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Difference between an M3 and M4 Enigma
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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 weather report messages. 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:
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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 is shown on the right.
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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, 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.
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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 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 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 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.
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© Copyright 2009-2013, Paul Reuvers & Marc Simons. Last changed: Thursday, 15 March 2012 - 17:20 CET
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