Lecture on Naval Enigma

The Breaking of German Naval Enigma
by Tony Sale | Back to Naval Enigma Index |

The difficulties in breaking German Naval Enigma.

It was one of the triumphs of WW II that it was broken.

At first sight it is not obvious why Naval Enigma was so difficult, it initially used the same version of the Enigma as the German Army and Air Force and these were broken virtually throughout the War.

The difficulty lay in the indicator system. This was unique to the German Navy and involved a seperate coding system, bigrams and trigrams, for concealing the message setting. As will be explained, it was this indicator system which made the breaking of Naval Enigma so difficult.

The story starts with Arther Scherbius who invented the Enigma in 1922.
He produced a relatively simple commercial machine which had three rotors and a rotatable reflector without plug board.

It was bought by the German Navy and used from about 1926 onwards more or less unchanged from the commercial machine.

Then the German Navy changed over to the main machine, the Heimsoeth & Rinke as used by the German Army and later the Air Force.
That was a slightly different kettle of fish because first of all the wiring inside the rotors of the machine had been changed and although the reflector was fixed, a stecker or plug board had been added at the front of the machine .

The German Enigma Machine in summary.

1. "Reciprocal" : If A -> J then J -> A at the same wheel position.
2. "Non-crashing" : A cannot encipher as A.
3. Turnover notches on the alphabet rings. The rings can be set to different positions relative to the core containing the cross wiring. (the Ringstellung).

4. These are the letters showing in the windows when turnover was about to occur, together with the BP rhyme.
.
.(R)oyal (F)lags (W)ave (K)ings (A)bove.
..I......II......III....IV.......V.
.
This was a mistake. All wheels should have had the same turnover points, and the later wheels 6,7 and 8 did. With the turnover points being different the wheels could be identified.

Wheels 6,7 and 8 were added by the German Navy. These wheels all had two turnover points which were the same on all of them.
This shows the wheel turnovers plotted on an alphabet.
.
5,6,7,8.. 2.........4.....6,7,8...1.........3.
|.........|.........|.....|.......|.........|.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z.
.
5. For each wheel number always the same letter shows in the window at turnover.

6. The electric current flows through the wiring and lights a lamp AFTER the wheel(s) have turned.

Early Enigma - the Poles.

The first Enigma machine, used by the German Navy in the 1920s had three wheels, a rotatable reflector and no Steckers. The early Naval machine was soon worked out because the Poles had purchased a commercial version.

In 1931 the German Navy changed to the "Heimsoeth and Rinke" Enigma as used throughout the German forces for the rest of the war. This initially had three wheels and a Stecker board with only 6 letter pairs being Steckered.

The Poles had already recruited the three young mathematicians , Zygalski Rozeki and Rejewski, and they were given the task of finding how to break it.

Marian Rejewski developed mathematical theories of the Enigma and with the aid of spy material, worked out the wheel wirings. The Poles then built replica Enigmas and were able to read traffic currently for many years.

The Polish copy has a plug board at the back with ordinary telephone jacks.
When the Poles were involved only 6 letter pairs were being steckered. Later it increased to 10 which was the optimum number. But with only 6 steckers the machine behaved very nearly like an unsteckered Enigma, this made breaking much easier.

Concealing the Message Setting.

The message setting gave the starting wheel positions for encoding or decoding a message.

The German Navy used two different methods to conceal the message setting from an interceptor.

a) the Enigma machine itself.

A message setting, chosen by the operator, was enciphered twice on the Enigma machine starting from the Grund and the resultant six letters transmitted to the intended recipient in a header to the message. This was the system used by the German Army and Air Force.

b) a completely separate coding system

On May 1st 1937 the German Navy introduced a new system encoding the message setting by bigram substitution.
This was the main Naval system and involved selecting a trigram from a book (the Kennbuch, or K Book), enciphering this at the Grund to get the message setting, then performing a bigram substitution on the trigram and then transmitting the result as a header to the message.

The intended recipient then performed the inverse bigram substitution to recover the trigram then enciphered this at the Grund to get the message setting.

The Poles suddenly found that their systems for breaking the double message setting system, no longer worked and they did not know why.

The Grundstellung for 8th May was found as a result of a typical German error. A torpedo boat, call sign AFA, had not received its instructions on the new system and was therefore told, in a message sent in another cipher which the Poles could break, to use the old system. Two or three messages from AFA were enough to find the Grundstellung using Forty Weepy Weepy cribs. The Grund was the same as for April 30th and the intermediate days were then got out.

But despite having solved about 15 messages for each day, the Poles could not work out the new indicator system. They suspected that it was a bigram substitution but got no further.

Forty Weepy Weepy cribs.

Continuation messages: FORT 23.30.
Top row of Keyboard used for numbers. .
Q W E R T Z U I O P.
1 2 3 4 5 6 7 8 9 0 .
Letter Y used to indicate figures.
FORTYWEEPYYWEEPY.

The Daily Key,(Tagschluessel)

1. The Wheel Order (WO).
The numbers of the wheels to be placed in the Enigma machine from left to right.
( 336 possible from eight wheels, reduced by the "rules" for inclusion of at least one of wheels 6,7 or 8).
2. Ringstellung,
(Tyre or Ring setting) for each wheel, left to right.( 17,576 combinations ).
Note: the Wheel Order and Ringstellung were set for two days, (so called paired days), the Stecker and Grundstellung changed every day.

3. Stecker, or plug-board connections. Usually ten pairs involving 20 letters.( 140 million million ).
4. The Grundstellung (Grund). The three (or four) letters showing the position of the wheels to be used for enciphering the message setting.

single Grund on right hand side.
illustrative only, not an original.

This page was originally created by the late Tony Sale
the original curator of the Bletchley Park Museum