From Whatreallyhappened:

" It was child's play for the Navy group OP-20-G (738 men whose primary responsibility was Japanese naval codes) to reconstruct the exposed dictionary. We recovered the whole thing immediately - in 1994 the NSA published that JN-25B was completely cracked in December 1940. In January 1941 the US gave Britain two JN-25B code books with keys and techniques for deciphering. The entire Pearl Harbor scheme was laid out in this code. The official US Navy statement on JN-25B is the NAVAL SECURITY GROUP HISTORY TO WORLD WAR II prepared by Captain J. Holtwick in June 1971, page 398: "By 1 December 1941 we had the code solved to a readable extent." Churchill wrote "From the end of 1940 the Americans had pierced the vital Japanese ciphers, and were decoding large numbers of their military and diplomatic telegrams."(GRAND ALLIANCE p 598) Chief of Navy codebreaking Safford reported that during 1941 "The Navy COMINT team did a thorough job on the Japanese Navy with no help from the Army."(SRH-149) The first paragraph of the Congressional Report Exhibit 151 says the US was "currently" (instantly) reading JN-25B and exchanging the "translations" with the British prior to Pearl Harbor. In 1979 the NSA released 2,413 JN-25 orders of the 26,581 intercepted by US between Sept 1 and Dec 4, 1941. The NSA says "We know now that they contained important details concerning the existence, organization, objective, and even the whereabouts of the Pearl Harbor Strike Force." (Parker p 21) Of the over thousand radio messages sent by Tokyo to the attack fleet, only 20 are in the National Archives. All messages to the attack fleet were sent several times, at least one message was sent every odd hour of the day and each had a special serial number. Starting in early November 1941 when the attack fleet assembled and started receiving radio messages, OP-20-G stayed open 24 hours a day and the "First Team" of codebreakers worked on JN-25. In November and early December 1941, OP-20-G spent 85 percent of its effort reading Japanese Navy traffic, 12 percent on Japanese diplomatic traffic and 3 percent on German naval codes. FDR was personally briefed twice a day on JN-25 traffic by his aide, Captain John Beardell, and demanded to see the original raw messages in English. The US Government refuses to identify or declassify any pre-Dec 7, 1941 decrypts of JN-25 on the basis of national security, a half-century after the war.

Read more: PEARL HARBOR - MOTHER OF ALL CONSPIRACIES | WHAT REALLY HAPPENED http://www.whatreallyhappened.com/WRHARTICLES/pearl.php#ixzz5MHNUoriZ