Following on from my Python Enigma cipher machine simulator, I re-wrote it in C++ for maximum speed.

The settings for each rotor - its start position and ring setting - can actually be combined into a single offset. This is perhaps why all the popular treatments of the subject just ignore the impact of the rings on the number of combinations. In truth, their impact is hard to explain. The rings change when rotors rotate, and do have an impact on the number of combinations, but many initial starting positions are equivilent for short messages because the rings cancel out the rotor start position and do not cause a rotation because the message is too short.

The M4 Enigma - as infamously used by the U-boats - is usually described as a ‘four rotor’ machine. And that’s strictly true, I suppose, but I think I have a better way of describing it:

The M4 was a normal 3-rotor M3 Enigma chassis. A new 'thin’ reflector design was developed that could take a new, thinner 'greek’ non-rotating wheel. The thin greek wheel could be placed in any of 26 positions and clicked into the thin reflector. This could then be placed in the chassis in the normal reflector position.

So basically the M4 is an M3 Enigma with a configurable reflector.

I wrote the fast Enigma sim to serve as a basis of a statistical attack on the Enigma. In fact, if you look at my test vectors, you’ll notice one of them is Simon Singh’s Cipher Challenge Enigma stage 8, which I cracked using my statistical attack! More on that in the next post :)

And here’s the code. As you can see, the vast majority of the code is just test vectors to ensure its correctness: