There was no information about an attack on Pearl Harbor. There were strong implications about an attack somewhere. Everybody thought that meant the Philippines. Hawaii was far out of range of the IJN, they had to draft every modern civilian tanker in their merchant marine to make the strike possible.
Exactly. Not only that, but Magic was not quite what people often think it is.
Japan did not use just one code, but at least five different codes. The one that gave the US the most trouble was PURPLE, which used a machine similar to ENIGMA to generate messages. It used another code system called BLUE to cypher messages, which was then run through the machines to give PURPLE messages. Essentially coding the message a second time. But these were not reliably decoded until mid-1942, and this was the code that the Japanese Navy used.
The Japanese Foreign Office however was not given PURPLE machines. They were still sending their messages using BLUE. This was a code Japan started using in 1930, and the US had broken in 1932. The US had been decoding so many messages that with BLUE they could actually decrypt them faster than the Embassies could.
In the months leading to the attack, they saw an increase in messages with BLUE, and the pattern indicated that an attack was coming. However, as there was absolutely no need for the Embassies to know when or where the attack was coming, they were never told. For months those foreign locations were doing things like reducing the number of staff, sending personnel home, and things like that. Years later Congress ordered an investigation into if Pearl Harbor was in any of the messages ever sent prior to the war breaking out.
And in a testament to Japanese OPSEC, not a single message (decoded or not) was ever found to have been sent about when an attack was going to happen, or where on 7 December. The closest thing ever found that Japan had transmitted is the infamous "14 Part Message", that was sent early in the morning on 7 December.
The full text of the Japanese 14-part message delivered to Secretary of State Cordell Hull on the afternoon of December 7, 1941 -- after the attack on Pearl Harbor. Note that the text does <b>not</b> declare war, or even break diplomatic relations -- it merely declares an impasse in the ongoing...
www.ibiblio.org
In fact, Secretary of State Cordell Hull and President Roosevelt both read transcripts of the message even before it was presented to them. The Navy had decrypted it even before the Embassy had. But nowhere in it did it say where the attack was going to happen. Only that the nations were at war.
What happened in MAGIC was a combination of building even more sophisticated decryption machines than even the Japanese had, as well as bringing in a huge number of translators that could read them. But under instructions hand delivered, all attack orders were hand delivered by courier to the respective invasion fleets (there were three main invasions), and not a single message was ever broadcast. The only messages ever sent was a final one that was not to be acknowledged from Japan telling them to proceed, and then a return message back to Japan once the attacks had started with the initial prognosis of the success of the attacks.
The command to continue the attack from Japan was "Climb Mount Nitaka", and this was sent from Japan to the fleets, and signaled they were to commence "Operation Z", their own code name for the attacks on 7 December. The attacks themselves also went out under radio silence. Never broken until the attack itself commenced, with the repeating of a single word repeated three times.
For an attack starting upon an unsuspecting and unprepared enemy, this was the Japanese word for "Tiger". Hence, the connection of "Tora, Tora, Tora" among historians. This was the message sent from the attacking airplanes to the fleet as the attack started. This was then repeated back to Japan after the attacks were concluded and the fleet was returning home..
It has been confirmed that the "Climb Mount Nitaka" message was received prior to the attack. However, it was so brief that it was ignored until afterwards. And even if it had been decoded in real time, without any context it literally would have meant nothing to the cryptographers. Simply one of many messages that all militaries would use on occasion, simply to ensure that the code systems were functioning properly. It would be like if the US was following similar protocols, and the Germans had broken our codes. And the only thing sent prior to 6 June was a message never acknowledged from Washington that said "Lewis and Clark". It would be completely nonsensical to anybody with that, as it was only the command to do the attack, without any connection to the attack itself.
Because there was not just one attack on 7 December (8 December in Japan), but three. The raid on Pearl Harbor, the Invasion of the Philippines, and the Invasion of British Malaysia. And not a single message relating to any of those three attacks was ever broadcast over the radio. All instructions and orders were hand delivered only.
And the true capability of MAGIC was only fully realized many months later, when through decodes shoed an attack was about to happen on an island they called "AF". But the US had no way of knowing what AF actually was. So a variation of the "Canary Trap" was used, where messages were sent relating to false emergencies at each of the various outposts, each one with a different issue that needed to be addressed immediately. The only one that mattered though was the message requesting emergency water purification at one outpost as their system had broken down.
Hence, to any student of the Battle of Midway, "AF is short of water" is of critical importance. Less than a day after the message was sent by the US Navy, the IJN was telling it's invasion commanders to bring extra water processing equipment as the equipment on their target were not working.