Enigma-M4


The Enigma-M4 is a rotor key machine that was used for encrypted communication by the German Kriegsmarine during World War II from October 1941.
In contrast to the previously used Enigma-M3 and the Enigma I and the Enigma-G used by the German Secret services, the Enigma-M4 is characterized by four rollers. This makes its encryption cryptographic significantly stronger than that of the other Enigma variants with only three rotors and therefore could not be broken by the Allies for a long time.

Previous history

All parts of the German Wehrmacht used the rotor cipher machine to encrypt their secret messages. Enigma. However, different models were used. While the army and air force used the Enigma I almost exclusively, there were different model variants of the Enigma-M for the navy, which usually referred to it as the "Schlüssel M". The main difference was the use of more rollers than in the Enigma I, where three rollers could be selected from a range of five. This resulted in 5-4-3 = 60 possible roller positions of the Enigma I.
The Enigma-M1, on the other hand, had an assortment of six different rollers, three of which were inserted into the machine. This increased the combinatorial complexity to 6-5-4 = 120 possible roller positions. In the Enigma-M2, the range of rollers had been increased by a further one, so that 7-6-5 = 210 roller layers were now possible. And with the Enigma-M3, which was used by the navy at the beginning of the war, there were eight rollers, three of which were used. This meant that the M3 had 8-7-6 = 336 possible roller positions. While the M1 to M3 models only ever used three rollers in the machine, the M4 had four rollers next to each other in the machine. This significantly increased the cryptographic security of the M4 compared to its predecessor models.
The operation was described in the naval service regulation M.Dv.Nr. 32/1 with the title "Der Schlüssel M - Verfahren M Allgemein". The key procedure used in the Enigma-M4 was precisely defined in this document.
A particularly important part of the key procedure, in addition to correct operation, was the agreement of a common cryptological key. As with all symmetric cryptosystems, the sender and receiver of an encrypted secret message not only had to have the same machine, but also had to set it identically to each other. For this purpose, secret key tables were distributed in advance to all authorized participants. To ensure security within such a large organization as the Kriegsmarine, there were many different key networks, as in the army and air force, for example Aegir for surface warships and auxiliary cruisers overseas, Hydra for warships near the coast, Medusa for U-boats in the Mediterranean, for battleships and heavy cruisers and Triton for the Atlantic submarines. There were other key nets. The Kriegsmarine initially used the Enigma-M3 exclusively for all of them.
However, the need was soon recognized to form a separate, specially secured key network for the Einheiten operating on the "Hohe See". In October 1941, a letter from the Oberkommando der Kriegsmarine to the Commander of the Battleships introduced the "Neptun key board" as a new key and ordered the use of the Enigma-M4 for this purpose. This took place four months before the M4 was put into service for the U-boats. Unlike Triton, the Neptun key network was not broken at the time.

Structure

The construction of the Enigma-M4 has some special features compared to the Enigma I. The most important difference is the use of four rollers compared to only three in the other models. The four rollers were selected from a range of eight plus two rollers. A distinction had to be made between rollers I to V, which were also used on the Enigma-I, and rollers VI to VIII, which were familiar from the Enigma-M3, and the two rollers specially designed for the Enigma-M4, which were thinner than the others and were therefore referred to as "thin" rollers.
The technical reason for the production of thin rollers was the Navy's desire to be able to continue using the same housing as Enigma I and Enigma-M3. For this purpose, the space previously occupied by a reversing roller now had to be used by a thin reversing roller and one of the newly added thin fourth rollers. Instead of Roman numerals, the thin rollers were marked with Greek letters, namely "β" and "γ". Although they could each be manually rotated to one of 26 positions, unlike rollers I to VIII, they did not rotate any further during the encryption process.
The wiring diagram of the entry roller and the eight rotating rollers of the Enigma-M4 was identical to the Enigma I and M3. In particular, the two non-rotating thin rollers "Beta" and "Gamma" and the two equally thin reversing rollers "Bruno" and "Caesar". The wiring of the two thin rollers and the reversing rollers was designed in such a way that the combination of the reversing roller with the "matching" roller results in exactly the same involutory character permutation as the reverse rollers B and C of the Enigma I and the Enigma-M3 alone. This served the purpose of backward compatibility with the earlier systems. The only requirement for this was that the spell key of the U-boats was chosen so that it began with "A". Then the left cylinder was in exactly the position in which it worked together with the matching VHF in the same way as the corresponding VHF of the other Enigma models.
As with the Model-D, the keys and lamps were arranged in a layout similar to QWERTZU. Only the letters P and L were shifted to the edges of the bottom row, in contrast to QWERTZU.

Q W E R T Z U I O
A S D F G H J K
P Y X C V B N M L

In contrast to other models such as the Model-D, Model-K or the Model-T, there was no support for digits on the keyboard. Special characters were also not provided.
For data compression, the Navy used signal groups and phrases from code books such as the Short signal bookletand the Weather short key which consisted purely of letters.

Operation

To fully set the key, the navy distinguished between "outer" and "inner" key parts. The inner key included the selection of the rollers, the roller position and the ring position. The inner key settings could only be made by an officer, who opened the housing and selected, set up and arranged the rollers accordingly. He then closed the Enigma again and handed it over to the radio operator.
The radio operator's task was to make the external key settings, i.e. to insert the ten pairs of plugs into the plug board on the front panel of the M4 according to the day key, close the front flap and then turn the four rollers to the correct starting position. While the inner settings were only changed every two days, the outer settings had to be changed every day. The key change also took place on the high seas at 12:00 D.G.Z., for example early in the morning for U-boats operating off the American east coast.
The keys ordered were listed on top-secret "key boards" at the time. Here, too, a distinction was made between internal and external settings. The key board intended for the officer with the inner settings looked something like this:
Key M " T r i t o n "
---------------------------
Month: J u n e 1945 Test number: 123
------ ----------------
Secret matter of command!
----------------------
Key panel M-General
---------------------------

Inner attitude
------------------
Change 1200 h D.G.Z.
--------------------------
----------------------------------------------
|Months- | |
| ayd | Inner attitude |
----------------------------------------------
| 29. |B Beta VII IV V |
| | A G N O |
----------------------------------------------
| 27. |B Beta II I VIII |
| | A T Y F |
----------------------------------------------
| 25. |B Beta V VI I |
| | A M Q T |
----------------------------------------------
Only a few days of the month are shown above as an example, with the days sorted in descending order, as was customary at the time. This made it easy to cut off and destroy the "used" codes from previous days. The other key board, which listed the outer key parts, was similarly structured.
Example for June 27, 1945: Internal setting "B Beta II I VIII" means that the officer first had to select roller B as the reversing roller. Then he had to set the non-rotating Greek roller Beta to ring position A, roller II to ring position T, roller I to ring position Y and finally roller VIII on the far right to ring position F and insert the rollers in the order from left to right. With a little feeling, the ring position could also be set on installed rollers. The officer locked the roller cover and handed the M4 over to the encryptor, who made the external settings using his own documents.
Key M " T r i t o n "
---------------------------
Month: J u n e 1945 Test number: 123
------ ----------------
Secret commando matter!
----------------------
Key panel M-General
---------------------------

External setting
------------------
Change 1200 h D.G.Z.
--------------------------
----------------------------------------------------------------------
| Mon- | | Home - |
| ths- | P l u g c o n n e c t i o n s | Posi- |
| day | | tion |
----------------------------------------------------------------------
| 30. |18/26 17/4 21/6 3/16 19/14 22/7 8/1 12/25 5/9 10/15 |H F K D |
| 29. |20/13 2/3 10/4 21/24 12/1 6/5 16/18 15/8 7/11 23/26 |O M S R |
| 28. |9/14 4/5 18/24 3/16 20/26 23/21 12/19 13/2 22/6 1/8 |E Y D X |
| 27. |16/2 25/21 6/20 9/17 22/1 15/4 18/26 8/23 3/14 5/19 |T C X K |
| 26. |20/13 26/11 3/4 7/24 14/9 16/10 8/17 12/5 2/6 15/23 |Y S R B |
The radio master had to connect the double-pole sockets on the front panel with corresponding double-pole cables. As a rule, exactly ten cables were plugged in. Six letters remained "unplugged". The plug connections in the navy were listed numerically and not alphabetically. In the corresponding secret Naval service regulations M.Dv.Nr. 32/1 with the title "Der Schlüssel M - Verfahren M Allgemein" a conversion table was given as an aid for the operator.
ABCDEFGHIJKLMNOPQRSTUVWXYZ
0102030405060708091011121314151617181920212223242526

Now, the key operator had to turn the four rollers to a defined starting position and the Enigma-M4 was ready to encrypt or decrypt radio messages.