In 1934 Mr Hagelin was asked by the head of the French cipher bureau to construct a ciphering machine, small enough to fit the pocket of an ordinary army coat and also being capable of printing the text. To settle the size, Hagelin carved a piece of wood to fit in a pocket.
The C-35 was accepted by the French, and in October 1937 six machines of type C-36 were ordered by the Swedish Navy for testing. The C-36 is virtually the same machine as the C-35, but C-36 has a housing that the C-35 lacks, and different lug patterns (see below). Some later C-36 models have movable lugs.
The plaintext letter (or cipher letter if the machine is used for decryption) is found on a circular knob bearing the alphabet, to the left of the machine. The knob is turned and the sought letter is placed against a benchmark, and then a lever is operated.
The C-35 has five pinwheels which have individual periods of 25, 23, 21, 19 and 17. Around the pinwheels the following sequences of letters are engraved:
Pinwheel I or "25 wheel": ABCDEFGHIJKLMNOPQRSTUVXYZ Pinwheel II or "23 wheel": ABCDEFGHIJKLMNOPQRSTUVX Pinwheel III or "21 wheel": ABCDEFGHIJKLMNOPQRSTU Pinwheel IV or "19 wheel": ABCDEFGHIJKLMNOPQRS Pinwheel V or "17 wheel": ABCDEFGHIJKLMNOPQ
(Note the absence of the letter W. This possibly has to do with the machine's Swedish origin, since the letter W is redundant in the Swedish language.)
The wheels have pins at all lettered positions, all of which can be set in either active or inactive position. The five pinwheels all move one step when the lever is operated and since the pinwheels are relatively prime to each other, the period of the machine will be the product of the wheel-sizes (25x23x21x19x17) or 3,900,225.
Behind the five pinwheels is a drum composed of two circular endplates holding 25 horizontal bars. The first bar has one lug in the position corresponding to pinwheel I, bars two and three have a lug each in the position corresponding to pinwheel II, bars 4-7 have one lug each in the position corresponding to wheel III, bars 8-15 have one lug each in the position corresponding to wheel IV, and bars 16-25, finally, have one lug each in the position corresponding to wheel V. The pins of the pinwheels, when in active position on a specific pinwheel, displaces the bars of the drum to the left when encountering a lug, and these bars act as cogs, displacing the circular knob a corresponding number of positions - the so called Displacement number.
Due to the number of lugs chosen (1, 2, 4, 8, and 10 - on the C-36 these are different: 1, 2, 3, 7, and 12) all displacement numbers from 0 (no pin in active position on any pinwheel) to 25 (all pins in active position on all five pinwheels) may arise. Finally the cipher (or plaintext) letter is printed on a paper strip.
Qbasic simulation of the Hagelin C-35 cryptomachine