Station CAST

Station CAST was the United States Navy signals monitoring and cryptographicintelligence unit at Cavite Navy Yard in the Philippines, until Cavite was captured by the Japanese forces in 1942, during World War II. It was an important part of the Allied intelligence effort, addressing Japanese communications as the War expanded from China into the rest of the Pacific theaters. As Japanese advances in the Philippines threatened CAST, its staff and services were progressively transferred to Corregidor in Manila Bay, and eventually to a newly-formed US-Australian station, FRUMEL in Melbourne, Australia. [ [http://www.ibiblio.net/hyperwar////PTO/Magic/COMINT-CoralSea/PartOne.html Parker, Frederick D. "A Priceless Advantage: U.S. Navy Communications Intelligence and the Battles of Coral Sea, Midway, and the Aleutians". Fort Meade MD: Center for Cryptologic History, National Security Agency, 1993.] ]

Prior to the war, CAST was the US Navy's Far East cryptographic operation, under the OP-20-G Naval Intelligence section in Washington. It was located at the Navy Yard in Manila and moved into the tunnels on Corregidor, as Japanese attacks increased.

Cryptanalytic problems facing the United States in the Pacific prior to WWII were largely Japanese. An early decision by OP-20-G divided responsibility for Japanese cryptanalysis amongst its various stations. Station CAST (at Manila in the Philippines), Station HYPO (Pearl Harbor, Hawaii), and OP-20-G itself in Washington, shared cryptanalytic duties. Other Stations (on Guam, in Puget Sound on Bainbridge Island, Guam, etc) were tasked and staffed for signals interception and traffic analysis.

PURPLE diplomatic traffic

The US Army SIS break into the highest security Japanese diplomatic cypher (called PURPLE by US analysts) produced very interesting intelligence, but very little of military value, indeed no tactical value, and not much more of direct political value as the Foreign Office in Japan was thought by the ultra-nationalists in effective charge of Japanese foreign and military policy, to be unreliable.Fact|date=August 2007 Furthermore, decrypts from PURPLE traffic, eventually called MAGIC, were rather capriciously distributed to high level officials in Washington, and in general, poorly used.Fact|date=August 2007 SIS was able to build several PURPLE machine equivalents and the distribution of those machines has since been thought controversial. One was sent to Station CAST, and after the US entered the War, one went to Bletchley Park, the center of British cryptographic work.

Japanese Navy crypto systems: JN-25

Stations HYPO and CAST were assigned responsibility for work on Japanese Navy systems, and after the agreement with the United Kingdom and Netherlands to share the effort, worked with Hong Kong and Batavia crypto groups. Prior to the attack on Pearl Harbor, the amount of available traffic was low, and little progress had been made on the most important Japanese Navy system, called by US analysts JN-25. JN-25 was used for high level operations: movement and planning commands, for instance. It was a superencrypted code, eventually a two-book system, and joint cryptanalytic progress was slow. Most references cite about 10% of messages partially (or sometimes completely) decrypted prior to 1 December 1941, at which time a new edition of the system went into effect and sent all the cryptanalysts back to the beginning.

After 7 December 1941, there was considerably more JN-25 traffic as the Japanese Navy operational tempo increased and geographically expanded, and progress against it went better. Hong Kong's contribution stopped until the crypto station there could be relocated, but CAST, HYPO, and the Dutch at Batavia, in conjunction with OP-20-G, made steady progress.

CAST during and after the invasion of the Philippines

Station CAST and its personnel and equipment were moved from Manila to the tunnels on Corregidor as the Japanese approached and spent the next months working there. Eventually, they destroyed their equipment (some IBM punch card machines are said to have been among the gear shoved into the harbor) and were evacuated by submarine to Australia, for service with FRUMEL. Some personnel also worked at the Central Bureau, supplying signals intelligence to MacArthur's South West Pacific Area (command).

References