Enigma procedure (1940)

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Introduction

On May 1, 1940, the Enigma German encryption manual published in January came into force.

The allies (English and French) could read Enigma messages since January thanks to Zygalski’s sheets because at the time the message key was encrypted twice (thrown-on method). In the new procedure, the message key is only encrypted once. It is then a complete blackout for the allies.

Structure of a message

Here is the beginning of the example message from the manual of January 13, 1940 (Lord 2019): 

1755 129 = WEP = HFI = ULZNU SGEXU NFOPR SALME YDRJG QARZU BHFEM…
  • 1755 = The hour.
  • 129 = The number of characters of the message (the group containing the Kenngruppe included).
  • WEP = Grundstellung chosen by the operator.
  • HFI = The message key (XFR), encrypted only once at the WEP position.
  • ULNZU = The group containing the Kenngruppe (NUZ) and the two letters (UL) which complete the group.
  • SGEXU… = The message encrypted using the XFR key.

It is in this manual that the maximum size of a message is increased to 250 letters (instead of 180).

Exercise

Trying to decipher this encrypted message on February 27. We give the key tables.

Note: Often the header was repeated to ensure transmission reliability. 

1140 - 131 == HUG HUG == DSJ DSJ == RVVAK GBCBI ZVIJJ FPSUB
VFXFR SAATW XRBJD NKCWQ RSUPF BKNZP KLJAS IIXTF HQBJD YVAAU
APUAW HAIOY CQBYU OJDWD GTANS ZLEPO TBKHW SJQNC HUWCM PSWNA
FWYOR LTFAM P 

Geheim - Luftwaffe Allgemeine – Maschinenschlüssel             -  N°083
                  für  Monat  Februar 1941    
Tag  Walzenlage      Ring.     Steckerverbind.          Kenngruppen
 31  I    V    II    21 25  5  HT MR CY VW JZ FN QX BP  yab mly ocl slg
 30  IV   V    III    3 14 12  WX GJ AC MZ DI FL OY NT  iog vwd ght cqv
 29  I    V    II     8 20 13  HQ WX CE PR AU BY MZ FK  otx rgh eps lgd
 28  III  V    IV     8  9 25  EL AV OZ KY BS IJ QU GR  kvg ibr ibi mbj
 27  I    II   V      6 16  7  RT KZ AU BN IQ PW JX MS  vct did bax xnr

Geheim - Luftwaffe Fürungnetz – Maschinensclüssel               - N°001
                  für  Monat  Februar 1941    
Tag  Walzenlage      Ring.     Steckerverbind.          Kenngruppen
 31  IV   I    II     3 22  2  FP KL CI BD NY RV GM OW  hwm osk oxi fjd
 30  I    II   III    6 20 13  BO LZ RS KU NQ AC FM DJ  lvk exb xux wqr
 29  V    III  I     10  1  9  KT CR HV LU DG JO IP AW  tcs xuj gpx lev
 28  I    IV   III   17 17 26  BN MU PX GY IO RV LT EQ  kfp tsj dlj deq
 27  III  II   I     19  4 19  NQ LP TZ FW AS GJ OU VX  fsi xky vka uxw