The Cipher Department of the High Command of the Wehrmacht () (also Oberkommando der Wehrmacht Chiffrierabteilung or Chiffrierabteilung of the High Command of the Wehrmacht or Chiffrierabteilung of the OKW or OKW/Chi or Chi) was the Signal Intelligence Agency of the Supreme Command of the Armed Forces of the German Armed Forces before and during World War II. OKW/Chi, within the formal order of battle hierarchy OKW/WFsT/Ag WNV/Chi, dealt with the cryptanalysis and deciphering of enemy and neutral states' message traffic and security control of its own key processes and machinery, such as the rotor cipher machine ENIGMA machine. It was the successor to the former Chi bureau () of the Reichswehr Ministry.
Short name
The letter "Chi" for the Chiffrierabteilung ("cipher department") is, contrary to what one might expect, not the Greek letter Chi, nor anything to do with the chi-squared test, a common cryptographic test used as part of deciphering of enciphered message, and invented by Solomon Kullback, but simply the first three letters of the word Chiffrierabteilung.
German cryptology service structure during World War II
From the early 1930s to the start of the war, Germany had a good understanding of, and indeed a lead in, both cryptoanalytic and cryptographic cryptology services. The various agencies had cracked the French–English inter-allied cipher, the Germans with some help from the Italian Communications Intelligence Organization stole American diplomatic codes, and codes taken from the British embassy in Rome, that enabled the breaking of the cipher, leading to some gains early in the war. Although the Germans worked to ensure its cryptologic services were effective at the outbreak of the war, the service offerings fragmented considerably among the German armed forces. OKW/Chi had jurisdiction over all the military cryptologic bureaus, chairing the executive committee. However, for several reasons, including specialisation against opposing forces of a similar type, the inherent independence of agencies, and agencies vying for power and favour from Hitler, it was inevitable that the three military branches of German forces operated independently.
In total eight organizations operated within the forces, each operating on its own terms, even though OKW/Chi was considered the premier organization controlling both cipher creation and decipherment of enemy crypts. These eight bureaus which practiced cryptology were split between military and civilian control:
Military
OKW/Chi: The highest cryptologic bureau for the Wehrmacht supreme command. General der Nachrichtenaufklärung: The high command of the German Army (Wehrmacht) OKH/Chi cipher bureau. Luftnachrichten Abteilung 350: The OKL/Chi cipher bureau for high command of the Luftwaffe. B-Dienst: (Observation Service) The Navy High Command OKM/Chi cipher bureau. Abwehr: Military intelligence gathering and cipher bureau from 1941 to 1944.
Civilian
AA/Pers Z S: (AA) Auswärtiges Amt (Foreign Office Personnel Department) (Pers Z) Sonderdienst (Special Service of Unit Z) The Foreign Office cipher bureau decrypting diplomatic signals. RSHA: The Reichssicherheitshauptamt (Reich Security Main Office), the main cipher bureau for Hitler's personal staff including the SS. Forschungsamt: (Research bureau). Part of OKL and Göring's personnel cipher bureau.
Although OKW/Chi continually pushed for the integration of all five military services, it was last blocked in Autumn 1943 by Ribbentrop, Göring, and Himmler. It was not until 9 November 1944, that OKW/Chi was formally made responsible for control of all signals intelligence activities across all forces, by the order of Hitler.
Background
The OKW/Chi was one of the highest military agencies of the Wehrmacht, but with a dual focus: on cryptography, creating Germany's own secure communication systems, and the monitoring of enemy broadcasts and news analysis. As far as cryptanalysis was concerned, OKW/Chi tended to act as troubleshooter, providing the highest service to the Wehrmacht, instead of setting policy as its power to set military SIGINT was limited.
The unit began as the cipher section of the German Defense Ministry () in 1922. The Cipher Bureau () followed later in the 1930s. They were more interested in diplomatic communications than foreign military communications, which were in short supply, and viewed the diplomatic communications as a way to train staff during peacetime. With the rise of the Nazis, the unit grew from 10 people in 1937 to almost 200 people by the outbreak of World War II. By the end of the war, it had almost 800 individuals working there, and its focus had changed to strategy.