Background Station HYPO

cryptanalytic problems facing united states in pacific prior world war ii largely related japan. decision op-20-g in washington divided responsibilities them among cast @ cavite , corregidor, in philippines, hypo in hawaii, , op-20-g in washington. other navy crypto stations, including guam , bainbridge island on puget sound tasked , staffed signals interception , traffic analysis.

the army s sis broke highest level japanese diplomatic cypher (called purple us) before attack on pearl harbor. purple produced little of military value, japanese foreign ministry thought ultra-nationalists unreliable. furthermore, decrypts purple, called magic, poorly distributed , used in washington. sis able build several purple machine equivalents. 1 sent cast, hypo s assigned responsibility did not include purple traffic, no purple machine ever sent there. absence of such machine on site in hawaii has long been seen conspiracy theorists reason unpreparedness in hawaii, and/or evidence of conspiracy high level officials deprive pearl harbor of intelligence known washington. however, no hard evidence such conspiracy exists.

japanese naval signals in 1941 , 1942

hypo assigned responsibility work on japanese navy systems, , after agreement australia, united kingdom , netherlands share effort, worked crypto groups based @ melbourne, hong kong , batavia. prior attack on pearl harbor, amount of available ijn traffic low, , little progress had been made on important japanese navy system, called jn-25 u.s. analysts. jn-25 used ijn high level operations: movement , planning commands, instance. state-of-the-art superencrypted code, two-book system. cryptanalytic progress slow. references cite 10% of messages partially (or completely) decrypted prior december 1, 1941, @ time new version of system went effect, requiring cryptanalysts start again. although references did set limit of percent of messages decrypted @ 10%, not privy latest information. wilford in decoding pearl harbor: usn cryptanalysis , challenge of jn-25b in 1941, suggests view untenable , jn-25 codes readable great extent , hence, lends support revisionist theories of toland , stinnett.

lcdr joseph j. rochefort led , handpicked many of key codebreakers @ hypo.

after attack on pearl harbor, there considerably more jn-25 traffic japanese navy operational tempo increased , geographically expanded, helped progress against it. hong kong s contribution stopped until crypto station there relocated (to ceylon , kenya), hypo , dutch @ batavia, in conjunction cast , op-20-g made steady progress. hypo in particular made significant contributions. people, including commander, joseph rochefort, thought forthcoming japanese attack in 1942 intended central pacific, while opinion @ op-20-g, backed cast, favored north pacific, perhaps in aleutians.

in 1942, in response japanese advances in philippines (which threatened cast), possibility of invasion of hawaii, , increasing demand intelligence, signals intelligence center, known negat formed in washington, using elements of op-20-g. in words of nsa historian frederick d. parker:

by middle of march 1942, 2 viable naval radio intelligence centers existed in pacific—one in melbourne, australia [frumel], , one, hypo, in pearl harbor, hawaii ... center on corregidor (cast) no longer affiliated fleet command, , collection , processing capabilities rapidly disintegrating result of evacuations of personnel australia , destruction of facilities bombing , gunfire.

japanese traffic intercepted regarding new offensive operation being planned against target identified af. lcdr. jasper homes @ hypo responsible ruse identified af: false report of fresh water shortage on midway radioed in clear, evoking encrypted japanese response noting af reporting water troubles; af had midway.

as mid-1942 approached, hypo under high pressure, , there tales of 36-hour stints, of rochefort working in bathrobe , appearing briefings late , disheveled besides. effort climaxed in last week of may decryption of enough jn-25 traffic understand japanese attack plan @ midway in some, not complete detail. allowed admiral nimitz gamble on ambush resulted in battle of midway, loss of 4 japanese carriers , many naval aviators lower allied losses, , agreed have been turning point of pacific war.