Station CAST
From Wikipedia, the free encyclopedia
In the history of United States cryptographic efforts, Station CAST (for Cavite Navy Yard) has a very important part. CAST was the US Navy's Far East cryptographic operation under the control of the OP-20-G Naval Intelligence section in Washington. It was located at the Navy Yard in Manila, prior to the War, and moved into the tunnels on Corregidor after the Japanese invasion of the Philippines.
Cryptanalytic problems facing the United States in the Pacific prior to WWII were largely those related to Japan. An early decision by OP-20-G divided responsibility for them 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, etc) were tasked and staffed for signals interception and traffic analysis.
[edit] 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 and was therefore kept 'out of the loop'. Furthermore, decrypts from PURPLE traffic, eventually called MAGIC, were rather capriciously distributed to high level officials in Washington and, in general, poorly used. 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.
[edit] 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 British and Dutch to share the effort, worked with Hong Kong and Batavia crypto groups on them. 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 Dec 41, at which time a new edition of the system went into effect and sent all the cryptanalysts back to the beginning.
After 7 Dec 41, 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.
[edit] 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. Some personnel went on to work at the Combined Bureau supplying crypto intelligence to MacArthur's command.
