Japanese cryptology has a documented history extending from cipher systems associated with Japan's feudal period to organized military cryptographic and cryptanalytic practices in the early 20th century. Historical examples include a substitution cipher associated with the Uesugi clan, and cooperation between the Imperial Japanese Army and Polish cryptologists during the 1920s.
Uesugi cipher The cipher system that the Uesugi are said to have used is a simple substitution cipher known as a Polybius square, or "checkerboard." The i-ro-ha alphabet contains forty-eight letters, requiring a seven-by-seven square with one cell left blank. The rows and columns are labeled with numbers or letters. In the example below, both the numbering and the i-ro-ha alphabet begin in the upper-left corner.
{| class="wikitable" |+Numbered i-ro-ha checkerboard cipher |- ! !! 1 !! 2 !! 3 !! 4 !! 5 !! 6 !! 7 |- ! 1 | i || ro || ha || ni || ho || he || to |- ! 2 | chi || ri || nu || ru || wo || wa || ka |- ! 3 | yo || ta || re || so || tsu || ne || na |- ! 4 | ra || mu || u || wi || no || o || ku |- ! 5 | ya || ma || ke || fu || ko || e || te |- ! 6 | a || sa || ki || yu || me || mi || shi |- ! 7 | we || hi || mo || se || su || n ||
To encipher a message, each plaintext syllable is located in the square and replaced by a two-digit coordinate. Syllables with pronunciation marks, such as ga, are represented by their base forms; for example, ga is encoded in the same way as ka. The rows and columns may also be identified by letters rather than numbers. In the Uesugi cipher, the final fourteen characters of the Iroha poem can be used as row and column headings, as shown in the example below.
{| class="wikitable" |+ Iroha-character checkerboard cipher |- ! re !! ku !! fu !! yu !! no !! ki !! a !! |- | we || a || ya || ra || yo || chi || i || tsu |- | hi || sa || ma || mu || ta || ri || ro || re |- | mo || ki || ke || u || re || nu || ha || na |- | se || yu || fu || wi || so || ru || ni || ku |- | su || me || ko || no || tsu || wo || ho || mi |- | n || mi || e || o || ne || wa || he || e |- | || shi || te || ku || na || ka || to || shi
The American Black Chamber The American Black Chamber, led by Herbert O. Yardley, broke key Japanese diplomatic codes before the 1921–1922 Washington Naval Conference. During the conference, decrypted Japanese communications revealed Tokyo's negotiating position on naval ratios, giving American negotiators insight into the minimum terms Japan was prepared to accept.
Yardley's team analyzed the frequency and relationships of two-letter code groups and tested probable Japanese words against the intercepted messages. According to historian David Kahn, clues including Airurando dokuritsu (“Ireland independence”), Doitsu ("Germany"), and owari ("stop") contributed to the breakthrough in solving the Japanese diplomatic code.
Polish cryptological training Herbert O. Yardley later attributed improvements in Japanese codes to a Polish cryptographic expert. Later research identified the expert as Jan Kowalewski, whose mission to Japan took place from January to March 1923. Kowalewski lectured in Tokyo on cryptanalysis and radio intelligence, training Japanese officers and contributing to the development of Japanese cryptological capabilities in both the army and navy.
Japanese interest in Polish cryptological assistance grew from intelligence failures during negotiations with Soviet Russia at Dairen in 1921–1922. Japanese officers were unable to decipher intercepted Russian diplomatic messages and sent the material to Warsaw, where Polish cryptologists under Jan Kowalewski broke several of the codes within about a week. The success demonstrated the capabilities of Polish cryptanalysis and contributed to the Japanese General Staff's decision to invite Kowalewski to Japan.
At the beginning of 1923, Kowalewski travelled to Tokyo for a three-month course organized by the Japanese General Staff. His instruction covered methods of deciphering Soviet codes as well as the structure of contemporary European diplomatic and intelligence codes. Japanese officers were subsequently sent to Poland for longer periods of cryptological training, including Hyakutake Haruyoshi and Kudō Katsuhiko in 1926, Sakai Naoji and Ōkubo Shunjirō in 1929, and Sakurai Nobuta and Fukai Eiichi in 1935.
Hyakutake's ten-chart code
In 1926, Hyakutake Harukichi became chief of the code section of the Third Department of the Imperial Japanese Army General Staff. He developed a two-letter system using ten separate code charts to replace an earlier four-letter code used by Japanese military attachés. According to Herbert O. Yardley, messages could switch between the different charts by means of indicators, and American cryptanalysts eventually identified the system and broke it.
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