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

Tony Sale's
Codes and Ciphers


Using the K Book and Bigram tables.

1. Select a trigram from the K Book, say YLA.
2. Look in the Zuteilungsliste to see which columns of the K Book are allocated to his particular key ( Home Waters, U Boat etc).
3. Select another trigram, (the Schluessel Kenngruppe), say YVT.

Message Sheet.

Write in the boxes at the top of the message form:.
.
, Y V T.
Y L A .
.
Fill in the "dots" with any letters, giving say.
.
Q Y V T.
Y L A G.

.

Now look up the vertical pairs of letters in the Bigram Tables,

writing down the resultant pairs.

UB LK RS PW.

These are transmitted as two four letter groups at the start and end of the enciphered message.

When the message is received by the intended recipient, the letter pairs are looked up in the Bigram Tables to recover the Trigram which is then deciphered on the Enigma machine starting at the Grund to reveal the true message start setting.

The Interceptors Problem.

1. working out the Bigram Tables.

This had to start with a "pinch", ie a capture of a set of tables.
Once message breaking had started, it was possible, with some difficulty, to work out new bigram tables. The tables were changed roughly once a year.

2. recovering the daily keys
ie Wheel Order and Wheel Start (the Grund).
There are 336 WO's and (26)^3 or (26)^4 start positions. ie between 6,000,000 and 150,000,000 combinations to examine to find the right one.

3. This requires a test to distinguish between a right and a wrong position, and a very rapid means of applying this test.

Tests to find the correct machine setting.


1. "Cribs".
A Crib in BP terminology was a guess at a section of the German text that was enciphered to give the intercepted enciphered message. Such a guess required clues and the Germans provided these in abundance.
a) because of the length, time of origin, call sign etc of a message it probably began with a phrase like:. VORHERSAGEBEREICH SIEBEN. (weather forecast for Area seven).
b) routine messages were sent out day after day at about the same time, from the same place, of the same length and starting in exactly the same way.
c) Re-encodements. These were retransmission of messages already sent on some other key.


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