JN-25: The Imperial Japanese Navy’s Primary World War II Naval Cipher

The National Museum of Computing (TNMOC) is home to the world's largest collection of working historic computers.Follow the development of computing: from the Turing-Welchman Bombe and Colossus of the 1940s through the large systems and mainframes of the 1950s, 60s and 70s, to the rise of personal computing and the rise of mobile computing and the internet.Recognised as one of England’s top 100 ‘irreplaceable places’, we welcome corporate and group visits, schools and individuals from all over the world.We are normally open Tuesday to Friday 10:30 -17:00, Saturday & Sunday 10:30 – 17:00. Please check tickets for actual opening days and times.TNMOC operates independently of Bletchley Park Trust, although we are located on the Bletchley Park Estate.To visit us, go through the main Bletchley Park entrance, through the barrier, walk or drive bearing left beyond the main car parks, to our separate entrance located at the top of the Park.Visit their website here for more information.https://www.tnmoc.org/Sat Nav users: please use the postcode for the railway station MK3 6DS on Sherwood Drive and then follow signs. The postcode for Bletchley Park will actually take you to an old entrance which is closed.

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A Virtual Talk by Professor Chris ChristensenA successor to the Red Book and Blue Book codes, JN-25 was introduced by the Imperial Japanese Navy on 1 June 1939. JN-25 was a 5-digit code that was enciphered by additives. It evolved through a sequence of codebooks, additive tables, and “Uses.” The cipher was first broken by British codebreaker John Tiltman, but after the Japanese attacks that led to war in the Far East, the primary responsibility for breaking JN-25 messages was assigned to the US Navy’s codebreaking section OP-20-G. This presentation will explore the structure and evolution of the cipher and describe the basic ideas of the codebreaking attacks on JN-25.About the speakerProfessor Christensen is Professor of Mathematics at Northern Kentucky University and takes a considerable interest in Japanese Cryptologic systems, and breaking them, during WWII. He is also the review editor for the prestigious Cryptologia magazine.

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