Japanese naval signals in 1941 and early 1942 Station HYPO

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.