Enigma-M4

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.

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".

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.

Instead of Roman numerals, the thin rollers were marked with Greek letters, namely "β" (Beta) and "γ" (Gamma).

[9] 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.

However, since the order of execution is not important here either (it is cryptographically equivalent if, for example, A is plugged in first with X and then B with Y or vice versa, first B with Y and then A with X), the corresponding cases must not be considered as key combinations.

However, as explained in more detail in the Main article about the Enigma, the size of the keyspace is only a necessary, but not a sufficient condition for the security of a cryptographic method.

British cryptanalysts had been working on the deciphering the Enigma at Bletchley Park, about 70 km (43 mi) northwest of London, since the outbreak of the war.

The cryptanalysts benefited from the German thoroughness in writing routine messages, such as weather reports, with recurring patterns that could be used for deciphering.

In addition to the use of a larger range of cylinders (eight instead of just five to choose from), this was also largely due to a particularly sophisticated Verfahren zur Spruchschlüsselvereinbarung, which the navy used.

The break-in in "Shark" was not achieved until December 1942,[17] after the British destroyer Petard took down the German U-boat U-559 in the Mediterranean Sea on October 30, 1942.

From 1943, the Americans came to the rescue, under the leadership of Joseph Desch at the United States Naval Computing Machine Laboratory (NCML), based at the National Cash Register Company (NCR) in Dayton (Ohio), more than 120 units as of April 1943[20] produced high-speed variants of the British bomb that were specifically targeted against the M4.

[24] This covers the period from the fall of 1941 until the unconditional surrender of the Wehrmacht in May 1945, whereby, with the exception of the "blackout" between February and December 1942, it was possible to maintain the uninterrupted nature of deciphering for most of the time.

The reports from the boats with precise position and course information provided the Allies with a complete strategic situation picture.

Radar as a means of radiolocation at sea and ASDIC, an early form of sonar, for sound detection under water was another important tactical tool for submarine hunting.

The immediate consequence of the American decryptions was the sinking of eleven of the eighteen German Versorgungs-U-Boote ("Milchkühe") within a few months in 1943.

Especially for the execution of Operation Overlord, the planned invasion of Normandy, it was crucial for the Allied leadership to have the most comprehensive, up-to-date and, of course, correct picture of the situation.

In fact, however, everything remained unchanged and so the day key could be broken in less than two hours after midnight using the Cribs "WETTERVORHERSAGEBISKAYA", which the British cryptanalysts could easily guess and assumed correctly[26] and the invasion succeeded.

Many German U-boat drivers, above all the former chief of the B-Dienst (observation service) of the Kriegsmarine, were still very sure long after the war that "their" four-roller key machine was "unbreakable".

The special historical significance of the Enigma-M4 and its decryption is underlined by a statement made by former British Prime Minister Winston Churchill: "The only thing that really frightened me during the war was the U-boat peril.

"[28] (German: "Das einzige, wovor ich im Krieg wirklich Angst hatte, war die U-Boot-Gefahr.")

A significant example of the German considerations, procedures, conclusions and measures derived from them can be found in an English-language, highly secret interrogation protocol classified at the time as TOP SECRET "ULTRA", which was published immediately after the war, on June 21, 1945, by the Allied (British-American) TICOM (Target Intelligence Committee) at the Marine Intelligence School in Flensburg-Mürwik.

The German naval command could not explain this, especially why U-boats were sunk in very specific positions, and again asked themselves the question: "Is the machine safe?"

They began their investigations with the assumption that the enemy knew the machine, including all the rollers, and that they had a suspected clear text (Crib) of 25 letters in length.

Although neither he nor any of his colleagues had any previous experience in the cryptanalysis of key machines, such as the commercial Enigma, they had succeeded in breaking the M4 within six months, at least in theory.

This would have required case distinctions in the cryptanalysis, which would have increased the workload by a factor of 26, which was considered to be practically unacceptably high for a potential attacker.

This measure halved the number of possible roller positions (from 8-7-6 = 336 to 8-7-3 = 168), which meant a weakening of the combinatorial complexity, but at the same time strengthened the machine against the recognized weakness.

The following figures are available:[33][34][35] An example is a communication from Lieutenant Captain Hartwig Looks, commander of the German U-boat U 264, which was encrypted with an Enigma-M4 on November 19, 1942, shortly before the end of the black-out.

Short radio messages and the almost unavoidable spelling and transmission errors in practice make deciphering based on statistical analyses difficult.

The weather information "Air pressure 1014 mb falling, wind north-north-east with 4 Bft, visibility 10 nm" would have been shortened by a weather abbreviation (WKS) to 8 characters[39] instead of the 44 characters used Details Documents Decipherments Exhibits Photos, videos and audios Reproduction projects Simulations of the M4

The Enigma-M4 key machine
Key manual of the Kriegsmarine "Der Schlüssel M".
With the rollers removed, the thin reversing roller with its 26 contact plates can be seen on the left.
Two rollers of the Marine Enigma shortly before assembly.
All the rollers are inserted here. On the far left is the thin FM C, called "Caesar" (indicated by the engraved "C-"), and immediately to the right is one of the "Greek rollers", which is not switched on during scrambling, but can be turned by hand to one of 26 positions (A to Z).
M4 (without cover) with fully inserted roller set.
When the cover is closed, the "Greek roller" located behind the left-hand roller window can hardly be distinguished from the other rollers from the outside.
The "thin" VHF B (far left), also known as "Bruno", in the roller box of an M4 together with some rollers.
On the M4, the sockets were marked with numbers rather than letters. The type plate shows the production code aye for Olympia Büromaschinenwerke in Erfurt and the year 44 (for 1944).
The reel marked with the Roman numeral VI is one of the three reels developed exclusively for the Navy.
U-110 und HMS Bulldog
The only surviving Desch bomb in the National Cryptologic Museum
Identification group booklet captured by the Americans from U-505
In this photo from March 1941, just under a year before the M4 entered service on February 1, 1942, an M3 can still be seen in the U-124 's radio locker, which also served as a key room.
This M4, captured from the German submarine U-505 , is equipped with the "Schreibmax", which made the laborious reading of the letter lamps a thing of the past.