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Difference between an M3 and M4 Enigma
Both the M3 and M4 Enigma machines were used by the German Navy during WWII. The M3 is based on a set of 3 cipher wheels and a replacable UKW. The M4 however, uses an extra cipher wheel and is therefore much more difficult to break. M4 machines were used exclusively by the U-boat section of the German Navy.

Although the U-boats were using the much more secure M4 Enigma, there was still a need to be able to exchange messages with a 3-wheel Enigma, e.g. for the weather report. 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 both machines:

 
M3
Let's first look at the wheels of an M3 machine. 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 can be seen on the right:   

 
M4
In February 1942, the 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 the M3 case, which means that the 4 wheels had to be fitted in a space that was designed for 3 wheels. This was done by replacing the existing UKW by a much smaller one, which left 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 would fit in the same space as before:   

 
Compatibility
The 4th wheel is never moved by any of the other wheels and will stay in place for the duration of the cipher session. However, the wheel can be setup in 26 positions, which in effect creates 26 different UKWs. Both the UKW and the 4th wheel were wired in such a way that, when set to A, the machine was compatible with an M3 machine. In other words: the combination of an M4 UKW + 4th wheel (set to A) is the same as 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 can be said 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.
 
How is this done in the Enigma-E?
The Enigma-E works exactly in the same way. In fact it just simulates an M4 Enigma. If we want to simulate an M3, all we need to do is to select the correct 4th wheel and set it to the fixed position A. Please note that due to a misfeature in the software of the Enigma-E, it is still possible to change the setting of the 4th wheel in an Enigma M3 (although it doesn't have a 4th wheel) which will lead to incorrect operation. When using the M3 emulation, you should leave the settings of the 4th wheel to A.

It is also possible to exchange messages with an Enigma-A, used by the German Army and Air Force. The A models are fully compatible with an M3 Enigma, but only wheels I to V were supplied with the machine. Wheels VI to VIII were used exclusively by the German Navy.
 
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Crypto Museum. Last changed: Monday, 01 February 2010 - 17:16 CET.
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