The Breaking of JN-25 and Its Impact in the War Against Japan
Total Page:16
File Type:pdf, Size:1020Kb
From AFIO's The Intelligencer Association of Former Intelligence Officers 7700 Leesburg Pike, Suite 324 Journal of U.S. Intelligence Studies Falls Church, Virginia 22043 Web: www.afio.com * E-mail: [email protected] Volume 26 • Number 2 • Winter-Spring 2021 $15 single copy price result was that we detected absolutely no warning When Intelligence Made a Difference of the impending attack on Pearl Harbor.”5 Imperial Japanese Navy (IJN) communications security and — WWII — misinterpreted radar indications on the morning of December 7 contributed to the lack of warning.6 Iron- ically, IJN messages intercepted prior to Pearl Harbor but not decoded until after the war strongly hinted at planned operations against Hawaii.7 JN-25 – the Imperial Japanese Navy’s Command and Control Code The Breaking of JN-25 The IJN introduced two new enciphered codes and its Impact for general purposes in 1939; American cryptanalysts in the War Against Japan dubbed one of the systems the “Flag Officers Code” and the other, JN-25. The US Navy got little intercept in the Flag Officers Code and the effort against it was by Peter C. Oleson abandoned in December 1941.8 As historian John Prados explains, to commu- Introduction nicate over radio the alphabet is reduced to a code, such as the dots and dashes of Morse Code. To protect hile US Navy radio intelligence efforts in the easily read plaintext the sender uses a letter or digital 1930s and 1940 and 1941 had been focused transposition or substitutes whole words or phrases on Japan, there was no specific forewarning that only the sender and receiver know. Numeric mes- W sages could have a random additive number, which of the surprise attack on Pearl Harbor. The decryption of the Japanese diplomatic PURPLE codes by the US then “were considered ‘enciphered’ or ‘encrypted.’” A Army’s Signal Intelligence Service (SIS) clearly showed cryptanalyst’s recovery of a message required getting a deteriorating relationship with an aggressive Japan, rid of the additives (“deciphering”) using mathemat- especially after the freezing of Japanese assets by the ical techniques, which were aided by having multiple US in July 1941.1 intercepted messages allowing comparison of com- Navy cryptanalytical resources were very limited.2 monalities to gain entry into the code.9 Priority went to the high-level diplomatic cables (i.e., “JN-25 consisted of a codebook with approxi- PURPLE). Station Hypo, “… the radio intelligence unit mately 27,500 entries and an additive book for super- at Pearl Harbor [was assigned] to work exclusively on enciphering the codebook values. The additive book the Imperial Navy’s Admirals’ Code rather than have consisted of 300 pages, each page containing 100 any role in deciphering diplomatic codes or the more random five-digit groups. It should be noted that this widely used JN-25,” the Imperial Japanese Navy’s (IJN) additive book for JN-25 was not a one-time pad: the command and control code.3 Station C (“Cast”), the five-digit groups were re-used, as needed. In studying radio intelligence unit in the Philippines, and the Navy JN-25, US cryptanalysts had to collate large numbers Department’s OP-20-G in Washington led the crypt- of Japanese messages over time. Their first goal was to analytical attack on JN-25b, introduced in December recover the indicator in each message which showed 1940.4 Before December 7, 1941 Navy cryptanalysts where in the additive book numbers were taken; then could not read any of the JN-25 messages. “The net recover and strip away the additives themselves to get 5. Capt. Biard, “Joining the Dungeon,” https://stationhypo.com. See 1. The US Navy devised an analogue machine to “strip the ciphers” also Stephen Budiansky. “The monthly reports filed by OP-20-G off of the Japanese Foreign Ministry’s Red code in the 1930s. After the confirm that at the time of the Pearl Harbor attack, not a single JN-25 Japanese changed to the more complex PURPLE code in 1939 it too message from the previous 12 months had been read.” “Too Late for was solved by September 1940, and the US developed an analogue Pearl Harbor.” U.S. Naval Institute Proceedings 125, no. 12 (Dec. 1999): machine to decipher up to seventy-five messages per day. (John p 47-51. Prados. Combined Fleet Decoded: The Secret History of American Intel- 6. For the story about the missed indications from radar see Harry A. ligence and the Japanese Navy in World War II. Annapolis, MD: Naval Butowsky, PhD, “Mystery of the Opana Radar Site,” http://npshistory Institute Press, 1995, p 163.) .com/publications/valr/opana-radar-site.pdf. 2. Army SIS assets were also limited in Washington. SIS and Op-20-G 7. Prados, p 156-7. shared the translation workload for PURPLE decrypts. 8. “JN-25,” https://www.nsa.gov/about/cryptologic-heritage/center 3. Prados, p 166. -cryptologic-history/pearl-harbor-review/jn25/. 4. Prados, p 175. 9. Prados, p 74. Winter-Spring 2021 Intelligencer: Journal of U.S. Intelligence Studies Page 79 down to the codebook values; and, finally, recover the Attacking JN-25 meanings in plaintext Japanese of the underlying code- On December 10, 1941 the Navy Department gave book values, which would allow messages to be read, Hypo permission to attack the Japanese “five-digit, at least in part. A second version of this system, known two-part, enciphered, widely used strategic code,” to US Navy cryptanalysts as JN-25b, was introduced JN-25.14 In early 1942, Commander Joseph Rochefort, on December 1, 1940. Six months later, the Japanese the commander of Station Hypo, and his staff began Navy replaced the additive book. The additive book to make progress, but painfully slowly.15 In early was replaced in August and again on 4 December 1941, January 1942 the initial breaks were made by Station three days before the Japanese attack on American Cast, OP-20-G in Washington, and soon thereafter bases in Hawaii. These rapid changes in the codebook by the British Far Eastern Combined Bureau (FECB), and its additive required that US cryptanalysts begin an outpost of Britain’s Government Code and Cipher again with each change — virtually at the beginning School at Bletchley Park. “By the end of January… we — to attack the system. It is estimated that prior to were obtaining significant bits of information in more the change of the additive book in August 1941, the and more messages.” By February “… bits of opera- cryptanalysts had recovered only 2,000 code groups in tional intelligence we did… succeed in pulling out of JN-25 — about 4% of the codebook — and these were JN-25 sometimes were verifiable, as in the case when mostly numerals and stereotyped phrases.”10 we could put a submarine on the track of the carrier US Navy cryptanalytic efforts were complicated Kaga, which was returning to Japan for repairs after by the scarcity of cryptanalysts and Japanese transla- hitting a shoal off Malay.”16 tors. The IJN prior to and during World War II devel- oped a form of Morse code which used a syllabary of seventy-three kana, each with one or two suffixes that might alter its meaning.”11 As explained by two Australian professors of mathematics and statistics, “The methods used in the code breaking behind the successes of Allied Pacific Signals Intelligence are quite different to those used against encryption machine ciphers such as the [Nazi’s] Enigma. The reason is that the main cipher systems used by both the… IJA and the… IJN were based on code books rather than a machine.”12 Much of insight gained from radio intelligence before the decryption of JN-25 came from the exter- nals of a transmitted message. Each radio intercept station could get a bearing of a transmission. By comparing two or more station’s bearings allowed By March, the US had cracked part of JN-25. calculation of a transmitter’s geographic position. However, it could intercept only about 60 percent of If bearings changed the transmitter was from a transmissions and by May had the resources to ana- ship or aircraft and its direction and speed could be lyze only about 40 percent of intercepted messages. calculated. As many messages were transmitted at Even then, code breakers typically could read only 10 high frequencies, the US Navy developed by 1940 a to 15 percent of the code groups in a message. Navy network of sixteen High-Frequency Direction Finding radio intelligence traffic analysts primarily used direc- (HFDF – “huff-duff”) stations covering the Atlantic tion-finding and message externals to learn about and Pacific Oceans.13 order of battle and disposition of forces and where ships were heading.17 14. Biard. “Joining the Dungeon.” 15. https://stationhypo.com/2020/03/05/march-5-1942-station-hypo 10. “JN-25,” NSA. -started-reading-japanese-jn-25-2/#more-11789. 11. Prados, p 9. “Katakana and hiragana are both syllabic forms of 16. CAPT Biard. “Doolittle Raid and the Battle of Coral Sea,” https:// writing developed in the ninth century to simplify written Japanese….” stationhypo.com/2016/12/26/capt-biard-doolittle-raid-and-the-battle-of 12. Peter Donovan & John Mack. Code Breaking in the Pacific. Switzer- -coral-sea-part-3-of-4/#more-3057. land: Springer International Publishing, 2014. Preface. 17. “Battle of Coral Sea – What was the ‘RZP’ Campaign?” https:// 13. Prados, p 75. stationhypo.com/2017/04/17/part-1-of-5-battle-of-coral-sea-what-was-the Page 80 Intelligencer: Journal of U.S. Intelligence Studies Winter-Spring 2021 A notable cryptanalytic breakthrough occurred On April 18, the Doolittle raid, launched from as early as January 18, 1942.