How soon did the Allies start breaking Japanese naval codes?

I know that before Pearl Harbor, the British and Americans had broken the Japanese diplomatic code, but not the naval code (since before war started there weren't enough messages to analyze). However, just a few months after the war began, both the British and Americans were able to decipher the Japanese naval code JN-25. This allowed the British to anticipate the Indian Ocean Raid in April 1942 and for the Americans to anticipate Coral Sea and Midway in May and June. But how early were the Allies able to decode Japanese naval communications? March? February seems too early, since the Darwin raid was a surprise.
 
The JN25 Code is the closest to what you are thinking of. There were several others used by the IJN. Several sources confirm it was 'broken' or penetrated before December 1941. The Japanese were uneven in their signals security, but as you say there was not much material to work with in 1940-41. i.e: The Japanese left the regular radio operators of Nagumos fleet ashore in Hokkaido, sending fake messages while he attacked the US fleet @ Oahu. They may have sent deception messages earlier in November & October.

'Secret Allies in the Pacific' has some references to Dutch, Commonwealth, & US exchange of code/encryption information previous to Dec 1941. The Dutch seem to have been focused on the Japanese naval codes & had official contacts with the USN signals intel.

'Code Breaker in the Far East' by Alan Stripp has some details from a actual codebreaker. Ditto for 'The Emperors Codes' the author Michael Smith also worked in the codebreaking section in the Far East signals intelligence.

Stripp gives the following 'hints' about when some of the codes/encryptions were broken. Note the ID is usually Allied, not Japanese. Some of the codes Stripp worked with, others he takes from other sources. This is not a complete list. Since the Japanese destroyed a large portion of their signals & message security system before Allied intel could examine its difficult to understand all their codes/encryption methods.

1. JN 14 "probably gave warning of the attack on Miline Bay in September 1942.

2. JN 147 Appeared in 1943. Broken then.

3. JN 23 For new ships construction administration.Post 1942 ?

4. JN 25 First penetrated 1939, "intermittently" This was the primary senior command code of the fleet commanders. Decrypts warned of the raid on Colombo - Operation C, of the IJN move into the Coral Sea & first attempt to invade Miline Bay, & the Midway operation. Stripp notes that the Allied name JN 25 may cover several different systems. The Japanese destroyed all evidence of it before occupation. A earlier Naval command code may have been penetrated as well. This code was under attack by the Brits & Dutch as well as the US. Breaking it may have been a cooperative effort?

5. JN 36 & 37. For weather. 1942.

6. JN 40 Cargo ship code. Nov 1942

7. Unnamed naval code, probably for cargo ships. Broken by capture of code book 1944?

8. Unamed naval cipher. Use & date of penetration not given. Possibly low grade admin traffic?

9. CORAL Naval attache machine cipher. 1943

10. Unnamed Naval attache machine cipher. 1944

11. Unnamed code for Army controlled cargo ships. 1943

12. KA KA KA -Japanese name. Tactical code used in New Guniea. Perhaps one of dozens of low grade small unit codes distributed. 1943.

13. Unnamed Army 1943

14. Unnamed Army address code, independent of the message code. Not clear when broken.

15. Unnamed Army administrative code 1943

16. JMA Military attache use. Mid 1942

17. J 19 Consular code. 1941?

18 PURPLE high grade diplomatic. A machine cipher. From when introduced in 1941. Machine was examined/photographed by spies when delivered to Japanese embassy in Washington as replacement for earlier system.

19. GEAM admin traffic for conquered territories. Date not given

20. Unnamed diplomatic code. Used for forwarding intelligence reports brought to neutral Kabul & sent to Japan. Date of breaking not clear but Stripp says read messages were used to arrest Japanese agents in India

21. Unnamed naval air code. 1943 or 1942

22. Unnamed weather code. Date not clear but Stripp remarks that is was invaluable to Allied Metrologists for the south Asia area.

23. Unnamed Army air force code. 1944

24. Unnamed Army AF code for admin messages. "Easily broken." Date not given

25. BULBUL Army AF operations code. Note clear when broken but at least in 1943.

26. Unnamed Army code or codes? Used at most command levels. Code book captured on Okinawa 1945.

27. 6633 (Japanese name) Army air force code. Versions common through S Asia & S Pacific. 1942
 
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According to his obituary in The Daily Telegraph (which I can't find online anywhere) Eric Nave was breaking Japanese codes years before hostilities- he was on the Shangai station, on Hawkins, and in December 1926 the death of the Emperor gave him an opportunity to match the coded text sent to various embassies, etc, with successor Hirohito's statements to the Japanese press.
By 1927 Nave had unravelled the entire Japanese naval radio organisation so that the GC&CS had a complete list of all their call signs and radio frequencies...he arrived in London in 1928 to form the Japanese naval section of GC&CS, where his main task was the breaking of the Japanese NA code...
[by a similar method to 1926; IJNAs sent back home the contents of articles on naval affairs, and Nave compared the messages to press pieces of similar length].
...in 1934, he finally broke the NA code, and the first Japanese machine cryptograph, known as the "Type 91", or "Red Machine", used for naval and diplomatic messages.
On June 1 1939 the Japanese introduced Kaigun Ango-sho D (Navy Code D), known to the Allies as JN-25.This transmitted the operational (as opposed to diplomatic) signals about the attack on PH...JN-25 was, in fact, an old type of book code used by the US Army and Navy in the Spanish-American War of 1898, and long since discarded as insecure. Nave found it tedious rather than difficult, and had broken into it by the autumn of 1939.
This info from The Daily Telegraph Book of Naval Obituaries, ed David Twiston Davies (2004). Obituary was probably by John Winton (nom de plume of LtCdr John Pratt, RN). Nave was an odd character, and went off the rails somewhat by co-authoring, with "the imaginative James Rusbridger", the dreadful Betrayal at Pearl Harbour: How Churchill Lured Roosevelt into War - conspiracy theory rubbish.
 
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A distinction must be made her about the difference between signals security & message security. By the first I mean making the radio signal itself difficult to track and analyze. Signals analysis seeks to pull important information out to things like the signal strength, radio frequency, transmitter characteristics, habits of the operators, location... and characteristics of the messages other than what the text reads.

Message security is essentially encoding or encryption.

The Japanese had a small but very good signals intelligence section in their navy. They also understood the vulnerability & sought to minimize theirs. Conversely they do not seem to have much for code breaking and do not seem to have understood very well their vulnerabilities. Like the Germans they over estimated the difficulties, and thought that regular key changes were sufficient.

Another point that must be understood, is long complex operational plans are not usually sent via radio. thats for urgent brief coordinating messages. The PH attack plans were distributed in paper copies via courier. So, the conspiracy theorists are frustrated that the 'attack order' for PH was not intercepted & decoded by the US.
 
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