How did code breakers help stop Japanese advances?
Japanese Naval codes differed from German codes because they primarily book ciphers, while the Germans used mechanical one- the famous Enigma and Lorenz machines.
The Book ciphers worked by have the sender produce his message and then consult the code book. Common words and phrases were replaced with numbers and letters and then transmitted. The receiver of the message looks up each group in the corresponding code book and reforms the message. Extra security measures were taken by enciphering the codes themselves, or superenciphering.
The Japanese Navy had been using this method since the 1920s but it was generally weak and the U.S. and it’s allies had so far been successful at deciphering them. The Japanese Naval code called JN25 by the U.S. had a book of 90,000 words and phrases. Even when the superencipherment was broken down, the meanings of the codes still had to be broken down and inferred. There were a group of codebreakers at Station HYPO, also known as the Fleet Radio Unit Pacific, in Hawaii that were known for being prodigies in solving puzzles and cryptic messages who used IBM punch card machines to find messages using certain code groups.
In 1942, the U.S. began detecting signs of an oncoming attack and the attack was coded as “AF”. No other code in the JN25 was represented by a code group known as AF. Other Naval intelligence Organizations disagreed on the meaning so the staff at HYPO came up with an ingenious experiment to confirm the identity of AF. An underwater cable connected Midway Island to Pearl Harbor and the cable was prone to Japanese interception. HYPO transmitted a message to Midway by cable giving them orders to broadcast a message saying the island’s desalinization plant had broken down. The message was sent out in a way that was easy for the Japanese to read.
Once the Japanese intercepted it, they reported in by sending a message in JN25 stating that AF’s desalinization plant was out of order. Station HYPO intercepted that message and AF was confirmed as Midway. At this point it was only a question of when the attack would take place. After much debate between the intelligence organization, Station HYPO concluded that the attack would come in late May to early June 1942 and was right on the money.
