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Difference between an M3 and M4 Enigma
3 vs 4-wheel machines

Both the M3 and M4 Enigma machines were used by the German Navy (Kriegsmarine) during WWII. The M3 is based on the Enigma I, has 3 cipher wheels and a replacable UKW. The M4 however, kas an extra cipher wheel and is therefore more difficult to break. M4 machines were used exclusively by the U-boat section of the German Navy. The rest of the Navy used the M3.


Although the U-boats were using the more secure M4 Enigma, there was still a need to be able to exchange messages with a 3-wheel Enigma, e.g. for weather reports. Thus it was made possible to set up the M4 Enigma in such a way that it became compatible with a 3-wheel machine. To understand how this was done, we need to take a closer look at the wheel section of each model:

M3
Let's first look at the wheels of an M3. From right to left we can see the ETW (Eintrittswalze), the three movable coding wheels and finally the UKW (Umkehrwalze). The three moving wheels can be fitted on a rod in any particular order.

In the drawing above, you can clearly see the spring-loaded contacts on each of the wheels. The ETW is shown with part of its body 'removed' so that you can see how the wires are connected to the contacts. Once placed on the rod, the entire movable section can be fitted in between the ETW and UKW. A lever is then used to move the UKW to the right, so that all wheels are pressed firmly together. The result is shown here.
  

M4
In February 1942, the German Navy introduced the M4 Enigma, featuring an extra weel. The design was based on a 'modified' M3, so that existing parts could be used. The case of the M4 has the same size as that of the M3, which means that the 4th wheel had to fit in the existing space.

This was done by replacing the existing UKW by a much smaller one, leaving space for an extra coding wheel. As the remainging space wasn't sufficient for a standard coding wheel, a thin wheel was designed to sit next to the UKW. For this reason the 4th wheel cannot be swapped with the other three wheels. When mounted together, all four wheels and the UKW will fit in the same space as before.
  

Compatibility
The 4th wheel is never moved by any of the other wheels and stays in place for the duration of the cipher session. The 4th wheel can, however, be setup in 26 positions, which is equivalent to 26 different UKWs. Both the UKW and the 4th wheel were wired in such a way that, when the 4th wheel is set to A, the machine was backwaards compatible with an M3 machine. In other words: the combination of an M4 UKW + 4th wheel (set to A) is identical to an M3 UKW.

The standard coding wheels are marked in Roman numbers I to VIII (1 - 8). The 4th wheel, which is different, it is marked with a Greek symbol. As there were two UKWs in operation, B and C, the 4th wheel is called Beta and Gamma respectively. When UKW-B is combined with 4th wheel Beta, it is equal to an M3 UKW-B. The same is true for UKW-C and the Gamma wheel.

The 4th wheel is known under different names. It is sometimes referred to as the 4th wheel, or thin wheel, but can also be called Zusatzwalze or, more commonly, the Griechenwalze. Please note that it is possible to use UKW-B with Gamma and UKW-C with Beta, which adds to the complexity of the cipher. There are documented cases in which these combinations were used.

Further information
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Crypto Museum. Last changed: Sunday, 03 May 2015 - 12:30 CET.
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