The HC-9 is a handheld mechanical cipher machine used by the Swedish armed forces during the postwar era.
It is of some interest in that, although it operates by a completely different principle, it and the original version of the Hagelin lug and pin machine are the only secure modern cipher machines that are both very compact and of all-mechanical construction.
It behaves as if it contained five pinwheels, of sizes 29, 31, 33, 34, and 35. Each "pinwheel" advances one space for every letter enciphered, and their outputs are used to select one of sixteen cipher alphabets as follows: four bits are formed from the five bits presented by the five sequences by taking the XOR of each pair of adjacent bits.
This result is treated as a four-bit number. Thus, the XOR of the bits from the 29-bit and 31-bit sequences controls displacing the list of alphabets by eight places.
But instead of pinwheels, the sequences are supplied by a punched card, which sits in the device and whose large round holes are sensed by little metal fingers inside the device.
Another replaceable card inside the device gives the sixteen alphabets in use. These alphabets were all chosen to be reciprocal in practice for ease of use.
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