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

Tony Sale's
Codes and Ciphers

Weaknesses of the German key system.

1. Paired Days.

The WO and Rings remained the same for two days, with one three day in a 31 month.

2. Wheel Order Rules.

a) the WO always contained one wheel 6,7 or 8.

b) the same wheel never used in the same position on consecutive day pairs.

c) no wheel used 3 times in a month on the left.

By taking account of these and other "rules", the number of wheel orders to be run could be reduced from 336 to as little as 10 or 20, a great saving in bombe time.

3. Consecutive Stecker Knock ( CSKO ).

Adjacent letters never Steckered.

The "Narvik Pinch".

A trawler intercepted on April 26th 1940 by the destroyer Arrow proved to be a disguised German ship.

A boarding party recovered one of two bags thrown overboard by the crew.

It contained the Stecker and Grundstellung for April 23rd and 24th.

An operators log giving letter for letter cribs for April 25th and 26th.

Exact details of the indicating system which confirmed Turing's deductions.

E tables and a description of how they worked;.

The "Long E bars" (Alfa - Funksignale) a system for rapid communication by ships in action.

A good source of cribs.

The doldrums - May '40 to February '41.

Following the Narvik Pinch, giving Stecker and Grund, April 23rd and 24th were easily broken and "paired days" ie the same WO and Ringstellung, soon followed.

April 26th proved difficult. Hand methods failed because of 10 Stecker pairs. However the first Bombe had just arrived and a crib from the operator's log was tried. After a series of missadventures and a fortnights work, the Bombe triumphantly produced the answer.

With the 26th out, the paired day, the 27th, was soon broken and both days were found to be on the same bigram table.

Every effort was then made to break all the messages on those days in order to recover as much as possible of the bigram table.

Banburismus could then be tried on days using this table. But Banburismus proved to be very difficult in practice. May the 8th, the most promising day, was worked on ad nauseum for months.

Foss's Day.

In August Mr Foss retuned from sick leave, was given May 8th and by sheer perseverence broke it in November. May 8th is immortalised as Foss's Day.

The reasons for this long period of the doldrums were:

Incomplete bigram tables, lack of cribs and a large number of "Dummy" messages.

August 25th, Frank Birch wrote to Travis saying:.

"I'm worried about Naval Enigma.Turing and Twinn are like people waiting for a miracle, without believing in miracles... ".

Then came the Lofoten raid and the Enigma keys for February 1941 from the Krebs.

The Heydays of Banburismus.

April 1941 - February 1942.

The capture of the February '41 keys allowed the bigram tables to built up completely.

All April and May except 6th May were broken, but not currently.

The capture of the June keys covered the change in bigram tables on June 15th.

With increased staff, although the first six days of August proved difficult, Banburismus was now so refined that September 18th/19th were the only days not broken on DOLPHIN for the rest of the war.

Banburismus was now breaking a few hours after the completion of a day's traffic and if the next day was a "paired day", breaking could be current.


U33 Feb 12th 1940 rotors VI and VII.

Rotor VIII and North Africa.

Polares (Narvik Pinch) April 26th 1940 Keys for April, Instruction Manual and Enigma Tables.

Krebs March 4th 1941 Enigma keys for February.

Muenchen May 7th 1941 Short Weather cipher, Enigma Keys for Home Waters for June.

U110 May 9th 1941 Key tables for E, Offizier procedures, a bigram book and instructions for emergency keys (Stichworts).

Lauenburg June 28th 1941 July keys.

? December 30th 1941 Keys, bigram tables and K Book.

U559 October 24th 1942 Wetterkurz schluessel complete with indicator tables.

These Pinches werre absolutely essential, there were just too many unknowns in Naval Enigma for it to be workred out cryptographically.

Offizier Keys.

These were used for very important or personal messages which the officer in charge did not want the Enigma operator to see.

They were enciphered by the officer himself using this special key. The resultant encipher then being enciphered again by the Enigma operator using his normal procedures.

The Offizier Key used the same WO and Ringstellung as the normal key, an entirely different Stecker plugging, one of a set of 26 start positions, valid for a month, and denoted by a letter of the alphabet.

For instance a message when decoded on the main key might begin with an address followed by "Offizier Sophie" and then go into nonsense. The recipient would pass the message to his commanding officer who would change the Steckers to the Offizier Steckers for the day, look up S in his setting list, find say PJX, set the wheels to PJX and decipher the message.

Cribs for Offizier messages were very rare because of the individual nature of the messages.

One Offizier message laboriously decrypted proved to be an order for his luggage to be sent to mistress's house.

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