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In cryptography, a pinwheel was a device for producing a short pseudorandom sequence of bits (determined by the machine's initial settings), as a component in a cipher machine. A pinwheel consisted of a rotating wheel with a certain number of positions on its periphery. Each position had a "pin", "cam" or "lug" which could be either "set" or "unset". As the wheel rotated, each of these pins would in turn affect other parts of the machine, producing a series of "on" or "off" pulses which would repeat after one full rotation of the wheel. If the machine contained more than one wheel, usually their periods would be relatively prime to maximize the combined period.
Pinwheels might be turned through a purely mechanical action (as in the M-209) or electromechanically (as in the Lorenz SZ 40/42). Other cipher machines which used pinwheels include the C-52, the CD-57 and the Siemens and Halske T52.
Pinwheels can be viewed as a predecessor to the electronic linear feedback shift register (LFSR), used in later cryptosystems.
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