How did the Schlüsselkasten work?
I wish I could explain in detail how the Schlüsselkasten worked, but the description in the NSA report is hard to understand. The ciphering was done with two fixed strips (GEH… and FRI…) and a movable two row strip in between:
The inscriptions of the strips could be changed. 13 characters of the alphabet were written on the fixed base, and the other 13 on the top and bottom of the sliding strip. Here, the letters on the fixed parts are arranged with the passwords GEHEIMSCHREIBER and FRITZMENZER. The sliding strip was driven by a spring controled by four notched wheels.
If I’m not wrong, the G1, G2, G3 and G4 strips only indidcate if a particular notch of the notched wheel is activated. In the final design of the machine these strips did not exist. Here’s the description of the functioning from the NSA report:
In use, the slide was pulled to the right until it stopped, winding the spring that drove the mechanism. Pressing a button released the slide to move left. When at either or both of the reading positions A1 or A2, the pins on all four pin wheels were all inactive, the slide stopped, and encipherment took place. If the step came from A1 alone or A1 and A2 together, the slide took an additional step. Whenthe slide stopped, either the top or the bottom alphabet would be in phase, and the cipher value could be read off. Pressing the button again would allow the slide to slide left to its next stop.
I certainly have understood the concept of this machine, but I still don’t get the details. If a reader knows a better description or knows another source for the Schlüsselkasten, I would be interested to know.
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Further reading: German cipher machines in World War 2: A complete (?) list
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