page_title: Cryptanalysis of the Lorenz cipher
section_title: Interception
hierarchical_section_title: Cryptanalysis of the Lorenz cipher / Interception
caption_attribution_description: English: A length of tape, 12mm (0.5in) wide, produced by an undulator similar to those used during the Second World War for intercepted 'Tunny' wireless telegraphic traffic at Knockholt, for translation into ITA2 characters to be sent to Bletchley Park
context_page_description: Cryptanalysis of the Lorenz cipher was the process that enabled the British to read high-level German army messages during World War II. The British Government Code and Cypher School at Bletchley Park decrypted many communications between the Oberkommando der Wehrmacht in Berlin and their army commands throughout occupied Europe, some of which were signed "Adolf Hitler, Führer". These were intercepted non-Morse radio transmissions that had been enciphered by the Lorenz SZ teleprinter rotor stream cipher attachments. Decrypts of this traffic became an important source of "Ultra" intelligence, which contributed significantly to Allied victory.
  For its high-level secret messages, the German armed services enciphered each character using various online Geheimschreiber stream cipher machines at both ends of a telegraph link using the 5-bit International Telegraphy Alphabet No. 2. These machines were subsequently discovered to be the Lorenz SZ for the army, the Siemens and Halske T52 for the air force and the Siemens T43, which was little used and never broken by the Allies.
context_section_description: Intercepting Tunny transmissions presented substantial problems. As the transmitters were directional, most of the signals were quite weak at receivers in Britain. Furthermore, there were some 25 different frequencies used for these transmissions, and the frequency would sometimes be changed part way through. After the initial discovery of the non-Morse signals in 1940, a radio intercept station called the Foreign Office Research and Development Establishment was set up on a hill at  Ivy Farm  at Knockholt in Kent, specifically to intercept this traffic. The centre was headed by Harold Kenworthy, had 30 receiving sets and employed some 600 staff. It became fully operational early in 1943.
  Because a single missed or corrupted character could make decryption impossible, the greatest accuracy was required. The undulator technology used to record the impulses had originally been developed for high-speed Morse. It produced a visible record of the impulses on narrow paper tape. This was then read by people employed as "slip readers" who interpreted the peaks and troughs as the marks and spaces of ITA2 characters. Perforated paper tape was then produced for telegraphic transmission to Bletchley Park where it was punched out.
caption: A length of tape, 12 millimetres (0.47 in) wide, produced by an undulator similar to those used during the Second World War for intercepted 'Tunny' wireless telegraphic traffic at Knockholt, for translation into ITA2 characters to be sent to Bletchley Park
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