Break an Enigma Key with Bombe

Steps to break a key:

Introduction

In cryptanalysis, there are methods which, starting with a cryptogram, provide the plain text and the key of the day. This is not the case with the Turing / Welchman Bombe at all.

First, to be able to configure a bombe you need to have a piece of the cryptogram and the corresponding plain text (called Crib). In addition, this information, presented in the form of a "Menu" must have specific features for a solution to be possible. Finally, the bombe provides only part of the key: a stecker, the order of the rotors and the initial position of the rotors for a reference Ringstellung. The rest of the key must then be researched (all steckers and the Ringstellung).

Perhaps the most complex part of using the bombe is finding and positioning Cribs. It takes magicians to do this, called cribsters and the corresponding art cribbery. Cribsters were part of Hut 6, the Crib Room and later the Watch. It is not an accident that Stuart Millner Bary, who first experimented with the art of cribbery, became the director of Hut 6.

Here are the main steps in key breaking using bombes, from radio interception of encrypted messages to their translation and interpretation.

1. Interception (Y-Station)

Y-stations are made up of operators who listen to enemy messages transmitted by radio. Not only they record cryptograms (most of them encrypted with the Enigma) but provide a lot of metadata (direction finding, trade volumes, central station, ...) very useful for Cryptologists and even sometimes also for generals in theaters of war including in the case where the messages cannot be deciphered (Traffic Analysis).

2. Registration (Hut 6, RR)

Hut 6 is tasked within BP to break the Enigma keys. The Registration Room (RR) is responsible for retrieving the messages intercepted by the Y Stations and for carrying out a pre-processing. In particular, to classify and register messages according to their belonging to a particular Enigma network.

3. Crib Search (Hut 6, Crib Room)

Within Hut 6, the Crib Room is responsible for finding Cribs. Although the use of Bombes is not the only possibility to break a key (there are for example also the Cillies), it is the most common weapon. But to use the bombes you have to find cryptograms whose meaning can be guessed (so to find Cribs). Finding and positioning a Crib is the most complex task. They require real magicians.

4. Menu making (Hut 6, Crib Room, MR)

After finding one or more Cribs, prepare the Menus from which the bombs will be configured. A draft of the menus is produced by the Cribster. The concrete realization of the menus deriving from a crib is carried out within the Machine Room (MR). She will send the menus to Hut 1, which houses the bombes, for testing (at the start of bombe use).

5. Reduction of the number of jobs, key rules (Hut 6, MR)

Testing a menu on a bombe takes time (about half an hour if you take into account the configuration) and at the start of the war the number of bombes is very small. Everything must be done to reduce the number of Wheel Orders (WO) to be tested. The discovery of the key rules will concretely make it possible to test only part of the theoretical WO (60 WO if we use three rotors out of five).

6. Use of bombes (Hut 6, Hut 1, MR and TR)

When the Machine Room has designed the menus and determined which WOs to test, it sends those menus to Hut 1, which from them wires the bombes and makes them work. From time to time, a stop occurs. It is either the solution (the correct Enigma configuration) or a false positive.

7. Test the stops (Hut 6, Hut 1, MR and TR)

Within Hut 1, each stop is tested on a checking machine. When a stop is considered good, it is transmitted to the Machine Room.

8. Obtaining the rest of the key (Hut 6, MR)

The machine room (MR) verify each correct stop received from Hut 1 and then tries to find the full key (all steckers and Ringstellung). Inded a valid Stop only gives the Walzenlage (WO) and one stecker.

9. Message decryption (Hut 6, DR)

When the machine room has broken a key, the decoding room (DR) uses transformed Type-X cipher machines to emulate the Enigma to decrypt the messages and then forwards them to Hut 3.

10. Translation, interpretation and transmission of messages (Hut 3)

Hut 3 translates the messages and interprets the many abbreviations present. As it tracks messages it can report information not only from decrypted messages but also from Y Stations, such as the fact that a German unit has moved or that its leader has changed. Or the fact that a new unit appears. In conclusion, it not only translates the messages but brings an interpretation loaded with significance from a military point of view. It carries messages to Whiteall and also to theaters of operation through Colonel Winterbottom’s SLUs.

11. The use of Ultra (Whitewall or war theater)

In the end, a general in a theater of operations receives the most important messages and their interpretation. He can take this into account when establishing his strategy.