The Kryha Machine

The Kryha machine was a clockwork-powered mechanical running key encipherment
device. Two alphabets were on movable tabs so that the sequences of the plain component
and the cipher component could be changed at will. The movement of the cipher alphabet disk was controlled by a cogwheel with 52 holes around the edge. A plunger on the end ofa lever served as a detent by dropping into each ofthe holes in succession. These holes could
be individually covered, thus changing the wheel pattern, and varying the "kick" with each step of the wheel. The total kick in one full revolution was prime to 26 so that the period of the machine was equal to 26 times the total kick. The cryptovariables of the
system were therefore the plain sequence, the cipher sequence, the wheel pattern, the initial setting of the cogwheel, and the initial setting of the cipher sequence against the plain sequence. The last two variables were changed with each message, the others less
frequently. A pair of indicators, enciphered with a multivalued number key, gave the alphabet setting (the number of the cipher letter set against 0 plain) in the "ab" positions, and the cogwheel setting in the "de" positions. The "c" position was a filler.

The clockwork-driven Kryha, a popular German made cipher machine that was widely used commercially in the 1930s.

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