Breaking Enigma with Index of Coincidence on a Commodore 64

Although he's trying to avoid using floating point, the dirty secret in many Microsoft-derived BASICs, including Commodore's, is that everything is floating point. In fact, even if you explicitly declare a variable as integer, it actually gets truncated and expanded: the native format for calculations is still 40-bit MBF. The only advantage integer variables have is smaller array storage. Every variable in his program is actually internally handled as a floating point value even though they're all integrals.

I think the author is too quick to dismiss the impact of the plugboard.

> IC attacks only the rotor settings [...] Once you’ve found the right rotors and positions, you can solve the plugboard separately using frequency analysis.

So two steps: first solve for rotor settings with IC analysis, then solve the plugboard.

But IC analysis can't find the solution, just propose a lot of candidates among which a human can spot the correct one by looking for intelligible German text. And how can you spot intelligible German text if some letters are swapped by the plugboard?

Freaking awesome... thank you very much!

Have you seen the film about Enigma, named "The Imitation Game" of 2014? I once or two asked about one of the main points of the film stated in there, yet no one still responded. Yes, I do sure rarely see films/movies nowadays (but wish so much...) for the time availability sake, but some may find the film interesting for that main point stated, I believe.

The fact that Index of Coincidence works against Enigma at all is the real puzzle.