One of the most important Allied cryptosystems compromised by the codebreakers of Germany, Finland and Japan was the State Department’s M-138-A strip cipher. This cipher system was used for important messages by US embassies around the world and also by the Office of Strategic Services and the Office of War Information.Unfortunately accurate information on the compromise of this system is limited and the statements made in some of the available TICOM reports are often contradictory. Still it is clear that from 1940 till late 1944 the Axis codebreakers were able to read a lot of the traffic sent on the ‘circular’ and ‘special’ strips.
In complicated cases like this one the only way to find more information is by checking all the available sources. During WWII there was an exchange of information between Germany, Finland and Japan on the State Department’s strip cipher. Some of these messages were intercepted and decoded by the Western Allies, so it is possible to track the progress of the Axis codebreakers through their decoded messages.
For example a message sent from the Japanese military attaché in Helsinki, Finland to Tokyo, Japan in January 1943 lists the alphabet strip solved by the Finnish codebreakers in the previous year (1).
This complicated system was not easy to solve and required extensive research on behalf of the Allied codebreakers. Breaks in the system were possible by taking advantage of the stereotyped form of the reports, the limitations of the Gronsfeld square and by using the Allied cipher material as a ‘crib’ (suspected plaintext in the ciphertext). When the Japanese announced in a message that specific Allied codes would be transmitted in the next ones it was often possible to locate the compromised system and use it to break the Japanese code. For example:
The exchange of Allied cipher material between Japan and Germany started in 1941 but did not lead to close cooperation between the two countries since the Germans distrusted the Japanese and in addition there were many difficulties in transmitting information from Europe to Japan. It seems that in 1944 the German leadership decided to share more information with the Japanese representatives and the Signal Intelligence Agency of the Supreme Command of the Armed Forces - OKW/Chi (Oberkommando der Wehrmacht/Chiffrier Abteilung) gave them decoded Polish and US messages (3).
Notes:(1). British national archives HW 40/132
(2). NARA - RG 457 - Entry 9032 - box 1018 - ‘JAT write up - selections from JMA traffic'(3). British national archives HW 40/132 and HW 40/221
(4). NARA- RG 457 - Entry 9032 - Boxes 205-213(5). NARA - RG 457 - Entry 9032 - Box 214
Acknowledgments: I have to thank Ralph Erskine for pointing out that the Japanese code used to transmit information on Allied codes was a hand system and not the Coral cipher machine.