JN25: The Imperial Japanese Navy's Primary World War II Naval Cipher | Virtual Talk 

A Virtual Talk by Professor Chris Christensen
A 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. 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.
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