during the fall of Singapore 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 crypto groups in Hong Kong then Singapore (Far East Combined Bureau) and Batavia (Kamer 14 or Room 14). 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. JN-25B was introduced on 1 December 1940, but was broken immediately by FECB as the additives were not changed. 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. The FECB contribution stopped until the crypto station could be relocated from Singapore to Ceylon, 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 punched 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 History of cryptography Cryptography organizations Locations in the history of espionage Signals intelligence of World War II