John Mack

Codebreaking in the Pacific: Cracking the Imperial Japanese Navy’s Main Operational Code, JN-25

The ability to decrypt Japanese naval codes in the Second World War was of great importance to the Allied effort in the Pacific. However despite both the army and navy using the same cipher system, a series of Japanese errors led to the naval codes being successfully attacked by codebreakers even before the start of hostilities. John Mack outlines the cryptographic system, and shows how the Imperial Japanese Navys operational codes were broken through a combination of skill and blunder.

William Friedman and the US Army

William Friedman (1891–1969) was introduced to ciphers in an unexpected way. As a young man he developed an interest in genetics and studied this field at Cornell University from 1911 to 1915. He was then put in contact with George Fabyan, a wealthy man with many interests. Friedman commenced work in the genetics section of Fabyan’s Riverbank Laboratories on his farm in Illinois, set up with the aim of improving some of the farm’s products. Fabyan had also brought Elizabeth Gallup to these Laboratories, with financial support to assist her attempts to prove that the real author of the body of works attributed to William Shakespeare was in fact Francis Bacon. Fabyan’s interest in this was to provide Friedman with a new career.

Guadalcanal and Papua

Two campaigns dominated the second half of 1942 in the Pacific, both being finalised only early in 1943. These were the battle for Guadalcanal in the Solomons and the Japanese attempt to attack and capture Port Moresby, via an overland offensive from Gona and Buna on the northern coast of Papua (the eastern part of New Guinea) across the central spine formed by the Owen Stanley Range (the Kokoda Trail campaign). These were certainly not the campaigns anticipated by the Allies as a consequence of the Coral Sea and Midway battles. Their thinking was already directed towards the recovery of strategically important captured territory. Towards this end, and also from the experience gained in the Coral Sea engagement, the strategic benefit to be obtained from having available an operational airfield close to the eastern tip of New Guinea was appreciated. Following surveillance, the Allied Command selected a coconut plantation at Milne Bay. This had the advantage of providing for the development of a deepwater port in the bay. So, by late June, airstrip construction had commenced, supported by a strong contingent of troops.

From Pearl Harbor to Midway

This chapter outlines the critical 6 months from the Pearl Harbor raid of 7 December 1941 to the end of June 1942. Twenty years earlier Winston Churchill had referred to ‘the incomparable advantage of reading the plans and orders of the enemy before they were executed’ (already quoted in Sect. 1.6). This applies with great force to the crucial Battle of Midway, 4–6 June 1942.

Making Additive Systems Secure

This chapter is to some extent a continuation of Sect. 8.19, which gave some general maxims about cipher security in the WW2 era, but its focus is now on additive systems. After some general remarks on secure use of such systems, subsequent sections investigate techniques for improving the security of their various components.

Communications and Sigint

This chapter gives a brief account of the early development of telegraphic and radio communication systems and of the use of message interception techniques to obtain intelligence of diplomatic or operational value. Thus modern Signals Intelligence was born.

Breaking Additive Systems

This chapter discusses methods of attacking additive systems whose book groups are randomly chosen, rather than all being multiples of three. The whole process is much harder. Captured documents often helped in the later years of the Pacific War.

Recovery of a Code Book

The recovery of the underlying code book forming part of an additive system depends upon the successful decryption of many intercepted messages, thus exposing the bare code equivalents. The skills required for successful decoding of a bare code message are more akin to the linguistic challenge of determining the nature and meaning of an unknown written language than to those needed for elucidating the operation of a cipher machine or the form of a superencipherment. This is particularly so when the underlying plain text is in a foreign language or in the jargon peculiar to a restricted class of users. Two historical examples are given.

Organisation and Reorganisation

The previous chapter gave a contemporary overview of the development of one major Sigint unit, Central Bureau. There were several others, all of which have already been mentioned. The overall Allied Sigint activity was patently extremely expensive, particularly at a time when resources were stretched. Moreover there were various other units working on other sources of intelligence. Decisions must have been made at high levels that the cost was more than justified by the results.

Central Bureau 1942–1945

The only satisfactory account of the operation of Central Bureau is that provided by its own report, the Central Bureau Technical Records (CBTR), written late in 1945 and from which this book’s account of work on the IJA Water Transport Code has been painstakingly derived. This chapter consists mainly (Sect. 17.1—17.11) of an annotated partial text of Part A (entitled Organisation) of the CBTR and gives an overview of its wartime activities. The texts of Parts A to K are available on line via the Recordsearch option on the NAA website. The reference code is B5436. Another copy is in NARA RG457, Box 1086 item 3432. The original text is in conventional Roman type with additional comments in slanted type.