Geheimschreiber

Geheimschreiber (secret writer), German name for a non-Morse cipher machine. Its encoded messages were transmitted by a teleprinter which used the Baudot-Murray code where patterns of holes on the teleprinter's paper tape represent the letters of the alphabet. The teleprinter translated the transmissions into pulses which could be automatically converted back into a plain language message at the receiving end. They were beamed point-to-point between German stations first detected by the British in 1940 and a number of military links, most of them on the Eastern Front, had been identified by 1942.

Work on decrypting the messages, known to the British by the codename FISH, began at Bletchley Park in the middle of 1942. Unlike the ENIGMA cipher machine, which had three, four, or five rotary wheels, the Geheimschreiber had as many as ten which made the cipher much harder to break. Nothing of much value was therefore achieved until new machines were devised during the following months to help decrypt the signals. Because of its completely experimental nature, the first of these was known as ‘Heath Robinson’; it was succeeded by Colossus I and then by Colossus II, the latter being now recognized as the first electronic digital computer. Colossus II came into operation just in time to decrypt messages between the German High Command (OKW) and C-in-C West on 6 June 1944, the day the Normandy landings were launched (see OVERLORD), and subsequently produced much valuable information.

FISH was used between senior headquarters, and gradually replaced ENIGMA transmissions at these levels. It was classified as ULTRA intelligence and handled in the same way, and because those signals released into the public domain carry no external marks of their origin it is at present impossible to distinguish between those derived from FISH and those derived from ENIGMA.

Ralph Bennett