The British cryptographers of Bletchley Park codenamed German teleprinter ciphers FISH. The Geheimfernschreiber in particular was codenamed STURGEON. Another FISH cipher was the Lorenz machine, or Tunny. (It is sometimes stated that FISH refers only to the Geheimfernschreiber, or that the Geheimfernschreiber is the same as the Lorenz machine. This is not correct.)
The teleprinters of the day output each character as five parallel bits on five lines, typically encoded in the Baudot code or something similar. The Geheimfernschreiber had ten pinwheels, which were stepped in a complex, nonlinear way based on their positions from various delays in the past, but in such a way they could never stall. Each of the five plaintext bits was then XORed with the XOR sum of 3 taps from the pinwheels, and then cyclically adjacent pairs of plaintext bits were swapped or not, according to XOR sums of three (different) output bits. The numbers of pins on all the wheels were relatively prime, and the triplets of bits that controlled each XOR or swap were selectable through a plugboard.
This produces a much more complex cipher than the Lorenz machine, and also means that the Geheimfernschreiber is not just a pseudorandom number generator + XOR cipher. For example, if a cipher clerk erred and sent two different messages using exactly the same settings - a depth of two in Bletchley jargon - this could be detected statistically but was not immediately and trivially solvable as it would be with Lorenz.
The British did not break STURGEON as regularly as they broke Enigma or Tunny. Partly this was because the Geheimfernschreiber was by far the most complex cipher of the three, but also because the Luftwaffe very often retransmitted STURGEON messages on easier circuits - thus attacking STURGEON was not the most economical way to get the plaintext!
Related Topics
World War II Era Encryption Devices
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