The "Luftwaffe" Kenngruppen System


The German Airforce ENIGMA Indicator System (By Dan Girard)

As an example of the Heer/Luftwaffe ENIGMA indicator system in use at the time of the Norrköping intercepts, here's the beginning of the message AYDJM, intercepted at 0039 on 7 March 1940:

0020 0020 = 171 171 = ERG ERG = AYDJM UMYXI MYMZA XOZBC WGZFI GIMWX QWGAK OHLIC KJOHF JFVNA   Picture

Image "German Airforce Enigma Intercept"
Luftwaffe Enigma message 1940
First the time group (0020), the letter-count (171), and the Grund (ERG) are each given twice. The Grund is chosen at the discretion of the enciphering operator, and is given in the clear. The first five-letter group (AYDJM) consists of two dummy letters chosen at random (AY), followed by a three-letter indicator group or Kenngruppe (DJM), which identifies which key-net the message belongs to. As part of the daily keys, each key-net was assigned four of these three-letter Kennruppen, and the enciphering operator could choose any one of these four trigrams as his indicator. He could put the three letters of his chosen trigram into the last three positions of the first five-letter group in any order; thus, in this case, AYDJM, AYDMJ, AYJDM, AYJMD, AYMDJ, and AYMJD would all have been equally valid.

The next six letters (UMYXIM) are the message key, enciphered twice. This day's Enigma settings are:

Wheels: 5 1 4
Rings:    K S H (or the equivalent setting 11-19-08 on the Heer/Luftwaffe Enigma, which had the numbers 01 to 26 on its wheels instead of the letters A to Z)
Plugs:    BF DV EJ GM IY KP LZ NX OQ TW

With the Enigma set to these settings, and the wheels set to the Grund position ERG, then typing the letters UMYXIM results in the letters OELOEL. The message key then is OEL (or the equivalent position 15-05-12). With the wheels re-set to this key, decryption of the message can begin, starting with the last four letters of the third group (YMZA). The decryption begins,

"TAETIGKEITSBERIQTVOMXSEQSXDREIXVIERNULL..."

This system was changed not long after these messages were sent. After 30 April 1940, the double encipherment of the message keys was dropped. Thereafter, the message key was enciphered only once, and the enciphered key was given in the message heading, immediately after the Grund.