  KO IP HY BZ AR CE   2   tuz iyf rrf kdd
 30  II   I    III    3 14 24  VZ AW KY GQ NU JL   1   jfa fiq fin rul
 29  II   III  I      1 21  8  GN DU JX EM FY KS   2   qwp xcd hhd mig
 28  I    III  II    22 16  9  FY VW LS PQ EZ BU   2   zfo hij rkt snq
 27  I    III  II     4  3 15  LT XY CR DI KW NV   2   mcu wmp rwn kmu
 26  II   I    III   16 20 11  KN HR WX BL UV IQ   3   wwb rzy qjg yoa
 25  I    II   III   15  4 15  JV FR UX HY GL IN   1   ebc hdh qys gah
 24  II   III  I     18 14 13  AO NX MS CT GH FI   2   nnf vcd iux jps
...

Note: from December 15, two rotors (IV and V) come into operation.

At midnight, we configure the Enigma to the daily key (November 24): Walzenlage: II-III-I, Ringstellung: RNM, Steckerverbindungen: AO, NX, MS, CT, GH, FI.

Suppose we receive this message:

1755 - 119 =BIL=  KMVPV LSVCD CBXPD IJQOG UXLKQ MWKVW VCZLY 
PAZIK NYGGK GACEM PPJJW GGZTD ZGRRH RTUQM WDSKA EFVVA XXREY 
UPGZM BDSOR RWVWQ SRYCH GLRWN NGYQC KWFX

The time the message was sent is indicated by 1755 (5:55 p.m.) and 119 is the length of the message.

The operator first checks whether the message belongs to his Enigma network. For this, he inspects the group which contains the Kenngruppe. The daily key specifies it is the second group (LSVCD). The first two letters were chosen randomly by the issuer. The next three letters (VCD) correspond to one of the four Kenngruppe indicated in the key of the day (nnf, vcd, iux, jps) or permutation of one of them. We deduce that the message is intended for him. He and the transmitter are part of the same Enigma network, and therefore, they use the same key.

Then, Then it deletes the group containing the Kenngruppe. Then he extracts the indicator, which corresponds to the first 6 letters (KMVPVC). He decrypts it at the basic position (GrundStellung). This basic position, as indicated at the end of the header in the message, is the BIL trigram. The recipient, thus, obtains the repeated message key RIGRIG.

The recipient removes the indicator. What remains of the message is the cryptogram (BXPD IJQOG...). He can decipher it with the RIG key. (we let the reader decipher the message).

As you can see, the main difference between the new and old system is that the cipher clerk chooses the GrundStellung in the new system instead of taking the one specified in the key table.