The German company Siemens developed a mechanical teleprinter cipher
machine in the 1930s that was the first in a series of such machines. The
generic name was ``Der Geheimschreiber'', which the crypto department called
``G-skrivare'' (G-writer). In addition to the previously
mentioned classical type
``overlaying'', it also made a permutation
of the pulse order as
another encryption function.
The crypto department later used the expression ``transposition'' for this permutation. A polarity inversion was made with five relays, and five others took care of the transposition.[1] The relays were controlled by ten coding wheels which, through a set of plugs and jacks, could be connected to relays in an arbitrary way. The principle of transposition of the teleprinter pulses is not suitable for all types of teleprinters. The five pulses must be available simultaneously in the transmitter and the receiver for a permutation to take place. In the transmitter this is not so difficult to arrange, but it is much more difficult in the receiver. However, Siemens had solved the problem for the receiver even without access to modern digital technology. All functions were mechanical in the cipher machines of those days. An incoming character's five pulses, positive or negative, charged five capacitors in sequence. When the fifth pulse was received the information stored in the capacitors was simultaneously transferred to five polarized relays. These relays were part of a circuit that selected the character to be printed. During this transfer it was possible to produce a transposition by changing the connections between the capacitors and the relays.
The Geheimschreiber's ten code wheels had the periods 47, 53, 59, 61, 64, 65, 67, 69, 71 and 73. In the first models all the wheels moved one step for each enciphered character. Since the wheel periods were relatively prime, that is they had no common factor, the total period of the machine -- the number of steps the machine must make to return to its starting position -- was equal to the product of all the individual wheel periods, that is 893 622 318 929 520 960 steps. This number also indicates the number of possible wheel starting positions.
The ``transposition circuit'', that is the insertion of the ``transposition relays'' between the rows, could be varied. Eight basic patterns were possible, each with 2 612 736 000 variations.[2] The combinations of connections and wheel adjustments were, before the creation of the computer, considered to be extremely large numbers. In addition there were the number of ways of connecting the code wheels to the relays. This may have given the Germans the impression that the Geheimschreiber was a very secure cipher machine. It was probably considered to be more secure than the Enigma machine which was intended for tactical use. The Enigma had, for example, a period of 17576.[3]
The Geheimschreiber was gradually developed and several models were brought into service. The first machine the crypto department came in contact with was called T52A/B. Later T52C, D and E came into service. There were also variants of the different models. However, they were all based on the same basic principle.
Photo 2 - ``Geheimschreiber'' or ``G-skrivaren''
(Schlüsselfernschreibmachine T52C).
At the end of 1941, a new machine designated Z appeared in the traffic. It did not belong to the A/B-serie and it was called Geheimzusatz 40[4] and not Geheimschreiber. It was a stand-alone attachment that was connected between the teleprinter and the transmission line. It could therefore have been used together with teleprinters other than the Siemens machine that the Geheimschreiber was based on.