Station HYPO
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In the history of United States cryptographic efforts, Station HYPO (for Hawaii) has a very important part, and has been the subject of considerable controversy, both at the time and since. HYPO was the US Navy's Hawaiian cryptographic operation under the control of the OP-20-G Naval Intelligence section in Washington. It was located, prior to the 7th, and for sometime afterwards, in the basement of the Old Administration Building at Pearl Harbor. Later on, a new building was constructed for the Station, though it had been reorganized and renamed by then.
Cryptanalytic problems facing the United States in the Pacific prior to World War II were largely those related to Japan. An early decision by OP-20-G divided responsibility for them amongst the 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 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. Station HYPO's assigned responsibility did not include PURPLE traffic, so no PURPLE machine equivalent was ever sent to it. The absence of such a machine on site in Hawaii has long been seen by some as a reason for Hawaiian unpreparedness, or perhaps as evidence of a conspiracy by assorted high level officials to deprive Pearl Harbor of intelligence known to Washington.
[edit] Japanese Navy crypto systems: JN-25
Station HYPO was 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 HYPO and the Dutch at Batavia, in conjunction with CAST and OP-20-G made steady progress. HYPO in particular made some significant contributions. Its people, including its commander, Joseph Rochefort thought that a forthcoming Japanese attack (spring of '42) was intended for the central Pacific, while opinion at OP-20-G, backed by CAST, was in favor of the North Pacific, perhaps the Aleutians. One of HYPOS's personnel was responsible for the ruse which identified a call sign involved in that prospective Japanese operation as Midway Island (this involved a false claim of a fresh water shortage on Midway, broadcast in clear, which evoked an encrypted Japanese response). As mid-year of 1942 approached, Station HYPO was under high pressure, and there are tales of 36-hour stints, of Rochefort working in his bathrobe and appearing for briefings late and dissheveled besides. This effort climaxed in the last week of May with the decryption of enough JN-25 traffic to understand the Japanese attack plan at Midway in some, but not complete, detail. This allowed Admiral Nimitz to gamble on the ambush which resulted in the Battle of Midway, the loss of four Japanese carriers and many naval aviators, and what is generally agreed to have been the turning point of the Pacific War.
[edit] Post Midway transfers and changes
After Midway, disputes between Rochefort, on behalf of Station HYPO, and OP-20-G in Washington, together some bureaucratic infighting in Washington, resulted in the removal of Rochefort from command of the Hawaiian operation and eventually in his assignment to a minor land command on the West Coast. He did no further crypto work for the rest of the War; many writers have expressed astonishment at thus discarding one of the two or three most qualified Navy cryptographic and Japanese language experts. Nimitz, when he learned of Rochefort's treatment some years later, was quite displeased.
After having commandeered a girls' school in Washington (the Army took over one too), Naval Intelligence expanded greatly, as did OP-20-G, and so Station HYPO became less important in any case.
