Station HYPO

Station HYPO, also known as Fleet Radio Unit Pacific (FRUPAC), was the United States Navy signals monitoring and cryptographic intelligence unit in Hawaii during World War II. It was one of two major Allied signals intelligence units, called Fleet Radio Units in the Pacific theaters, along with FRUMEL in Melbourne, Australia. The station took its initial name from the phonetic code at the time for "H" for Heʻeia, Hawaii radio tower. The precise importance and role of HYPO in penetrating the Japanese naval codes has been the subject of considerable controversy, reflecting internal tensions amongst US Navy cryptographic stations. HYPO was under the control of the OP 20 G Naval Intelligence section in Washington. Before the attack on Pearl Harbor of December 7, 1941, and for some time afterwards, HYPO was 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 in Washington divided responsibilities for them among CAST at Cavite and then Corregidor, in the Philippines, HYPO in Hawaii, and OP 20 G itself in Washington. Other Navy crypto stations, including Guam and Bainbridge Island on Puget Sound were tasked and staffed for signals interception and traffic analysis. The US Army's SIS broke into the highest level Japanese diplomatic cypher (called PURPLE by the US) well before the attack on Pearl Harbor. PURPLE produced little of military value, as the Japanese Foreign Ministry was thought by the ultra nationalists to be unreliable. Furthermore, decrypts from PURPLE, eventually called MAGIC, were poorly distributed and used in Washington. SIS was able to build several PURPLE machine equivalents. One was sent to CAST, but as HYPO's assigned responsibility did not include PURPLE traffic, no PURPLE machine was ever sent there. The absence of such a machine on site in Hawaii has long been seen by conspiracy theorists as a reason for US unpreparedness in Hawaii, and/or to be evidence of a conspiracy by high level officials to deprive Pearl Harbor of intelligence known to Washington. However, no hard evidence for any such conspiracy exists.

HYPO was assigned responsibility for work on Japanese Navy systems, and after an agreement with Australia, the United Kingdom and Netherlands to share the effort, worked with crypto groups based at Melbourne, Hong Kong and Batavia. Prior to the attack on Pearl Harbor, the amount of available IJN traffic was low, and little progress had been made on the most important Japanese Navy system, called JN 25 by U. S. analysts. JN 25 was used by the IJN for high level operations: movement and planning commands, for instance. It was a state of the art superencrypted code, eventually a two book system. Cryptanalytic progress was slow. Most references cite about 10% of messages partially (or sometimes completely) decrypted prior to December 1, 1941, at which time a new version of the system went into effect, requiring the cryptanalysts to start again. Although most references did set the limit of the percent of the messages that were decrypted at 10%, they were not privy to the latest information. Wilford in his Decoding Pearl Harbor: USN Cryptanalysis and the Challenge of JN 25B in 1941, suggests that this view is now untenable and that the JN 25 codes were readable to a great extent and hence, lends "support to the revisionist theories of Toland and Stinnett." After the attack on Pearl Harbor, there was considerably more JN 25 traffic as the Japanese Navy operational tempo increased and geographically expanded, which helped progress against it. Hong Kong's contribution stopped until the crypto station there could be relocated (to Ceylon and eventually Kenya), but HYPO and the Dutch at Batavia, in conjunction with CAST and OP 20 G made steady progress. HYPO in particular made significant contributions. Its people, including its commander, Joseph Rochefort, thought a forthcoming Japanese attack early in 1942 was intended for the central Pacific, while opinion at OP 20 G, backed by CAST, favored the North Pacific, perhaps in the Aleutians. In early 1942, in response to the Japanese advances in the Philippines (which threatened CAST), the possibility of an invasion of Hawaii, and the increasing demand for intelligence, another signals intelligence center, known as NEGAT was formed in Washington, using elements of OP 20 G.

As mid 1942 approached, 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 disheveled 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 that resulted in the Battle of Midway, the loss of four Japanese carriers and many naval aviators for much lower Allied losses, and what is generally agreed to have been the turning point of the Pacific War.

In October 1942 after Midway, power struggles within the Navy resulted in the sidelining of Laurance Safford, with the support of Admirals Ernest King and Richmond K. Turner (and Joseph Redman). Control of naval intercept and codebreaking was centralizing in OP 20 G in Washington, where two new sections were headed by John R. Redman (Communications Combat Intelligence section) and Joseph Wenger (Communications Cryptanalytical section; to handle decryption and translation). Safford was shifted to an administrative support and cryptographic research role; so was sidelined for the remainder of the war (doing no further crypto work); as was Joseph Rochefort in Hawaii (he was assigned to command a dry dock on the West Coast). After Midway, Rochefort was recommended for the Distinguished Service Medal to Admiral Ernest King. However, personal enmity from his chief of staff due to a prior passing encounter, and non support from his immediate supervisors, led King to deny the award. Nimitz, when he learned of Rochefort's treatment some years later, was quite displeased. Rochefort was posthumously awarded the medal after a campaign by his intelligence officers galvanized Admiral D. "Mac" Showers and eventually CIA head William Casey to rectify the oversight. Having commandeered a girls' school in Washington (the Army took over one too), Naval Intelligence and OP 20 G expanded greatly. HYPO became a smaller proportion of the total Navy signals intelligence effort in the Pacific.