Lorenz cipher

The SZ40 and SZ42 from C. Lorenz AG were attachments bolted onto an ordinary teleprinter. Each character of the Baudot code was XORed bit by bit with a key stream generated by twelve wheels, and the receiving machine XORed it back off. The British called the traffic Fish and this particular machine Tunny. There was far less of it than Enigma traffic, but it ran between Berlin and army group headquarters — the level at which plans get discussed rather than submarine positions.

In August 1941 a German operator retransmitted a 4,000-character message on the same wheel settings, with slightly different abbreviations. From that single depth John Tiltman recovered the key stream, and Bill Tutte then spent a couple of months with pencil and paper deducing the machine's entire internal structure — wheel count, wheel lengths, combining logic — without anyone in Britain having laid eyes on one. Only then did it make sense to build Heath Robinson and Colossus to find the settings mechanically.