How to make an M4 Enigma compatible with an M3
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The M4 Enigma was used exclusively by the German Navy in their
U-boats and, when introduced later in the war, caused the code
breakers at Bletchley Park many headaches. As the M4 features
an extra wheel, it greatly enhances the possible number of
permutations.

As always, there was a flaw. The U-boats had to receive and
broadcast a weather report every day. The problem however was
that the German Wetterdienst was not allowed to use M4 machines,
but were restricted to the M3. So the Germans deviced a method
to make the M4 temporarily compatible with an M3.

How this was done? Well, to understand that, we have to go back
a little in history to see how the Enigma M4 was designed. In
fact, it was based on an M4. The new extra wheel of an M4 will
fit, together with a new UKW, in the space that was previously
only used for the UKW. In other words, someone has slided the
old UKW in two halves... one half would become the new 'thin'
UKW and the other half would be the new 'thin' extra wheel.

Now there were to UKWs in use: UKW-B and UKW-C and, suprise...
there are two new 'thin' wheels, called Beta and Gamma (a.k.a
the 'Greek' wheels or Griechenwalze). The thin wheels cannot
be exchanged with the other 3 wheels as it simply wouldn't fit.

Although the UKW-B, UKW-C, Beta and Gamma could be used in any
particular combination, there are some special situations. If
UKB-B is used in combination with the Beta wheel, AND the wheels
are set to the initial position 'A', it would behave like an
M3 Enigma with its UKW-B in place. The same can be said for
the combination UKW-C and Gamma.

Simulation
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In this directory you'll find two machines: an M3 and an M4
with the same initial settings. Please try for yourself.
