TheCodeBreakers
little. Mere interception would not have helped much. The messages bore
no external indication of their intended recipient, and they could not
have been read. Rochefort's attack on Japanese naval codes had
achieved some minor successes in late October and November, but he
could read only about 10 per cent of the naval traffic, and much of this
consisted of weather and other minor systems. The information obtained,
Rochefort said, "was not in any sense vital." Cavite was spottily reading
JN25 messages—which revealed nothing about Pearl Harbor—until
December 4, when the superencipherment was suddenly changed. As a
message that moved on the monitor channel put it: "Five numeral
intercepts subsequent to zero six hundred today indicate change of
cipher system including complete change differentials and indicator
subtracters X All intercepts received since time indicated checked against
all differentials three previous systems X No dupes." Corregidor was not
annaabi.ee 
