These were comparatively easily broken by British codebreakers in Singapore and are believed to have been the source of early indications of imminent naval war preparations.
JN-20 exploitation produced the "AF is short of water" message that established the main target of the Japanese Fleet, leading to a decisive U.S. victory at the Battle of Midway in 1942.
[9]: p.155 JN-25 is the name given by codebreakers to the main, and most secure, command and control communications scheme used by the IJN during World War II.
[10] Named as the 25th Japanese Navy system identified, it was initially given the designation AN-1 as a "research project" rather than a "current decryption" job.
John Tiltman with some help from Alan Turing (at GCSB, Government Communications Security Bureau) had "solved" JN25 by 1941, i.e. they knew that it was a five-digit code with a codebook to translate words into five digits and there was a second "additive" book that the sender used to add to the original numbers "But knowing all this didn’t help them read a single message".
British, Australian, Dutch and American cryptanalysts co-operated on breaking JN-25 well before the Pearl Harbor attack, but because the Japanese Navy was not engaged in significant battle operations before then, there was little traffic available to use as raw material.
Publicly available accounts differ, but the most credible agree that the JN-25 version in use before December 1941 was not more than perhaps 10% broken at the time of the attack,[17] and that primarily in stripping away its super-encipherment.
JN-25 traffic increased immensely with the outbreak of naval warfare at the end of 1941 and provided the cryptographic "depth" needed to succeed in substantially breaking the existing and subsequent versions of JN-25.
JN-25d was introduced from 1 April 1943, and while the additive had been changed, large portions had been recovered two weeks later, which provided details of Yamamoto's plans that were used in Operation Vengeance, the shooting-down of his plane.
However, in September 1942, an error by the Japanese gave clues to John MacInnes and Brian Townend, codebreakers at the British FECB, Kilindini.
For various reasons, including the desire not to bring more attention to the article and because the Espionage Act did not cover enemy secrets, the charges were dropped.
A grand jury investigation did not result in prosecution but generated further publicity and, according to Walter Winchell, "tossed security out of the window".
[29] In early August, a RAN intercept unit in Melbourne (FRUMEL) heard Japanese messages, using a superseded lower-grade code.